Adding upstream version 6.6.15.upstream/6.6.15

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

18 files changed, 4519 insertions, 0 deletions

diff --git a/Documentation/locking/futex-requeue-pi.rst b/Documentation/locking/futex-requeue-pi.rst
new file mode 100644
index 000000000..dd4ecf452
--- /dev/null
+++ b/Documentation/locking/futex-requeue-pi.rst
@@ -0,0 +1,132 @@
+================
+Futex Requeue PI
+================
+
+Requeueing of tasks from a non-PI futex to a PI futex requires
+special handling in order to ensure the underlying rt_mutex is never
+left without an owner if it has waiters; doing so would break the PI
+boosting logic [see rt-mutex-design.rst] For the purposes of
+brevity, this action will be referred to as "requeue_pi" throughout
+this document.  Priority inheritance is abbreviated throughout as
+"PI".
+
+Motivation
+----------
+
+Without requeue_pi, the glibc implementation of
+pthread_cond_broadcast() must resort to waking all the tasks waiting
+on a pthread_condvar and letting them try to sort out which task
+gets to run first in classic thundering-herd formation.  An ideal
+implementation would wake the highest-priority waiter, and leave the
+rest to the natural wakeup inherent in unlocking the mutex
+associated with the condvar.
+
+Consider the simplified glibc calls::
+
+	/* caller must lock mutex */
+	pthread_cond_wait(cond, mutex)
+	{
+		lock(cond->__data.__lock);
+		unlock(mutex);
+		do {
+		unlock(cond->__data.__lock);
+		futex_wait(cond->__data.__futex);
+		lock(cond->__data.__lock);
+		} while(...)
+		unlock(cond->__data.__lock);
+		lock(mutex);
+	}
+
+	pthread_cond_broadcast(cond)
+	{
+		lock(cond->__data.__lock);
+		unlock(cond->__data.__lock);
+		futex_requeue(cond->data.__futex, cond->mutex);
+	}
+
+Once pthread_cond_broadcast() requeues the tasks, the cond->mutex
+has waiters. Note that pthread_cond_wait() attempts to lock the
+mutex only after it has returned to user space.  This will leave the
+underlying rt_mutex with waiters, and no owner, breaking the
+previously mentioned PI-boosting algorithms.
+
+In order to support PI-aware pthread_condvar's, the kernel needs to
+be able to requeue tasks to PI futexes.  This support implies that
+upon a successful futex_wait system call, the caller would return to
+user space already holding the PI futex.  The glibc implementation
+would be modified as follows::
+
+
+	/* caller must lock mutex */
+	pthread_cond_wait_pi(cond, mutex)
+	{
+		lock(cond->__data.__lock);
+		unlock(mutex);
+		do {
+		unlock(cond->__data.__lock);
+		futex_wait_requeue_pi(cond->__data.__futex);
+		lock(cond->__data.__lock);
+		} while(...)
+		unlock(cond->__data.__lock);
+		/* the kernel acquired the mutex for us */
+	}
+
+	pthread_cond_broadcast_pi(cond)
+	{
+		lock(cond->__data.__lock);
+		unlock(cond->__data.__lock);
+		futex_requeue_pi(cond->data.__futex, cond->mutex);
+	}
+
+The actual glibc implementation will likely test for PI and make the
+necessary changes inside the existing calls rather than creating new
+calls for the PI cases.  Similar changes are needed for
+pthread_cond_timedwait() and pthread_cond_signal().
+
+Implementation
+--------------
+
+In order to ensure the rt_mutex has an owner if it has waiters, it
+is necessary for both the requeue code, as well as the waiting code,
+to be able to acquire the rt_mutex before returning to user space.
+The requeue code cannot simply wake the waiter and leave it to
+acquire the rt_mutex as it would open a race window between the
+requeue call returning to user space and the waiter waking and
+starting to run.  This is especially true in the uncontended case.
+
+The solution involves two new rt_mutex helper routines,
+rt_mutex_start_proxy_lock() and rt_mutex_finish_proxy_lock(), which
+allow the requeue code to acquire an uncontended rt_mutex on behalf
+of the waiter and to enqueue the waiter on a contended rt_mutex.
+Two new system calls provide the kernel<->user interface to
+requeue_pi: FUTEX_WAIT_REQUEUE_PI and FUTEX_CMP_REQUEUE_PI.
+
+FUTEX_WAIT_REQUEUE_PI is called by the waiter (pthread_cond_wait()
+and pthread_cond_timedwait()) to block on the initial futex and wait
+to be requeued to a PI-aware futex.  The implementation is the
+result of a high-speed collision between futex_wait() and
+futex_lock_pi(), with some extra logic to check for the additional
+wake-up scenarios.
+
+FUTEX_CMP_REQUEUE_PI is called by the waker
+(pthread_cond_broadcast() and pthread_cond_signal()) to requeue and
+possibly wake the waiting tasks. Internally, this system call is
+still handled by futex_requeue (by passing requeue_pi=1).  Before
+requeueing, futex_requeue() attempts to acquire the requeue target
+PI futex on behalf of the top waiter.  If it can, this waiter is
+woken.  futex_requeue() then proceeds to requeue the remaining
+nr_wake+nr_requeue tasks to the PI futex, calling
+rt_mutex_start_proxy_lock() prior to each requeue to prepare the
+task as a waiter on the underlying rt_mutex.  It is possible that
+the lock can be acquired at this stage as well, if so, the next
+waiter is woken to finish the acquisition of the lock.
+
+FUTEX_CMP_REQUEUE_PI accepts nr_wake and nr_requeue as arguments, but
+their sum is all that really matters.  futex_requeue() will wake or
+requeue up to nr_wake + nr_requeue tasks.  It will wake only as many
+tasks as it can acquire the lock for, which in the majority of cases
+should be 0 as good programming practice dictates that the caller of
+either pthread_cond_broadcast() or pthread_cond_signal() acquire the
+mutex prior to making the call. FUTEX_CMP_REQUEUE_PI requires that
+nr_wake=1.  nr_requeue should be INT_MAX for broadcast and 0 for
+signal.
diff --git a/Documentation/locking/hwspinlock.rst b/Documentation/locking/hwspinlock.rst
new file mode 100644
index 000000000..6f03713b7
--- /dev/null
+++ b/Documentation/locking/hwspinlock.rst
@@ -0,0 +1,485 @@
+===========================
+Hardware Spinlock Framework
+===========================
+
+Introduction
+============
+
+Hardware spinlock modules provide hardware assistance for synchronization
+and mutual exclusion between heterogeneous processors and those not operating
+under a single, shared operating system.
+
+For example, OMAP4 has dual Cortex-A9, dual Cortex-M3 and a C64x+ DSP,
+each of which is running a different Operating System (the master, A9,
+is usually running Linux and the slave processors, the M3 and the DSP,
+are running some flavor of RTOS).
+
+A generic hwspinlock framework allows platform-independent drivers to use
+the hwspinlock device in order to access data structures that are shared
+between remote processors, that otherwise have no alternative mechanism
+to accomplish synchronization and mutual exclusion operations.
+
+This is necessary, for example, for Inter-processor communications:
+on OMAP4, cpu-intensive multimedia tasks are offloaded by the host to the
+remote M3 and/or C64x+ slave processors (by an IPC subsystem called Syslink).
+
+To achieve fast message-based communications, a minimal kernel support
+is needed to deliver messages arriving from a remote processor to the
+appropriate user process.
+
+This communication is based on simple data structures that is shared between
+the remote processors, and access to it is synchronized using the hwspinlock
+module (remote processor directly places new messages in this shared data
+structure).
+
+A common hwspinlock interface makes it possible to have generic, platform-
+independent, drivers.
+
+User API
+========
+
+::
+
+  struct hwspinlock *hwspin_lock_request(void);
+
+Dynamically assign an hwspinlock and return its address, or NULL
+in case an unused hwspinlock isn't available. Users of this
+API will usually want to communicate the lock's id to the remote core
+before it can be used to achieve synchronization.
+
+Should be called from a process context (might sleep).
+
+::
+
+  struct hwspinlock *hwspin_lock_request_specific(unsigned int id);
+
+Assign a specific hwspinlock id and return its address, or NULL
+if that hwspinlock is already in use. Usually board code will
+be calling this function in order to reserve specific hwspinlock
+ids for predefined purposes.
+
+Should be called from a process context (might sleep).
+
+::
+
+  int of_hwspin_lock_get_id(struct device_node *np, int index);
+
+Retrieve the global lock id for an OF phandle-based specific lock.
+This function provides a means for DT users of a hwspinlock module
+to get the global lock id of a specific hwspinlock, so that it can
+be requested using the normal hwspin_lock_request_specific() API.
+
+The function returns a lock id number on success, -EPROBE_DEFER if
+the hwspinlock device is not yet registered with the core, or other
+error values.
+
+Should be called from a process context (might sleep).
+
+::
+
+  int hwspin_lock_free(struct hwspinlock *hwlock);
+
+Free a previously-assigned hwspinlock; returns 0 on success, or an
+appropriate error code on failure (e.g. -EINVAL if the hwspinlock
+is already free).
+
+Should be called from a process context (might sleep).
+
+::
+
+  int hwspin_lock_timeout(struct hwspinlock *hwlock, unsigned int timeout);
+
+Lock a previously-assigned hwspinlock with a timeout limit (specified in
+msecs). If the hwspinlock is already taken, the function will busy loop
+waiting for it to be released, but give up when the timeout elapses.
+Upon a successful return from this function, preemption is disabled so
+the caller must not sleep, and is advised to release the hwspinlock as
+soon as possible, in order to minimize remote cores polling on the
+hardware interconnect.
+
+Returns 0 when successful and an appropriate error code otherwise (most
+notably -ETIMEDOUT if the hwspinlock is still busy after timeout msecs).
+The function will never sleep.
+
+::
+
+  int hwspin_lock_timeout_irq(struct hwspinlock *hwlock, unsigned int timeout);
+
+Lock a previously-assigned hwspinlock with a timeout limit (specified in
+msecs). If the hwspinlock is already taken, the function will busy loop
+waiting for it to be released, but give up when the timeout elapses.
+Upon a successful return from this function, preemption and the local
+interrupts are disabled, so the caller must not sleep, and is advised to
+release the hwspinlock as soon as possible.
+
+Returns 0 when successful and an appropriate error code otherwise (most
+notably -ETIMEDOUT if the hwspinlock is still busy after timeout msecs).
+The function will never sleep.
+
+::
+
+  int hwspin_lock_timeout_irqsave(struct hwspinlock *hwlock, unsigned int to,
+				  unsigned long *flags);
+
+Lock a previously-assigned hwspinlock with a timeout limit (specified in
+msecs). If the hwspinlock is already taken, the function will busy loop
+waiting for it to be released, but give up when the timeout elapses.
+Upon a successful return from this function, preemption is disabled,
+local interrupts are disabled and their previous state is saved at the
+given flags placeholder. The caller must not sleep, and is advised to
+release the hwspinlock as soon as possible.
+
+Returns 0 when successful and an appropriate error code otherwise (most
+notably -ETIMEDOUT if the hwspinlock is still busy after timeout msecs).
+
+The function will never sleep.
+
+::
+
+  int hwspin_lock_timeout_raw(struct hwspinlock *hwlock, unsigned int timeout);
+
+Lock a previously-assigned hwspinlock with a timeout limit (specified in
+msecs). If the hwspinlock is already taken, the function will busy loop
+waiting for it to be released, but give up when the timeout elapses.
+
+Caution: User must protect the routine of getting hardware lock with mutex
+or spinlock to avoid dead-lock, that will let user can do some time-consuming
+or sleepable operations under the hardware lock.
+
+Returns 0 when successful and an appropriate error code otherwise (most
+notably -ETIMEDOUT if the hwspinlock is still busy after timeout msecs).
+
+The function will never sleep.
+
+::
+
+  int hwspin_lock_timeout_in_atomic(struct hwspinlock *hwlock, unsigned int to);
+
+Lock a previously-assigned hwspinlock with a timeout limit (specified in
+msecs). If the hwspinlock is already taken, the function will busy loop
+waiting for it to be released, but give up when the timeout elapses.
+
+This function shall be called only from an atomic context and the timeout
+value shall not exceed a few msecs.
+
+Returns 0 when successful and an appropriate error code otherwise (most
+notably -ETIMEDOUT if the hwspinlock is still busy after timeout msecs).
+
+The function will never sleep.
+
+::
+
+  int hwspin_trylock(struct hwspinlock *hwlock);
+
+
+Attempt to lock a previously-assigned hwspinlock, but immediately fail if
+it is already taken.
+
+Upon a successful return from this function, preemption is disabled so
+caller must not sleep, and is advised to release the hwspinlock as soon as
+possible, in order to minimize remote cores polling on the hardware
+interconnect.
+
+Returns 0 on success and an appropriate error code otherwise (most
+notably -EBUSY if the hwspinlock was already taken).
+The function will never sleep.
+
+::
+
+  int hwspin_trylock_irq(struct hwspinlock *hwlock);
+
+
+Attempt to lock a previously-assigned hwspinlock, but immediately fail if
+it is already taken.
+
+Upon a successful return from this function, preemption and the local
+interrupts are disabled so caller must not sleep, and is advised to
+release the hwspinlock as soon as possible.
+
+Returns 0 on success and an appropriate error code otherwise (most
+notably -EBUSY if the hwspinlock was already taken).
+
+The function will never sleep.
+
+::
+
+  int hwspin_trylock_irqsave(struct hwspinlock *hwlock, unsigned long *flags);
+
+Attempt to lock a previously-assigned hwspinlock, but immediately fail if
+it is already taken.
+
+Upon a successful return from this function, preemption is disabled,
+the local interrupts are disabled and their previous state is saved
+at the given flags placeholder. The caller must not sleep, and is advised
+to release the hwspinlock as soon as possible.
+
+Returns 0 on success and an appropriate error code otherwise (most
+notably -EBUSY if the hwspinlock was already taken).
+The function will never sleep.
+
+::
+
+  int hwspin_trylock_raw(struct hwspinlock *hwlock);
+
+Attempt to lock a previously-assigned hwspinlock, but immediately fail if
+it is already taken.
+
+Caution: User must protect the routine of getting hardware lock with mutex
+or spinlock to avoid dead-lock, that will let user can do some time-consuming
+or sleepable operations under the hardware lock.
+
+Returns 0 on success and an appropriate error code otherwise (most
+notably -EBUSY if the hwspinlock was already taken).
+The function will never sleep.
+
+::
+
+  int hwspin_trylock_in_atomic(struct hwspinlock *hwlock);
+
+Attempt to lock a previously-assigned hwspinlock, but immediately fail if
+it is already taken.
+
+This function shall be called only from an atomic context.
+
+Returns 0 on success and an appropriate error code otherwise (most
+notably -EBUSY if the hwspinlock was already taken).
+The function will never sleep.
+
+::
+
+  void hwspin_unlock(struct hwspinlock *hwlock);
+
+Unlock a previously-locked hwspinlock. Always succeed, and can be called
+from any context (the function never sleeps).
+
+.. note::
+
+  code should **never** unlock an hwspinlock which is already unlocked
+  (there is no protection against this).
+
+::
+
+  void hwspin_unlock_irq(struct hwspinlock *hwlock);
+
+Unlock a previously-locked hwspinlock and enable local interrupts.
+The caller should **never** unlock an hwspinlock which is already unlocked.
+
+Doing so is considered a bug (there is no protection against this).
+Upon a successful return from this function, preemption and local
+interrupts are enabled. This function will never sleep.
+
+::
+
+  void
+  hwspin_unlock_irqrestore(struct hwspinlock *hwlock, unsigned long *flags);
+
+Unlock a previously-locked hwspinlock.
+
+The caller should **never** unlock an hwspinlock which is already unlocked.
+Doing so is considered a bug (there is no protection against this).
+Upon a successful return from this function, preemption is reenabled,
+and the state of the local interrupts is restored to the state saved at
+the given flags. This function will never sleep.
+
+::
+
+  void hwspin_unlock_raw(struct hwspinlock *hwlock);
+
+Unlock a previously-locked hwspinlock.
+
+The caller should **never** unlock an hwspinlock which is already unlocked.
+Doing so is considered a bug (there is no protection against this).
+This function will never sleep.
+
+::
+
+  void hwspin_unlock_in_atomic(struct hwspinlock *hwlock);
+
+Unlock a previously-locked hwspinlock.
+
+The caller should **never** unlock an hwspinlock which is already unlocked.
+Doing so is considered a bug (there is no protection against this).
+This function will never sleep.
+
+::
+
+  int hwspin_lock_get_id(struct hwspinlock *hwlock);
+
+Retrieve id number of a given hwspinlock. This is needed when an
+hwspinlock is dynamically assigned: before it can be used to achieve
+mutual exclusion with a remote cpu, the id number should be communicated
+to the remote task with which we want to synchronize.
+
+Returns the hwspinlock id number, or -EINVAL if hwlock is null.
+
+Typical usage
+=============
+
+::
+
+	#include <linux/hwspinlock.h>
+	#include <linux/err.h>
+
+	int hwspinlock_example1(void)
+	{
+		struct hwspinlock *hwlock;
+		int ret;
+
+		/* dynamically assign a hwspinlock */
+		hwlock = hwspin_lock_request();
+		if (!hwlock)
+			...
+
+		id = hwspin_lock_get_id(hwlock);
+		/* probably need to communicate id to a remote processor now */
+
+		/* take the lock, spin for 1 sec if it's already taken */
+		ret = hwspin_lock_timeout(hwlock, 1000);
+		if (ret)
+			...
+
+		/*
+		* we took the lock, do our thing now, but do NOT sleep
+		*/
+
+		/* release the lock */
+		hwspin_unlock(hwlock);
+
+		/* free the lock */
+		ret = hwspin_lock_free(hwlock);
+		if (ret)
+			...
+
+		return ret;
+	}
+
+	int hwspinlock_example2(void)
+	{
+		struct hwspinlock *hwlock;
+		int ret;
+
+		/*
+		* assign a specific hwspinlock id - this should be called early
+		* by board init code.
+		*/
+		hwlock = hwspin_lock_request_specific(PREDEFINED_LOCK_ID);
+		if (!hwlock)
+			...
+
+		/* try to take it, but don't spin on it */
+		ret = hwspin_trylock(hwlock);
+		if (!ret) {
+			pr_info("lock is already taken\n");
+			return -EBUSY;
+		}
+
+		/*
+		* we took the lock, do our thing now, but do NOT sleep
+		*/
+
+		/* release the lock */
+		hwspin_unlock(hwlock);
+
+		/* free the lock */
+		ret = hwspin_lock_free(hwlock);
+		if (ret)
+			...
+
+		return ret;
+	}
+
+
+API for implementors
+====================
+
+::
+
+  int hwspin_lock_register(struct hwspinlock_device *bank, struct device *dev,
+		const struct hwspinlock_ops *ops, int base_id, int num_locks);
+
+To be called from the underlying platform-specific implementation, in
+order to register a new hwspinlock device (which is usually a bank of
+numerous locks). Should be called from a process context (this function
+might sleep).
+
+Returns 0 on success, or appropriate error code on failure.
+
+::
+
+  int hwspin_lock_unregister(struct hwspinlock_device *bank);
+
+To be called from the underlying vendor-specific implementation, in order
+to unregister an hwspinlock device (which is usually a bank of numerous
+locks).
+
+Should be called from a process context (this function might sleep).
+
+Returns the address of hwspinlock on success, or NULL on error (e.g.
+if the hwspinlock is still in use).
+
+Important structs
+=================
+
+struct hwspinlock_device is a device which usually contains a bank
+of hardware locks. It is registered by the underlying hwspinlock
+implementation using the hwspin_lock_register() API.
+
+::
+
+	/**
+	* struct hwspinlock_device - a device which usually spans numerous hwspinlocks
+	* @dev: underlying device, will be used to invoke runtime PM api
+	* @ops: platform-specific hwspinlock handlers
+	* @base_id: id index of the first lock in this device
+	* @num_locks: number of locks in this device
+	* @lock: dynamically allocated array of 'struct hwspinlock'
+	*/
+	struct hwspinlock_device {
+		struct device *dev;
+		const struct hwspinlock_ops *ops;
+		int base_id;
+		int num_locks;
+		struct hwspinlock lock[0];
+	};
+
+struct hwspinlock_device contains an array of hwspinlock structs, each
+of which represents a single hardware lock::
+
+	/**
+	* struct hwspinlock - this struct represents a single hwspinlock instance
+	* @bank: the hwspinlock_device structure which owns this lock
+	* @lock: initialized and used by hwspinlock core
+	* @priv: private data, owned by the underlying platform-specific hwspinlock drv
+	*/
+	struct hwspinlock {
+		struct hwspinlock_device *bank;
+		spinlock_t lock;
+		void *priv;
+	};
+
+When registering a bank of locks, the hwspinlock driver only needs to
+set the priv members of the locks. The rest of the members are set and
+initialized by the hwspinlock core itself.
+
+Implementation callbacks
+========================
+
+There are three possible callbacks defined in 'struct hwspinlock_ops'::
+
+	struct hwspinlock_ops {
+		int (*trylock)(struct hwspinlock *lock);
+		void (*unlock)(struct hwspinlock *lock);
+		void (*relax)(struct hwspinlock *lock);
+	};
+
+The first two callbacks are mandatory:
+
+The ->trylock() callback should make a single attempt to take the lock, and
+return 0 on failure and 1 on success. This callback may **not** sleep.
+
+The ->unlock() callback releases the lock. It always succeed, and it, too,
+may **not** sleep.
+
+The ->relax() callback is optional. It is called by hwspinlock core while
+spinning on a lock, and can be used by the underlying implementation to force
+a delay between two successive invocations of ->trylock(). It may **not** sleep.
diff --git a/Documentation/locking/index.rst b/Documentation/locking/index.rst
new file mode 100644
index 000000000..6a9ea96c8
--- /dev/null
+++ b/Documentation/locking/index.rst
@@ -0,0 +1,33 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=======
+Locking
+=======
+
+.. toctree::
+    :maxdepth: 1
+
+    locktypes
+    lockdep-design
+    lockstat
+    locktorture
+    mutex-design
+    rt-mutex-design
+    rt-mutex
+    seqlock
+    spinlocks
+    ww-mutex-design
+    preempt-locking
+    pi-futex
+    futex-requeue-pi
+    hwspinlock
+    percpu-rw-semaphore
+    robust-futexes
+    robust-futex-ABI
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`
diff --git a/Documentation/locking/lockdep-design.rst b/Documentation/locking/lockdep-design.rst
new file mode 100644
index 000000000..56b90eea2
--- /dev/null
+++ b/Documentation/locking/lockdep-design.rst
@@ -0,0 +1,663 @@
+Runtime locking correctness validator
+=====================================
+
+started by Ingo Molnar <mingo@redhat.com>
+
+additions by Arjan van de Ven <arjan@linux.intel.com>
+
+Lock-class
+----------
+
+The basic object the validator operates upon is a 'class' of locks.
+
+A class of locks is a group of locks that are logically the same with
+respect to locking rules, even if the locks may have multiple (possibly
+tens of thousands of) instantiations. For example a lock in the inode
+struct is one class, while each inode has its own instantiation of that
+lock class.
+
+The validator tracks the 'usage state' of lock-classes, and it tracks
+the dependencies between different lock-classes. Lock usage indicates
+how a lock is used with regard to its IRQ contexts, while lock
+dependency can be understood as lock order, where L1 -> L2 suggests that
+a task is attempting to acquire L2 while holding L1. From lockdep's
+perspective, the two locks (L1 and L2) are not necessarily related; that
+dependency just means the order ever happened. The validator maintains a
+continuing effort to prove lock usages and dependencies are correct or
+the validator will shoot a splat if incorrect.
+
+A lock-class's behavior is constructed by its instances collectively:
+when the first instance of a lock-class is used after bootup the class
+gets registered, then all (subsequent) instances will be mapped to the
+class and hence their usages and dependencies will contribute to those of
+the class. A lock-class does not go away when a lock instance does, but
+it can be removed if the memory space of the lock class (static or
+dynamic) is reclaimed, this happens for example when a module is
+unloaded or a workqueue is destroyed.
+
+State
+-----
+
+The validator tracks lock-class usage history and divides the usage into
+(4 usages * n STATEs + 1) categories:
+
+where the 4 usages can be:
+
+- 'ever held in STATE context'
+- 'ever held as readlock in STATE context'
+- 'ever held with STATE enabled'
+- 'ever held as readlock with STATE enabled'
+
+where the n STATEs are coded in kernel/locking/lockdep_states.h and as of
+now they include:
+
+- hardirq
+- softirq
+
+where the last 1 category is:
+
+- 'ever used'                                       [ == !unused        ]
+
+When locking rules are violated, these usage bits are presented in the
+locking error messages, inside curlies, with a total of 2 * n STATEs bits.
+A contrived example::
+
+   modprobe/2287 is trying to acquire lock:
+    (&sio_locks[i].lock){-.-.}, at: [<c02867fd>] mutex_lock+0x21/0x24
+
+   but task is already holding lock:
+    (&sio_locks[i].lock){-.-.}, at: [<c02867fd>] mutex_lock+0x21/0x24
+
+
+For a given lock, the bit positions from left to right indicate the usage
+of the lock and readlock (if exists), for each of the n STATEs listed
+above respectively, and the character displayed at each bit position
+indicates:
+
+   ===  ===================================================
+   '.'  acquired while irqs disabled and not in irq context
+   '-'  acquired in irq context
+   '+'  acquired with irqs enabled
+   '?'  acquired in irq context with irqs enabled.
+   ===  ===================================================
+
+The bits are illustrated with an example::
+
+    (&sio_locks[i].lock){-.-.}, at: [<c02867fd>] mutex_lock+0x21/0x24
+                         ||||
+                         ||| \-> softirq disabled and not in softirq context
+                         || \--> acquired in softirq context
+                         | \---> hardirq disabled and not in hardirq context
+                          \----> acquired in hardirq context
+
+
+For a given STATE, whether the lock is ever acquired in that STATE
+context and whether that STATE is enabled yields four possible cases as
+shown in the table below. The bit character is able to indicate which
+exact case is for the lock as of the reporting time.
+
+  +--------------+-------------+--------------+
+  |              | irq enabled | irq disabled |
+  +--------------+-------------+--------------+
+  | ever in irq  |     '?'     |      '-'     |
+  +--------------+-------------+--------------+
+  | never in irq |     '+'     |      '.'     |
+  +--------------+-------------+--------------+
+
+The character '-' suggests irq is disabled because if otherwise the
+character '?' would have been shown instead. Similar deduction can be
+applied for '+' too.
+
+Unused locks (e.g., mutexes) cannot be part of the cause of an error.
+
+
+Single-lock state rules:
+------------------------
+
+A lock is irq-safe means it was ever used in an irq context, while a lock
+is irq-unsafe means it was ever acquired with irq enabled.
+
+A softirq-unsafe lock-class is automatically hardirq-unsafe as well. The
+following states must be exclusive: only one of them is allowed to be set
+for any lock-class based on its usage::
+
+ <hardirq-safe> or <hardirq-unsafe>
+ <softirq-safe> or <softirq-unsafe>
+
+This is because if a lock can be used in irq context (irq-safe) then it
+cannot be ever acquired with irq enabled (irq-unsafe). Otherwise, a
+deadlock may happen. For example, in the scenario that after this lock
+was acquired but before released, if the context is interrupted this
+lock will be attempted to acquire twice, which creates a deadlock,
+referred to as lock recursion deadlock.
+
+The validator detects and reports lock usage that violates these
+single-lock state rules.
+
+Multi-lock dependency rules:
+----------------------------
+
+The same lock-class must not be acquired twice, because this could lead
+to lock recursion deadlocks.
+
+Furthermore, two locks can not be taken in inverse order::
+
+ <L1> -> <L2>
+ <L2> -> <L1>
+
+because this could lead to a deadlock - referred to as lock inversion
+deadlock - as attempts to acquire the two locks form a circle which
+could lead to the two contexts waiting for each other permanently. The
+validator will find such dependency circle in arbitrary complexity,
+i.e., there can be any other locking sequence between the acquire-lock
+operations; the validator will still find whether these locks can be
+acquired in a circular fashion.
+
+Furthermore, the following usage based lock dependencies are not allowed
+between any two lock-classes::
+
+   <hardirq-safe>   ->  <hardirq-unsafe>
+   <softirq-safe>   ->  <softirq-unsafe>
+
+The first rule comes from the fact that a hardirq-safe lock could be
+taken by a hardirq context, interrupting a hardirq-unsafe lock - and
+thus could result in a lock inversion deadlock. Likewise, a softirq-safe
+lock could be taken by an softirq context, interrupting a softirq-unsafe
+lock.
+
+The above rules are enforced for any locking sequence that occurs in the
+kernel: when acquiring a new lock, the validator checks whether there is
+any rule violation between the new lock and any of the held locks.
+
+When a lock-class changes its state, the following aspects of the above
+dependency rules are enforced:
+
+- if a new hardirq-safe lock is discovered, we check whether it
+  took any hardirq-unsafe lock in the past.
+
+- if a new softirq-safe lock is discovered, we check whether it took
+  any softirq-unsafe lock in the past.
+
+- if a new hardirq-unsafe lock is discovered, we check whether any
+  hardirq-safe lock took it in the past.
+
+- if a new softirq-unsafe lock is discovered, we check whether any
+  softirq-safe lock took it in the past.
+
+(Again, we do these checks too on the basis that an interrupt context
+could interrupt _any_ of the irq-unsafe or hardirq-unsafe locks, which
+could lead to a lock inversion deadlock - even if that lock scenario did
+not trigger in practice yet.)
+
+Exception: Nested data dependencies leading to nested locking
+-------------------------------------------------------------
+
+There are a few cases where the Linux kernel acquires more than one
+instance of the same lock-class. Such cases typically happen when there
+is some sort of hierarchy within objects of the same type. In these
+cases there is an inherent "natural" ordering between the two objects
+(defined by the properties of the hierarchy), and the kernel grabs the
+locks in this fixed order on each of the objects.
+
+An example of such an object hierarchy that results in "nested locking"
+is that of a "whole disk" block-dev object and a "partition" block-dev
+object; the partition is "part of" the whole device and as long as one
+always takes the whole disk lock as a higher lock than the partition
+lock, the lock ordering is fully correct. The validator does not
+automatically detect this natural ordering, as the locking rule behind
+the ordering is not static.
+
+In order to teach the validator about this correct usage model, new
+versions of the various locking primitives were added that allow you to
+specify a "nesting level". An example call, for the block device mutex,
+looks like this::
+
+  enum bdev_bd_mutex_lock_class
+  {
+       BD_MUTEX_NORMAL,
+       BD_MUTEX_WHOLE,
+       BD_MUTEX_PARTITION
+  };
+
+  mutex_lock_nested(&bdev->bd_contains->bd_mutex, BD_MUTEX_PARTITION);
+
+In this case the locking is done on a bdev object that is known to be a
+partition.
+
+The validator treats a lock that is taken in such a nested fashion as a
+separate (sub)class for the purposes of validation.
+
+Note: When changing code to use the _nested() primitives, be careful and
+check really thoroughly that the hierarchy is correctly mapped; otherwise
+you can get false positives or false negatives.
+
+Annotations
+-----------
+
+Two constructs can be used to annotate and check where and if certain locks
+must be held: lockdep_assert_held*(&lock) and lockdep_*pin_lock(&lock).
+
+As the name suggests, lockdep_assert_held* family of macros assert that a
+particular lock is held at a certain time (and generate a WARN() otherwise).
+This annotation is largely used all over the kernel, e.g. kernel/sched/
+core.c::
+
+  void update_rq_clock(struct rq *rq)
+  {
+	s64 delta;
+
+	lockdep_assert_held(&rq->lock);
+	[...]
+  }
+
+where holding rq->lock is required to safely update a rq's clock.
+
+The other family of macros is lockdep_*pin_lock(), which is admittedly only
+used for rq->lock ATM. Despite their limited adoption these annotations
+generate a WARN() if the lock of interest is "accidentally" unlocked. This turns
+out to be especially helpful to debug code with callbacks, where an upper
+layer assumes a lock remains taken, but a lower layer thinks it can maybe drop
+and reacquire the lock ("unwittingly" introducing races). lockdep_pin_lock()
+returns a 'struct pin_cookie' that is then used by lockdep_unpin_lock() to check
+that nobody tampered with the lock, e.g. kernel/sched/sched.h::
+
+  static inline void rq_pin_lock(struct rq *rq, struct rq_flags *rf)
+  {
+	rf->cookie = lockdep_pin_lock(&rq->lock);
+	[...]
+  }
+
+  static inline void rq_unpin_lock(struct rq *rq, struct rq_flags *rf)
+  {
+	[...]
+	lockdep_unpin_lock(&rq->lock, rf->cookie);
+  }
+
+While comments about locking requirements might provide useful information,
+the runtime checks performed by annotations are invaluable when debugging
+locking problems and they carry the same level of details when inspecting
+code.  Always prefer annotations when in doubt!
+
+Proof of 100% correctness:
+--------------------------
+
+The validator achieves perfect, mathematical 'closure' (proof of locking
+correctness) in the sense that for every simple, standalone single-task
+locking sequence that occurred at least once during the lifetime of the
+kernel, the validator proves it with a 100% certainty that no
+combination and timing of these locking sequences can cause any class of
+lock related deadlock. [1]_
+
+I.e. complex multi-CPU and multi-task locking scenarios do not have to
+occur in practice to prove a deadlock: only the simple 'component'
+locking chains have to occur at least once (anytime, in any
+task/context) for the validator to be able to prove correctness. (For
+example, complex deadlocks that would normally need more than 3 CPUs and
+a very unlikely constellation of tasks, irq-contexts and timings to
+occur, can be detected on a plain, lightly loaded single-CPU system as
+well!)
+
+This radically decreases the complexity of locking related QA of the
+kernel: what has to be done during QA is to trigger as many "simple"
+single-task locking dependencies in the kernel as possible, at least
+once, to prove locking correctness - instead of having to trigger every
+possible combination of locking interaction between CPUs, combined with
+every possible hardirq and softirq nesting scenario (which is impossible
+to do in practice).
+
+.. [1]
+
+    assuming that the validator itself is 100% correct, and no other
+    part of the system corrupts the state of the validator in any way.
+    We also assume that all NMI/SMM paths [which could interrupt
+    even hardirq-disabled codepaths] are correct and do not interfere
+    with the validator. We also assume that the 64-bit 'chain hash'
+    value is unique for every lock-chain in the system. Also, lock
+    recursion must not be higher than 20.
+
+Performance:
+------------
+
+The above rules require **massive** amounts of runtime checking. If we did
+that for every lock taken and for every irqs-enable event, it would
+render the system practically unusably slow. The complexity of checking
+is O(N^2), so even with just a few hundred lock-classes we'd have to do
+tens of thousands of checks for every event.
+
+This problem is solved by checking any given 'locking scenario' (unique
+sequence of locks taken after each other) only once. A simple stack of
+held locks is maintained, and a lightweight 64-bit hash value is
+calculated, which hash is unique for every lock chain. The hash value,
+when the chain is validated for the first time, is then put into a hash
+table, which hash-table can be checked in a lockfree manner. If the
+locking chain occurs again later on, the hash table tells us that we
+don't have to validate the chain again.
+
+Troubleshooting:
+----------------
+
+The validator tracks a maximum of MAX_LOCKDEP_KEYS number of lock classes.
+Exceeding this number will trigger the following lockdep warning::
+
+	(DEBUG_LOCKS_WARN_ON(id >= MAX_LOCKDEP_KEYS))
+
+By default, MAX_LOCKDEP_KEYS is currently set to 8191, and typical
+desktop systems have less than 1,000 lock classes, so this warning
+normally results from lock-class leakage or failure to properly
+initialize locks.  These two problems are illustrated below:
+
+1.	Repeated module loading and unloading while running the validator
+	will result in lock-class leakage.  The issue here is that each
+	load of the module will create a new set of lock classes for
+	that module's locks, but module unloading does not remove old
+	classes (see below discussion of reuse of lock classes for why).
+	Therefore, if that module is loaded and unloaded repeatedly,
+	the number of lock classes will eventually reach the maximum.
+
+2.	Using structures such as arrays that have large numbers of
+	locks that are not explicitly initialized.  For example,
+	a hash table with 8192 buckets where each bucket has its own
+	spinlock_t will consume 8192 lock classes -unless- each spinlock
+	is explicitly initialized at runtime, for example, using the
+	run-time spin_lock_init() as opposed to compile-time initializers
+	such as __SPIN_LOCK_UNLOCKED().  Failure to properly initialize
+	the per-bucket spinlocks would guarantee lock-class overflow.
+	In contrast, a loop that called spin_lock_init() on each lock
+	would place all 8192 locks into a single lock class.
+
+	The moral of this story is that you should always explicitly
+	initialize your locks.
+
+One might argue that the validator should be modified to allow
+lock classes to be reused.  However, if you are tempted to make this
+argument, first review the code and think through the changes that would
+be required, keeping in mind that the lock classes to be removed are
+likely to be linked into the lock-dependency graph.  This turns out to
+be harder to do than to say.
+
+Of course, if you do run out of lock classes, the next thing to do is
+to find the offending lock classes.  First, the following command gives
+you the number of lock classes currently in use along with the maximum::
+
+	grep "lock-classes" /proc/lockdep_stats
+
+This command produces the following output on a modest system::
+
+	lock-classes:                          748 [max: 8191]
+
+If the number allocated (748 above) increases continually over time,
+then there is likely a leak.  The following command can be used to
+identify the leaking lock classes::
+
+	grep "BD" /proc/lockdep
+
+Run the command and save the output, then compare against the output from
+a later run of this command to identify the leakers.  This same output
+can also help you find situations where runtime lock initialization has
+been omitted.
+
+Recursive read locks:
+---------------------
+The whole of the rest document tries to prove a certain type of cycle is equivalent
+to deadlock possibility.
+
+There are three types of lockers: writers (i.e. exclusive lockers, like
+spin_lock() or write_lock()), non-recursive readers (i.e. shared lockers, like
+down_read()) and recursive readers (recursive shared lockers, like rcu_read_lock()).
+And we use the following notations of those lockers in the rest of the document:
+
+	W or E:	stands for writers (exclusive lockers).
+	r:	stands for non-recursive readers.
+	R:	stands for recursive readers.
+	S:	stands for all readers (non-recursive + recursive), as both are shared lockers.
+	N:	stands for writers and non-recursive readers, as both are not recursive.
+
+Obviously, N is "r or W" and S is "r or R".
+
+Recursive readers, as their name indicates, are the lockers allowed to acquire
+even inside the critical section of another reader of the same lock instance,
+in other words, allowing nested read-side critical sections of one lock instance.
+
+While non-recursive readers will cause a self deadlock if trying to acquire inside
+the critical section of another reader of the same lock instance.
+
+The difference between recursive readers and non-recursive readers is because:
+recursive readers get blocked only by a write lock *holder*, while non-recursive
+readers could get blocked by a write lock *waiter*. Considering the follow
+example::
+
+	TASK A:			TASK B:
+
+	read_lock(X);
+				write_lock(X);
+	read_lock_2(X);
+
+Task A gets the reader (no matter whether recursive or non-recursive) on X via
+read_lock() first. And when task B tries to acquire writer on X, it will block
+and become a waiter for writer on X. Now if read_lock_2() is recursive readers,
+task A will make progress, because writer waiters don't block recursive readers,
+and there is no deadlock. However, if read_lock_2() is non-recursive readers,
+it will get blocked by writer waiter B, and cause a self deadlock.
+
+Block conditions on readers/writers of the same lock instance:
+--------------------------------------------------------------
+There are simply four block conditions:
+
+1.	Writers block other writers.
+2.	Readers block writers.
+3.	Writers block both recursive readers and non-recursive readers.
+4.	And readers (recursive or not) don't block other recursive readers but
+	may block non-recursive readers (because of the potential co-existing
+	writer waiters)
+
+Block condition matrix, Y means the row blocks the column, and N means otherwise.
+
+	+---+---+---+---+
+	|   | W | r | R |
+	+---+---+---+---+
+	| W | Y | Y | Y |
+	+---+---+---+---+
+	| r | Y | Y | N |
+	+---+---+---+---+
+	| R | Y | Y | N |
+	+---+---+---+---+
+
+	(W: writers, r: non-recursive readers, R: recursive readers)
+
+
+acquired recursively. Unlike non-recursive read locks, recursive read locks
+only get blocked by current write lock *holders* other than write lock
+*waiters*, for example::
+
+	TASK A:			TASK B:
+
+	read_lock(X);
+
+				write_lock(X);
+
+	read_lock(X);
+
+is not a deadlock for recursive read locks, as while the task B is waiting for
+the lock X, the second read_lock() doesn't need to wait because it's a recursive
+read lock. However if the read_lock() is non-recursive read lock, then the above
+case is a deadlock, because even if the write_lock() in TASK B cannot get the
+lock, but it can block the second read_lock() in TASK A.
+
+Note that a lock can be a write lock (exclusive lock), a non-recursive read
+lock (non-recursive shared lock) or a recursive read lock (recursive shared
+lock), depending on the lock operations used to acquire it (more specifically,
+the value of the 'read' parameter for lock_acquire()). In other words, a single
+lock instance has three types of acquisition depending on the acquisition
+functions: exclusive, non-recursive read, and recursive read.
+
+To be concise, we call that write locks and non-recursive read locks as
+"non-recursive" locks and recursive read locks as "recursive" locks.
+
+Recursive locks don't block each other, while non-recursive locks do (this is
+even true for two non-recursive read locks). A non-recursive lock can block the
+corresponding recursive lock, and vice versa.
+
+A deadlock case with recursive locks involved is as follow::
+
+	TASK A:			TASK B:
+
+	read_lock(X);
+				read_lock(Y);
+	write_lock(Y);
+				write_lock(X);
+
+Task A is waiting for task B to read_unlock() Y and task B is waiting for task
+A to read_unlock() X.
+
+Dependency types and strong dependency paths:
+---------------------------------------------
+Lock dependencies record the orders of the acquisitions of a pair of locks, and
+because there are 3 types for lockers, there are, in theory, 9 types of lock
+dependencies, but we can show that 4 types of lock dependencies are enough for
+deadlock detection.
+
+For each lock dependency::
+
+	L1 -> L2
+
+, which means lockdep has seen L1 held before L2 held in the same context at runtime.
+And in deadlock detection, we care whether we could get blocked on L2 with L1 held,
+IOW, whether there is a locker L3 that L1 blocks L3 and L2 gets blocked by L3. So
+we only care about 1) what L1 blocks and 2) what blocks L2. As a result, we can combine
+recursive readers and non-recursive readers for L1 (as they block the same types) and
+we can combine writers and non-recursive readers for L2 (as they get blocked by the
+same types).
+
+With the above combination for simplification, there are 4 types of dependency edges
+in the lockdep graph:
+
+1) -(ER)->:
+	    exclusive writer to recursive reader dependency, "X -(ER)-> Y" means
+	    X -> Y and X is a writer and Y is a recursive reader.
+
+2) -(EN)->:
+	    exclusive writer to non-recursive locker dependency, "X -(EN)-> Y" means
+	    X -> Y and X is a writer and Y is either a writer or non-recursive reader.
+
+3) -(SR)->:
+	    shared reader to recursive reader dependency, "X -(SR)-> Y" means
+	    X -> Y and X is a reader (recursive or not) and Y is a recursive reader.
+
+4) -(SN)->:
+	    shared reader to non-recursive locker dependency, "X -(SN)-> Y" means
+	    X -> Y and X is a reader (recursive or not) and Y is either a writer or
+	    non-recursive reader.
+
+Note that given two locks, they may have multiple dependencies between them,
+for example::
+
+	TASK A:
+
+	read_lock(X);
+	write_lock(Y);
+	...
+
+	TASK B:
+
+	write_lock(X);
+	write_lock(Y);
+
+, we have both X -(SN)-> Y and X -(EN)-> Y in the dependency graph.
+
+We use -(xN)-> to represent edges that are either -(EN)-> or -(SN)->, the
+similar for -(Ex)->, -(xR)-> and -(Sx)->
+
+A "path" is a series of conjunct dependency edges in the graph. And we define a
+"strong" path, which indicates the strong dependency throughout each dependency
+in the path, as the path that doesn't have two conjunct edges (dependencies) as
+-(xR)-> and -(Sx)->. In other words, a "strong" path is a path from a lock
+walking to another through the lock dependencies, and if X -> Y -> Z is in the
+path (where X, Y, Z are locks), and the walk from X to Y is through a -(SR)-> or
+-(ER)-> dependency, the walk from Y to Z must not be through a -(SN)-> or
+-(SR)-> dependency.
+
+We will see why the path is called "strong" in next section.
+
+Recursive Read Deadlock Detection:
+----------------------------------
+
+We now prove two things:
+
+Lemma 1:
+
+If there is a closed strong path (i.e. a strong circle), then there is a
+combination of locking sequences that causes deadlock. I.e. a strong circle is
+sufficient for deadlock detection.
+
+Lemma 2:
+
+If there is no closed strong path (i.e. strong circle), then there is no
+combination of locking sequences that could cause deadlock. I.e.  strong
+circles are necessary for deadlock detection.
+
+With these two Lemmas, we can easily say a closed strong path is both sufficient
+and necessary for deadlocks, therefore a closed strong path is equivalent to
+deadlock possibility. As a closed strong path stands for a dependency chain that
+could cause deadlocks, so we call it "strong", considering there are dependency
+circles that won't cause deadlocks.
+
+Proof for sufficiency (Lemma 1):
+
+Let's say we have a strong circle::
+
+	L1 -> L2 ... -> Ln -> L1
+
+, which means we have dependencies::
+
+	L1 -> L2
+	L2 -> L3
+	...
+	Ln-1 -> Ln
+	Ln -> L1
+
+We now can construct a combination of locking sequences that cause deadlock:
+
+Firstly let's make one CPU/task get the L1 in L1 -> L2, and then another get
+the L2 in L2 -> L3, and so on. After this, all of the Lx in Lx -> Lx+1 are
+held by different CPU/tasks.
+
+And then because we have L1 -> L2, so the holder of L1 is going to acquire L2
+in L1 -> L2, however since L2 is already held by another CPU/task, plus L1 ->
+L2 and L2 -> L3 are not -(xR)-> and -(Sx)-> (the definition of strong), which
+means either L2 in L1 -> L2 is a non-recursive locker (blocked by anyone) or
+the L2 in L2 -> L3, is writer (blocking anyone), therefore the holder of L1
+cannot get L2, it has to wait L2's holder to release.
+
+Moreover, we can have a similar conclusion for L2's holder: it has to wait L3's
+holder to release, and so on. We now can prove that Lx's holder has to wait for
+Lx+1's holder to release, and note that Ln+1 is L1, so we have a circular
+waiting scenario and nobody can get progress, therefore a deadlock.
+
+Proof for necessary (Lemma 2):
+
+Lemma 2 is equivalent to: If there is a deadlock scenario, then there must be a
+strong circle in the dependency graph.
+
+According to Wikipedia[1], if there is a deadlock, then there must be a circular
+waiting scenario, means there are N CPU/tasks, where CPU/task P1 is waiting for
+a lock held by P2, and P2 is waiting for a lock held by P3, ... and Pn is waiting
+for a lock held by P1. Let's name the lock Px is waiting as Lx, so since P1 is waiting
+for L1 and holding Ln, so we will have Ln -> L1 in the dependency graph. Similarly,
+we have L1 -> L2, L2 -> L3, ..., Ln-1 -> Ln in the dependency graph, which means we
+have a circle::
+
+	Ln -> L1 -> L2 -> ... -> Ln
+
+, and now let's prove the circle is strong:
+
+For a lock Lx, Px contributes the dependency Lx-1 -> Lx and Px+1 contributes
+the dependency Lx -> Lx+1, and since Px is waiting for Px+1 to release Lx,
+so it's impossible that Lx on Px+1 is a reader and Lx on Px is a recursive
+reader, because readers (no matter recursive or not) don't block recursive
+readers, therefore Lx-1 -> Lx and Lx -> Lx+1 cannot be a -(xR)-> -(Sx)-> pair,
+and this is true for any lock in the circle, therefore, the circle is strong.
+
+References:
+-----------
+[1]: https://en.wikipedia.org/wiki/Deadlock
+[2]: Shibu, K. (2009). Intro To Embedded Systems (1st ed.). Tata McGraw-Hill
diff --git a/Documentation/locking/lockstat.rst b/Documentation/locking/lockstat.rst
new file mode 100644
index 000000000..536eab8db
--- /dev/null
+++ b/Documentation/locking/lockstat.rst
@@ -0,0 +1,204 @@
+===============
+Lock Statistics
+===============
+
+What
+====
+
+As the name suggests, it provides statistics on locks.
+
+
+Why
+===
+
+Because things like lock contention can severely impact performance.
+
+How
+===
+
+Lockdep already has hooks in the lock functions and maps lock instances to
+lock classes. We build on that (see Documentation/locking/lockdep-design.rst).
+The graph below shows the relation between the lock functions and the various
+hooks therein::
+
+        __acquire
+            |
+           lock _____
+            |        \
+            |    __contended
+            |         |
+            |       <wait>
+            | _______/
+            |/
+            |
+       __acquired
+            |
+            .
+          <hold>
+            .
+            |
+       __release
+            |
+         unlock
+
+  lock, unlock	- the regular lock functions
+  __*		- the hooks
+  <> 		- states
+
+With these hooks we provide the following statistics:
+
+ con-bounces
+	- number of lock contention that involved x-cpu data
+ contentions
+	- number of lock acquisitions that had to wait
+ wait time
+     min
+	- shortest (non-0) time we ever had to wait for a lock
+     max
+	- longest time we ever had to wait for a lock
+     total
+	- total time we spend waiting on this lock
+     avg
+	- average time spent waiting on this lock
+ acq-bounces
+	- number of lock acquisitions that involved x-cpu data
+ acquisitions
+	- number of times we took the lock
+ hold time
+     min
+	- shortest (non-0) time we ever held the lock
+     max
+	- longest time we ever held the lock
+     total
+	- total time this lock was held
+     avg
+	- average time this lock was held
+
+These numbers are gathered per lock class, per read/write state (when
+applicable).
+
+It also tracks 4 contention points per class. A contention point is a call site
+that had to wait on lock acquisition.
+
+Configuration
+-------------
+
+Lock statistics are enabled via CONFIG_LOCK_STAT.
+
+Usage
+-----
+
+Enable collection of statistics::
+
+	# echo 1 >/proc/sys/kernel/lock_stat
+
+Disable collection of statistics::
+
+	# echo 0 >/proc/sys/kernel/lock_stat
+
+Look at the current lock statistics::
+
+  ( line numbers not part of actual output, done for clarity in the explanation
+    below )
+
+  # less /proc/lock_stat
+
+  01 lock_stat version 0.4
+  02-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+  03                              class name    con-bounces    contentions   waittime-min   waittime-max waittime-total   waittime-avg    acq-bounces   acquisitions   holdtime-min   holdtime-max holdtime-total   holdtime-avg
+  04-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+  05
+  06                         &mm->mmap_sem-W:            46             84           0.26         939.10       16371.53         194.90          47291        2922365           0.16     2220301.69 17464026916.32        5975.99
+  07                         &mm->mmap_sem-R:            37            100           1.31      299502.61      325629.52        3256.30         212344       34316685           0.10        7744.91    95016910.20           2.77
+  08                         ---------------
+  09                           &mm->mmap_sem              1          [<ffffffff811502a7>] khugepaged_scan_mm_slot+0x57/0x280
+  10                           &mm->mmap_sem             96          [<ffffffff815351c4>] __do_page_fault+0x1d4/0x510
+  11                           &mm->mmap_sem             34          [<ffffffff81113d77>] vm_mmap_pgoff+0x87/0xd0
+  12                           &mm->mmap_sem             17          [<ffffffff81127e71>] vm_munmap+0x41/0x80
+  13                         ---------------
+  14                           &mm->mmap_sem              1          [<ffffffff81046fda>] dup_mmap+0x2a/0x3f0
+  15                           &mm->mmap_sem             60          [<ffffffff81129e29>] SyS_mprotect+0xe9/0x250
+  16                           &mm->mmap_sem             41          [<ffffffff815351c4>] __do_page_fault+0x1d4/0x510
+  17                           &mm->mmap_sem             68          [<ffffffff81113d77>] vm_mmap_pgoff+0x87/0xd0
+  18
+  19.............................................................................................................................................................................................................................
+  20
+  21                         unix_table_lock:           110            112           0.21          49.24         163.91           1.46          21094          66312           0.12         624.42       31589.81           0.48
+  22                         ---------------
+  23                         unix_table_lock             45          [<ffffffff8150ad8e>] unix_create1+0x16e/0x1b0
+  24                         unix_table_lock             47          [<ffffffff8150b111>] unix_release_sock+0x31/0x250
+  25                         unix_table_lock             15          [<ffffffff8150ca37>] unix_find_other+0x117/0x230
+  26                         unix_table_lock              5          [<ffffffff8150a09f>] unix_autobind+0x11f/0x1b0
+  27                         ---------------
+  28                         unix_table_lock             39          [<ffffffff8150b111>] unix_release_sock+0x31/0x250
+  29                         unix_table_lock             49          [<ffffffff8150ad8e>] unix_create1+0x16e/0x1b0
+  30                         unix_table_lock             20          [<ffffffff8150ca37>] unix_find_other+0x117/0x230
+  31                         unix_table_lock              4          [<ffffffff8150a09f>] unix_autobind+0x11f/0x1b0
+
+
+This excerpt shows the first two lock class statistics. Line 01 shows the
+output version - each time the format changes this will be updated. Line 02-04
+show the header with column descriptions. Lines 05-18 and 20-31 show the actual
+statistics. These statistics come in two parts; the actual stats separated by a
+short separator (line 08, 13) from the contention points.
+
+Lines 09-12 show the first 4 recorded contention points (the code
+which tries to get the lock) and lines 14-17 show the first 4 recorded
+contended points (the lock holder). It is possible that the max
+con-bounces point is missing in the statistics.
+
+The first lock (05-18) is a read/write lock, and shows two lines above the
+short separator. The contention points don't match the column descriptors,
+they have two: contentions and [<IP>] symbol. The second set of contention
+points are the points we're contending with.
+
+The integer part of the time values is in us.
+
+Dealing with nested locks, subclasses may appear::
+
+  32...........................................................................................................................................................................................................................
+  33
+  34                               &rq->lock:       13128          13128           0.43         190.53      103881.26           7.91          97454        3453404           0.00         401.11    13224683.11           3.82
+  35                               ---------
+  36                               &rq->lock          645          [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75
+  37                               &rq->lock          297          [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
+  38                               &rq->lock          360          [<ffffffff8103c4c5>] select_task_rq_fair+0x1f0/0x74a
+  39                               &rq->lock          428          [<ffffffff81045f98>] scheduler_tick+0x46/0x1fb
+  40                               ---------
+  41                               &rq->lock           77          [<ffffffff8103bfc4>] task_rq_lock+0x43/0x75
+  42                               &rq->lock          174          [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
+  43                               &rq->lock         4715          [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54
+  44                               &rq->lock          893          [<ffffffff81340524>] schedule+0x157/0x7b8
+  45
+  46...........................................................................................................................................................................................................................
+  47
+  48                             &rq->lock/1:        1526          11488           0.33         388.73      136294.31          11.86          21461          38404           0.00          37.93      109388.53           2.84
+  49                             -----------
+  50                             &rq->lock/1        11526          [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54
+  51                             -----------
+  52                             &rq->lock/1         5645          [<ffffffff8103ed4b>] double_rq_lock+0x42/0x54
+  53                             &rq->lock/1         1224          [<ffffffff81340524>] schedule+0x157/0x7b8
+  54                             &rq->lock/1         4336          [<ffffffff8103ed58>] double_rq_lock+0x4f/0x54
+  55                             &rq->lock/1          181          [<ffffffff8104ba65>] try_to_wake_up+0x127/0x25a
+
+Line 48 shows statistics for the second subclass (/1) of &rq->lock class
+(subclass starts from 0), since in this case, as line 50 suggests,
+double_rq_lock actually acquires a nested lock of two spinlocks.
+
+View the top contending locks::
+
+  # grep : /proc/lock_stat | head
+			clockevents_lock:       2926159        2947636           0.15       46882.81  1784540466.34         605.41        3381345        3879161           0.00        2260.97    53178395.68          13.71
+		     tick_broadcast_lock:        346460         346717           0.18        2257.43    39364622.71         113.54        3642919        4242696           0.00        2263.79    49173646.60          11.59
+		  &mapping->i_mmap_mutex:        203896         203899           3.36      645530.05 31767507988.39      155800.21        3361776        8893984           0.17        2254.15    14110121.02           1.59
+			       &rq->lock:        135014         136909           0.18         606.09      842160.68           6.15        1540728       10436146           0.00         728.72    17606683.41           1.69
+	       &(&zone->lru_lock)->rlock:         93000          94934           0.16          59.18      188253.78           1.98        1199912        3809894           0.15         391.40     3559518.81           0.93
+			 tasklist_lock-W:         40667          41130           0.23        1189.42      428980.51          10.43         270278         510106           0.16         653.51     3939674.91           7.72
+			 tasklist_lock-R:         21298          21305           0.20        1310.05      215511.12          10.12         186204         241258           0.14        1162.33     1179779.23           4.89
+			      rcu_node_1:         47656          49022           0.16         635.41      193616.41           3.95         844888        1865423           0.00         764.26     1656226.96           0.89
+       &(&dentry->d_lockref.lock)->rlock:         39791          40179           0.15        1302.08       88851.96           2.21        2790851       12527025           0.10        1910.75     3379714.27           0.27
+			      rcu_node_0:         29203          30064           0.16         786.55     1555573.00          51.74          88963         244254           0.00         398.87      428872.51           1.76
+
+Clear the statistics::
+
+  # echo 0 > /proc/lock_stat
diff --git a/Documentation/locking/locktorture.rst b/Documentation/locking/locktorture.rst
new file mode 100644
index 000000000..3e763f77a
--- /dev/null
+++ b/Documentation/locking/locktorture.rst
@@ -0,0 +1,169 @@
+==================================
+Kernel Lock Torture Test Operation
+==================================
+
+CONFIG_LOCK_TORTURE_TEST
+========================
+
+The CONFIG_LOCK_TORTURE_TEST config option provides a kernel module
+that runs torture tests on core kernel locking primitives. The kernel
+module, 'locktorture', may be built after the fact on the running
+kernel to be tested, if desired. The tests periodically output status
+messages via printk(), which can be examined via the dmesg (perhaps
+grepping for "torture").  The test is started when the module is loaded,
+and stops when the module is unloaded. This program is based on how RCU
+is tortured, via rcutorture.
+
+This torture test consists of creating a number of kernel threads which
+acquire the lock and hold it for specific amount of time, thus simulating
+different critical region behaviors. The amount of contention on the lock
+can be simulated by either enlarging this critical region hold time and/or
+creating more kthreads.
+
+
+Module Parameters
+=================
+
+This module has the following parameters:
+
+
+Locktorture-specific
+--------------------
+
+nwriters_stress
+		  Number of kernel threads that will stress exclusive lock
+		  ownership (writers). The default value is twice the number
+		  of online CPUs.
+
+nreaders_stress
+		  Number of kernel threads that will stress shared lock
+		  ownership (readers). The default is the same amount of writer
+		  locks. If the user did not specify nwriters_stress, then
+		  both readers and writers be the amount of online CPUs.
+
+torture_type
+		  Type of lock to torture. By default, only spinlocks will
+		  be tortured. This module can torture the following locks,
+		  with string values as follows:
+
+		     - "lock_busted":
+				Simulates a buggy lock implementation.
+
+		     - "spin_lock":
+				spin_lock() and spin_unlock() pairs.
+
+		     - "spin_lock_irq":
+				spin_lock_irq() and spin_unlock_irq() pairs.
+
+		     - "rw_lock":
+				read/write lock() and unlock() rwlock pairs.
+
+		     - "rw_lock_irq":
+				read/write lock_irq() and unlock_irq()
+				rwlock pairs.
+
+		     - "mutex_lock":
+				mutex_lock() and mutex_unlock() pairs.
+
+		     - "rtmutex_lock":
+				rtmutex_lock() and rtmutex_unlock() pairs.
+				Kernel must have CONFIG_RT_MUTEXES=y.
+
+		     - "rwsem_lock":
+				read/write down() and up() semaphore pairs.
+
+
+Torture-framework (RCU + locking)
+---------------------------------
+
+shutdown_secs
+		  The number of seconds to run the test before terminating
+		  the test and powering off the system.  The default is
+		  zero, which disables test termination and system shutdown.
+		  This capability is useful for automated testing.
+
+onoff_interval
+		  The number of seconds between each attempt to execute a
+		  randomly selected CPU-hotplug operation.  Defaults
+		  to zero, which disables CPU hotplugging.  In
+		  CONFIG_HOTPLUG_CPU=n kernels, locktorture will silently
+		  refuse to do any CPU-hotplug operations regardless of
+		  what value is specified for onoff_interval.
+
+onoff_holdoff
+		  The number of seconds to wait until starting CPU-hotplug
+		  operations.  This would normally only be used when
+		  locktorture was built into the kernel and started
+		  automatically at boot time, in which case it is useful
+		  in order to avoid confusing boot-time code with CPUs
+		  coming and going. This parameter is only useful if
+		  CONFIG_HOTPLUG_CPU is enabled.
+
+stat_interval
+		  Number of seconds between statistics-related printk()s.
+		  By default, locktorture will report stats every 60 seconds.
+		  Setting the interval to zero causes the statistics to
+		  be printed -only- when the module is unloaded.
+
+stutter
+		  The length of time to run the test before pausing for this
+		  same period of time.  Defaults to "stutter=5", so as
+		  to run and pause for (roughly) five-second intervals.
+		  Specifying "stutter=0" causes the test to run continuously
+		  without pausing.
+
+shuffle_interval
+		  The number of seconds to keep the test threads affinitized
+		  to a particular subset of the CPUs, defaults to 3 seconds.
+		  Used in conjunction with test_no_idle_hz.
+
+verbose
+		  Enable verbose debugging printing, via printk(). Enabled
+		  by default. This extra information is mostly related to
+		  high-level errors and reports from the main 'torture'
+		  framework.
+
+
+Statistics
+==========
+
+Statistics are printed in the following format::
+
+  spin_lock-torture: Writes:  Total: 93746064  Max/Min: 0/0   Fail: 0
+     (A)		    (B)		   (C)		  (D)	       (E)
+
+  (A): Lock type that is being tortured -- torture_type parameter.
+
+  (B): Number of writer lock acquisitions. If dealing with a read/write
+       primitive a second "Reads" statistics line is printed.
+
+  (C): Number of times the lock was acquired.
+
+  (D): Min and max number of times threads failed to acquire the lock.
+
+  (E): true/false values if there were errors acquiring the lock. This should
+       -only- be positive if there is a bug in the locking primitive's
+       implementation. Otherwise a lock should never fail (i.e., spin_lock()).
+       Of course, the same applies for (C), above. A dummy example of this is
+       the "lock_busted" type.
+
+Usage
+=====
+
+The following script may be used to torture locks::
+
+	#!/bin/sh
+
+	modprobe locktorture
+	sleep 3600
+	rmmod locktorture
+	dmesg | grep torture:
+
+The output can be manually inspected for the error flag of "!!!".
+One could of course create a more elaborate script that automatically
+checked for such errors.  The "rmmod" command forces a "SUCCESS",
+"FAILURE", or "RCU_HOTPLUG" indication to be printk()ed.  The first
+two are self-explanatory, while the last indicates that while there
+were no locking failures, CPU-hotplug problems were detected.
+
+Also see: Documentation/RCU/torture.rst
diff --git a/Documentation/locking/locktypes.rst b/Documentation/locking/locktypes.rst
new file mode 100644
index 000000000..80c914f6e
--- /dev/null
+++ b/Documentation/locking/locktypes.rst
@@ -0,0 +1,534 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _kernel_hacking_locktypes:
+
+==========================
+Lock types and their rules
+==========================
+
+Introduction
+============
+
+The kernel provides a variety of locking primitives which can be divided
+into three categories:
+
+ - Sleeping locks
+ - CPU local locks
+ - Spinning locks
+
+This document conceptually describes these lock types and provides rules
+for their nesting, including the rules for use under PREEMPT_RT.
+
+
+Lock categories
+===============
+
+Sleeping locks
+--------------
+
+Sleeping locks can only be acquired in preemptible task context.
+
+Although implementations allow try_lock() from other contexts, it is
+necessary to carefully evaluate the safety of unlock() as well as of
+try_lock().  Furthermore, it is also necessary to evaluate the debugging
+versions of these primitives.  In short, don't acquire sleeping locks from
+other contexts unless there is no other option.
+
+Sleeping lock types:
+
+ - mutex
+ - rt_mutex
+ - semaphore
+ - rw_semaphore
+ - ww_mutex
+ - percpu_rw_semaphore
+
+On PREEMPT_RT kernels, these lock types are converted to sleeping locks:
+
+ - local_lock
+ - spinlock_t
+ - rwlock_t
+
+
+CPU local locks
+---------------
+
+ - local_lock
+
+On non-PREEMPT_RT kernels, local_lock functions are wrappers around
+preemption and interrupt disabling primitives. Contrary to other locking
+mechanisms, disabling preemption or interrupts are pure CPU local
+concurrency control mechanisms and not suited for inter-CPU concurrency
+control.
+
+
+Spinning locks
+--------------
+
+ - raw_spinlock_t
+ - bit spinlocks
+
+On non-PREEMPT_RT kernels, these lock types are also spinning locks:
+
+ - spinlock_t
+ - rwlock_t
+
+Spinning locks implicitly disable preemption and the lock / unlock functions
+can have suffixes which apply further protections:
+
+ ===================  ====================================================
+ _bh()                Disable / enable bottom halves (soft interrupts)
+ _irq()               Disable / enable interrupts
+ _irqsave/restore()   Save and disable / restore interrupt disabled state
+ ===================  ====================================================
+
+
+Owner semantics
+===============
+
+The aforementioned lock types except semaphores have strict owner
+semantics:
+
+  The context (task) that acquired the lock must release it.
+
+rw_semaphores have a special interface which allows non-owner release for
+readers.
+
+
+rtmutex
+=======
+
+RT-mutexes are mutexes with support for priority inheritance (PI).
+
+PI has limitations on non-PREEMPT_RT kernels due to preemption and
+interrupt disabled sections.
+
+PI clearly cannot preempt preemption-disabled or interrupt-disabled
+regions of code, even on PREEMPT_RT kernels.  Instead, PREEMPT_RT kernels
+execute most such regions of code in preemptible task context, especially
+interrupt handlers and soft interrupts.  This conversion allows spinlock_t
+and rwlock_t to be implemented via RT-mutexes.
+
+
+semaphore
+=========
+
+semaphore is a counting semaphore implementation.
+
+Semaphores are often used for both serialization and waiting, but new use
+cases should instead use separate serialization and wait mechanisms, such
+as mutexes and completions.
+
+semaphores and PREEMPT_RT
+----------------------------
+
+PREEMPT_RT does not change the semaphore implementation because counting
+semaphores have no concept of owners, thus preventing PREEMPT_RT from
+providing priority inheritance for semaphores.  After all, an unknown
+owner cannot be boosted. As a consequence, blocking on semaphores can
+result in priority inversion.
+
+
+rw_semaphore
+============
+
+rw_semaphore is a multiple readers and single writer lock mechanism.
+
+On non-PREEMPT_RT kernels the implementation is fair, thus preventing
+writer starvation.
+
+rw_semaphore complies by default with the strict owner semantics, but there
+exist special-purpose interfaces that allow non-owner release for readers.
+These interfaces work independent of the kernel configuration.
+
+rw_semaphore and PREEMPT_RT
+---------------------------
+
+PREEMPT_RT kernels map rw_semaphore to a separate rt_mutex-based
+implementation, thus changing the fairness:
+
+ Because an rw_semaphore writer cannot grant its priority to multiple
+ readers, a preempted low-priority reader will continue holding its lock,
+ thus starving even high-priority writers.  In contrast, because readers
+ can grant their priority to a writer, a preempted low-priority writer will
+ have its priority boosted until it releases the lock, thus preventing that
+ writer from starving readers.
+
+
+local_lock
+==========
+
+local_lock provides a named scope to critical sections which are protected
+by disabling preemption or interrupts.
+
+On non-PREEMPT_RT kernels local_lock operations map to the preemption and
+interrupt disabling and enabling primitives:
+
+ ===============================  ======================
+ local_lock(&llock)               preempt_disable()
+ local_unlock(&llock)             preempt_enable()
+ local_lock_irq(&llock)           local_irq_disable()
+ local_unlock_irq(&llock)         local_irq_enable()
+ local_lock_irqsave(&llock)       local_irq_save()
+ local_unlock_irqrestore(&llock)  local_irq_restore()
+ ===============================  ======================
+
+The named scope of local_lock has two advantages over the regular
+primitives:
+
+  - The lock name allows static analysis and is also a clear documentation
+    of the protection scope while the regular primitives are scopeless and
+    opaque.
+
+  - If lockdep is enabled the local_lock gains a lockmap which allows to
+    validate the correctness of the protection. This can detect cases where
+    e.g. a function using preempt_disable() as protection mechanism is
+    invoked from interrupt or soft-interrupt context. Aside of that
+    lockdep_assert_held(&llock) works as with any other locking primitive.
+
+local_lock and PREEMPT_RT
+-------------------------
+
+PREEMPT_RT kernels map local_lock to a per-CPU spinlock_t, thus changing
+semantics:
+
+  - All spinlock_t changes also apply to local_lock.
+
+local_lock usage
+----------------
+
+local_lock should be used in situations where disabling preemption or
+interrupts is the appropriate form of concurrency control to protect
+per-CPU data structures on a non PREEMPT_RT kernel.
+
+local_lock is not suitable to protect against preemption or interrupts on a
+PREEMPT_RT kernel due to the PREEMPT_RT specific spinlock_t semantics.
+
+
+raw_spinlock_t and spinlock_t
+=============================
+
+raw_spinlock_t
+--------------
+
+raw_spinlock_t is a strict spinning lock implementation in all kernels,
+including PREEMPT_RT kernels.  Use raw_spinlock_t only in real critical
+core code, low-level interrupt handling and places where disabling
+preemption or interrupts is required, for example, to safely access
+hardware state.  raw_spinlock_t can sometimes also be used when the
+critical section is tiny, thus avoiding RT-mutex overhead.
+
+spinlock_t
+----------
+
+The semantics of spinlock_t change with the state of PREEMPT_RT.
+
+On a non-PREEMPT_RT kernel spinlock_t is mapped to raw_spinlock_t and has
+exactly the same semantics.
+
+spinlock_t and PREEMPT_RT
+-------------------------
+
+On a PREEMPT_RT kernel spinlock_t is mapped to a separate implementation
+based on rt_mutex which changes the semantics:
+
+ - Preemption is not disabled.
+
+ - The hard interrupt related suffixes for spin_lock / spin_unlock
+   operations (_irq, _irqsave / _irqrestore) do not affect the CPU's
+   interrupt disabled state.
+
+ - The soft interrupt related suffix (_bh()) still disables softirq
+   handlers.
+
+   Non-PREEMPT_RT kernels disable preemption to get this effect.
+
+   PREEMPT_RT kernels use a per-CPU lock for serialization which keeps
+   preemption enabled. The lock disables softirq handlers and also
+   prevents reentrancy due to task preemption.
+
+PREEMPT_RT kernels preserve all other spinlock_t semantics:
+
+ - Tasks holding a spinlock_t do not migrate.  Non-PREEMPT_RT kernels
+   avoid migration by disabling preemption.  PREEMPT_RT kernels instead
+   disable migration, which ensures that pointers to per-CPU variables
+   remain valid even if the task is preempted.
+
+ - Task state is preserved across spinlock acquisition, ensuring that the
+   task-state rules apply to all kernel configurations.  Non-PREEMPT_RT
+   kernels leave task state untouched.  However, PREEMPT_RT must change
+   task state if the task blocks during acquisition.  Therefore, it saves
+   the current task state before blocking and the corresponding lock wakeup
+   restores it, as shown below::
+
+    task->state = TASK_INTERRUPTIBLE
+     lock()
+       block()
+         task->saved_state = task->state
+	 task->state = TASK_UNINTERRUPTIBLE
+	 schedule()
+					lock wakeup
+					  task->state = task->saved_state
+
+   Other types of wakeups would normally unconditionally set the task state
+   to RUNNING, but that does not work here because the task must remain
+   blocked until the lock becomes available.  Therefore, when a non-lock
+   wakeup attempts to awaken a task blocked waiting for a spinlock, it
+   instead sets the saved state to RUNNING.  Then, when the lock
+   acquisition completes, the lock wakeup sets the task state to the saved
+   state, in this case setting it to RUNNING::
+
+    task->state = TASK_INTERRUPTIBLE
+     lock()
+       block()
+         task->saved_state = task->state
+	 task->state = TASK_UNINTERRUPTIBLE
+	 schedule()
+					non lock wakeup
+					  task->saved_state = TASK_RUNNING
+
+					lock wakeup
+					  task->state = task->saved_state
+
+   This ensures that the real wakeup cannot be lost.
+
+
+rwlock_t
+========
+
+rwlock_t is a multiple readers and single writer lock mechanism.
+
+Non-PREEMPT_RT kernels implement rwlock_t as a spinning lock and the
+suffix rules of spinlock_t apply accordingly. The implementation is fair,
+thus preventing writer starvation.
+
+rwlock_t and PREEMPT_RT
+-----------------------
+
+PREEMPT_RT kernels map rwlock_t to a separate rt_mutex-based
+implementation, thus changing semantics:
+
+ - All the spinlock_t changes also apply to rwlock_t.
+
+ - Because an rwlock_t writer cannot grant its priority to multiple
+   readers, a preempted low-priority reader will continue holding its lock,
+   thus starving even high-priority writers.  In contrast, because readers
+   can grant their priority to a writer, a preempted low-priority writer
+   will have its priority boosted until it releases the lock, thus
+   preventing that writer from starving readers.
+
+
+PREEMPT_RT caveats
+==================
+
+local_lock on RT
+----------------
+
+The mapping of local_lock to spinlock_t on PREEMPT_RT kernels has a few
+implications. For example, on a non-PREEMPT_RT kernel the following code
+sequence works as expected::
+
+  local_lock_irq(&local_lock);
+  raw_spin_lock(&lock);
+
+and is fully equivalent to::
+
+   raw_spin_lock_irq(&lock);
+
+On a PREEMPT_RT kernel this code sequence breaks because local_lock_irq()
+is mapped to a per-CPU spinlock_t which neither disables interrupts nor
+preemption. The following code sequence works perfectly correct on both
+PREEMPT_RT and non-PREEMPT_RT kernels::
+
+  local_lock_irq(&local_lock);
+  spin_lock(&lock);
+
+Another caveat with local locks is that each local_lock has a specific
+protection scope. So the following substitution is wrong::
+
+  func1()
+  {
+    local_irq_save(flags);    -> local_lock_irqsave(&local_lock_1, flags);
+    func3();
+    local_irq_restore(flags); -> local_unlock_irqrestore(&local_lock_1, flags);
+  }
+
+  func2()
+  {
+    local_irq_save(flags);    -> local_lock_irqsave(&local_lock_2, flags);
+    func3();
+    local_irq_restore(flags); -> local_unlock_irqrestore(&local_lock_2, flags);
+  }
+
+  func3()
+  {
+    lockdep_assert_irqs_disabled();
+    access_protected_data();
+  }
+
+On a non-PREEMPT_RT kernel this works correctly, but on a PREEMPT_RT kernel
+local_lock_1 and local_lock_2 are distinct and cannot serialize the callers
+of func3(). Also the lockdep assert will trigger on a PREEMPT_RT kernel
+because local_lock_irqsave() does not disable interrupts due to the
+PREEMPT_RT-specific semantics of spinlock_t. The correct substitution is::
+
+  func1()
+  {
+    local_irq_save(flags);    -> local_lock_irqsave(&local_lock, flags);
+    func3();
+    local_irq_restore(flags); -> local_unlock_irqrestore(&local_lock, flags);
+  }
+
+  func2()
+  {
+    local_irq_save(flags);    -> local_lock_irqsave(&local_lock, flags);
+    func3();
+    local_irq_restore(flags); -> local_unlock_irqrestore(&local_lock, flags);
+  }
+
+  func3()
+  {
+    lockdep_assert_held(&local_lock);
+    access_protected_data();
+  }
+
+
+spinlock_t and rwlock_t
+-----------------------
+
+The changes in spinlock_t and rwlock_t semantics on PREEMPT_RT kernels
+have a few implications.  For example, on a non-PREEMPT_RT kernel the
+following code sequence works as expected::
+
+   local_irq_disable();
+   spin_lock(&lock);
+
+and is fully equivalent to::
+
+   spin_lock_irq(&lock);
+
+Same applies to rwlock_t and the _irqsave() suffix variants.
+
+On PREEMPT_RT kernel this code sequence breaks because RT-mutex requires a
+fully preemptible context.  Instead, use spin_lock_irq() or
+spin_lock_irqsave() and their unlock counterparts.  In cases where the
+interrupt disabling and locking must remain separate, PREEMPT_RT offers a
+local_lock mechanism.  Acquiring the local_lock pins the task to a CPU,
+allowing things like per-CPU interrupt disabled locks to be acquired.
+However, this approach should be used only where absolutely necessary.
+
+A typical scenario is protection of per-CPU variables in thread context::
+
+  struct foo *p = get_cpu_ptr(&var1);
+
+  spin_lock(&p->lock);
+  p->count += this_cpu_read(var2);
+
+This is correct code on a non-PREEMPT_RT kernel, but on a PREEMPT_RT kernel
+this breaks. The PREEMPT_RT-specific change of spinlock_t semantics does
+not allow to acquire p->lock because get_cpu_ptr() implicitly disables
+preemption. The following substitution works on both kernels::
+
+  struct foo *p;
+
+  migrate_disable();
+  p = this_cpu_ptr(&var1);
+  spin_lock(&p->lock);
+  p->count += this_cpu_read(var2);
+
+migrate_disable() ensures that the task is pinned on the current CPU which
+in turn guarantees that the per-CPU access to var1 and var2 are staying on
+the same CPU while the task remains preemptible.
+
+The migrate_disable() substitution is not valid for the following
+scenario::
+
+  func()
+  {
+    struct foo *p;
+
+    migrate_disable();
+    p = this_cpu_ptr(&var1);
+    p->val = func2();
+
+This breaks because migrate_disable() does not protect against reentrancy from
+a preempting task. A correct substitution for this case is::
+
+  func()
+  {
+    struct foo *p;
+
+    local_lock(&foo_lock);
+    p = this_cpu_ptr(&var1);
+    p->val = func2();
+
+On a non-PREEMPT_RT kernel this protects against reentrancy by disabling
+preemption. On a PREEMPT_RT kernel this is achieved by acquiring the
+underlying per-CPU spinlock.
+
+
+raw_spinlock_t on RT
+--------------------
+
+Acquiring a raw_spinlock_t disables preemption and possibly also
+interrupts, so the critical section must avoid acquiring a regular
+spinlock_t or rwlock_t, for example, the critical section must avoid
+allocating memory.  Thus, on a non-PREEMPT_RT kernel the following code
+works perfectly::
+
+  raw_spin_lock(&lock);
+  p = kmalloc(sizeof(*p), GFP_ATOMIC);
+
+But this code fails on PREEMPT_RT kernels because the memory allocator is
+fully preemptible and therefore cannot be invoked from truly atomic
+contexts.  However, it is perfectly fine to invoke the memory allocator
+while holding normal non-raw spinlocks because they do not disable
+preemption on PREEMPT_RT kernels::
+
+  spin_lock(&lock);
+  p = kmalloc(sizeof(*p), GFP_ATOMIC);
+
+
+bit spinlocks
+-------------
+
+PREEMPT_RT cannot substitute bit spinlocks because a single bit is too
+small to accommodate an RT-mutex.  Therefore, the semantics of bit
+spinlocks are preserved on PREEMPT_RT kernels, so that the raw_spinlock_t
+caveats also apply to bit spinlocks.
+
+Some bit spinlocks are replaced with regular spinlock_t for PREEMPT_RT
+using conditional (#ifdef'ed) code changes at the usage site.  In contrast,
+usage-site changes are not needed for the spinlock_t substitution.
+Instead, conditionals in header files and the core locking implementation
+enable the compiler to do the substitution transparently.
+
+
+Lock type nesting rules
+=======================
+
+The most basic rules are:
+
+  - Lock types of the same lock category (sleeping, CPU local, spinning)
+    can nest arbitrarily as long as they respect the general lock ordering
+    rules to prevent deadlocks.
+
+  - Sleeping lock types cannot nest inside CPU local and spinning lock types.
+
+  - CPU local and spinning lock types can nest inside sleeping lock types.
+
+  - Spinning lock types can nest inside all lock types
+
+These constraints apply both in PREEMPT_RT and otherwise.
+
+The fact that PREEMPT_RT changes the lock category of spinlock_t and
+rwlock_t from spinning to sleeping and substitutes local_lock with a
+per-CPU spinlock_t means that they cannot be acquired while holding a raw
+spinlock.  This results in the following nesting ordering:
+
+  1) Sleeping locks
+  2) spinlock_t, rwlock_t, local_lock
+  3) raw_spinlock_t and bit spinlocks
+
+Lockdep will complain if these constraints are violated, both in
+PREEMPT_RT and otherwise.
diff --git a/Documentation/locking/mutex-design.rst b/Documentation/locking/mutex-design.rst
new file mode 100644
index 000000000..78540cd7f
--- /dev/null
+++ b/Documentation/locking/mutex-design.rst
@@ -0,0 +1,152 @@
+=======================
+Generic Mutex Subsystem
+=======================
+
+started by Ingo Molnar <mingo@redhat.com>
+
+updated by Davidlohr Bueso <davidlohr@hp.com>
+
+What are mutexes?
+-----------------
+
+In the Linux kernel, mutexes refer to a particular locking primitive
+that enforces serialization on shared memory systems, and not only to
+the generic term referring to 'mutual exclusion' found in academia
+or similar theoretical text books. Mutexes are sleeping locks which
+behave similarly to binary semaphores, and were introduced in 2006[1]
+as an alternative to these. This new data structure provided a number
+of advantages, including simpler interfaces, and at that time smaller
+code (see Disadvantages).
+
+[1] https://lwn.net/Articles/164802/
+
+Implementation
+--------------
+
+Mutexes are represented by 'struct mutex', defined in include/linux/mutex.h
+and implemented in kernel/locking/mutex.c. These locks use an atomic variable
+(->owner) to keep track of the lock state during its lifetime.  Field owner
+actually contains `struct task_struct *` to the current lock owner and it is
+therefore NULL if not currently owned. Since task_struct pointers are aligned
+to at least L1_CACHE_BYTES, low bits (3) are used to store extra state (e.g.,
+if waiter list is non-empty).  In its most basic form it also includes a
+wait-queue and a spinlock that serializes access to it. Furthermore,
+CONFIG_MUTEX_SPIN_ON_OWNER=y systems use a spinner MCS lock (->osq), described
+below in (ii).
+
+When acquiring a mutex, there are three possible paths that can be
+taken, depending on the state of the lock:
+
+(i) fastpath: tries to atomically acquire the lock by cmpxchg()ing the owner with
+    the current task. This only works in the uncontended case (cmpxchg() checks
+    against 0UL, so all 3 state bits above have to be 0). If the lock is
+    contended it goes to the next possible path.
+
+(ii) midpath: aka optimistic spinning, tries to spin for acquisition
+     while the lock owner is running and there are no other tasks ready
+     to run that have higher priority (need_resched). The rationale is
+     that if the lock owner is running, it is likely to release the lock
+     soon. The mutex spinners are queued up using MCS lock so that only
+     one spinner can compete for the mutex.
+
+     The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spinlock
+     with the desirable properties of being fair and with each cpu trying
+     to acquire the lock spinning on a local variable. It avoids expensive
+     cacheline bouncing that common test-and-set spinlock implementations
+     incur. An MCS-like lock is specially tailored for optimistic spinning
+     for sleeping lock implementation. An important feature of the customized
+     MCS lock is that it has the extra property that spinners are able to exit
+     the MCS spinlock queue when they need to reschedule. This further helps
+     avoid situations where MCS spinners that need to reschedule would continue
+     waiting to spin on mutex owner, only to go directly to slowpath upon
+     obtaining the MCS lock.
+
+
+(iii) slowpath: last resort, if the lock is still unable to be acquired,
+      the task is added to the wait-queue and sleeps until woken up by the
+      unlock path. Under normal circumstances it blocks as TASK_UNINTERRUPTIBLE.
+
+While formally kernel mutexes are sleepable locks, it is path (ii) that
+makes them more practically a hybrid type. By simply not interrupting a
+task and busy-waiting for a few cycles instead of immediately sleeping,
+the performance of this lock has been seen to significantly improve a
+number of workloads. Note that this technique is also used for rw-semaphores.
+
+Semantics
+---------
+
+The mutex subsystem checks and enforces the following rules:
+
+    - Only one task can hold the mutex at a time.
+    - Only the owner can unlock the mutex.
+    - Multiple unlocks are not permitted.
+    - Recursive locking/unlocking is not permitted.
+    - A mutex must only be initialized via the API (see below).
+    - A task may not exit with a mutex held.
+    - Memory areas where held locks reside must not be freed.
+    - Held mutexes must not be reinitialized.
+    - Mutexes may not be used in hardware or software interrupt
+      contexts such as tasklets and timers.
+
+These semantics are fully enforced when CONFIG DEBUG_MUTEXES is enabled.
+In addition, the mutex debugging code also implements a number of other
+features that make lock debugging easier and faster:
+
+    - Uses symbolic names of mutexes, whenever they are printed
+      in debug output.
+    - Point-of-acquire tracking, symbolic lookup of function names,
+      list of all locks held in the system, printout of them.
+    - Owner tracking.
+    - Detects self-recursing locks and prints out all relevant info.
+    - Detects multi-task circular deadlocks and prints out all affected
+      locks and tasks (and only those tasks).
+
+
+Interfaces
+----------
+Statically define the mutex::
+
+   DEFINE_MUTEX(name);
+
+Dynamically initialize the mutex::
+
+   mutex_init(mutex);
+
+Acquire the mutex, uninterruptible::
+
+   void mutex_lock(struct mutex *lock);
+   void mutex_lock_nested(struct mutex *lock, unsigned int subclass);
+   int  mutex_trylock(struct mutex *lock);
+
+Acquire the mutex, interruptible::
+
+   int mutex_lock_interruptible_nested(struct mutex *lock,
+				       unsigned int subclass);
+   int mutex_lock_interruptible(struct mutex *lock);
+
+Acquire the mutex, interruptible, if dec to 0::
+
+   int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock);
+
+Unlock the mutex::
+
+   void mutex_unlock(struct mutex *lock);
+
+Test if the mutex is taken::
+
+   int mutex_is_locked(struct mutex *lock);
+
+Disadvantages
+-------------
+
+Unlike its original design and purpose, 'struct mutex' is among the largest
+locks in the kernel. E.g: on x86-64 it is 32 bytes, where 'struct semaphore'
+is 24 bytes and rw_semaphore is 40 bytes. Larger structure sizes mean more CPU
+cache and memory footprint.
+
+When to use mutexes
+-------------------
+
+Unless the strict semantics of mutexes are unsuitable and/or the critical
+region prevents the lock from being shared, always prefer them to any other
+locking primitive.
diff --git a/Documentation/locking/percpu-rw-semaphore.rst b/Documentation/locking/percpu-rw-semaphore.rst
new file mode 100644
index 000000000..247de6410
--- /dev/null
+++ b/Documentation/locking/percpu-rw-semaphore.rst
@@ -0,0 +1,28 @@
+====================
+Percpu rw semaphores
+====================
+
+Percpu rw semaphores is a new read-write semaphore design that is
+optimized for locking for reading.
+
+The problem with traditional read-write semaphores is that when multiple
+cores take the lock for reading, the cache line containing the semaphore
+is bouncing between L1 caches of the cores, causing performance
+degradation.
+
+Locking for reading is very fast, it uses RCU and it avoids any atomic
+instruction in the lock and unlock path. On the other hand, locking for
+writing is very expensive, it calls synchronize_rcu() that can take
+hundreds of milliseconds.
+
+The lock is declared with "struct percpu_rw_semaphore" type.
+The lock is initialized percpu_init_rwsem, it returns 0 on success and
+-ENOMEM on allocation failure.
+The lock must be freed with percpu_free_rwsem to avoid memory leak.
+
+The lock is locked for read with percpu_down_read, percpu_up_read and
+for write with percpu_down_write, percpu_up_write.
+
+The idea of using RCU for optimized rw-lock was introduced by
+Eric Dumazet <eric.dumazet@gmail.com>.
+The code was written by Mikulas Patocka <mpatocka@redhat.com>
diff --git a/Documentation/locking/pi-futex.rst b/Documentation/locking/pi-futex.rst
new file mode 100644
index 000000000..c33ba2bef
--- /dev/null
+++ b/Documentation/locking/pi-futex.rst
@@ -0,0 +1,122 @@
+======================
+Lightweight PI-futexes
+======================
+
+We are calling them lightweight for 3 reasons:
+
+ - in the user-space fastpath a PI-enabled futex involves no kernel work
+   (or any other PI complexity) at all. No registration, no extra kernel
+   calls - just pure fast atomic ops in userspace.
+
+ - even in the slowpath, the system call and scheduling pattern is very
+   similar to normal futexes.
+
+ - the in-kernel PI implementation is streamlined around the mutex
+   abstraction, with strict rules that keep the implementation
+   relatively simple: only a single owner may own a lock (i.e. no
+   read-write lock support), only the owner may unlock a lock, no
+   recursive locking, etc.
+
+Priority Inheritance - why?
+---------------------------
+
+The short reply: user-space PI helps achieving/improving determinism for
+user-space applications. In the best-case, it can help achieve
+determinism and well-bound latencies. Even in the worst-case, PI will
+improve the statistical distribution of locking related application
+delays.
+
+The longer reply
+----------------
+
+Firstly, sharing locks between multiple tasks is a common programming
+technique that often cannot be replaced with lockless algorithms. As we
+can see it in the kernel [which is a quite complex program in itself],
+lockless structures are rather the exception than the norm - the current
+ratio of lockless vs. locky code for shared data structures is somewhere
+between 1:10 and 1:100. Lockless is hard, and the complexity of lockless
+algorithms often endangers to ability to do robust reviews of said code.
+I.e. critical RT apps often choose lock structures to protect critical
+data structures, instead of lockless algorithms. Furthermore, there are
+cases (like shared hardware, or other resource limits) where lockless
+access is mathematically impossible.
+
+Media players (such as Jack) are an example of reasonable application
+design with multiple tasks (with multiple priority levels) sharing
+short-held locks: for example, a highprio audio playback thread is
+combined with medium-prio construct-audio-data threads and low-prio
+display-colory-stuff threads. Add video and decoding to the mix and
+we've got even more priority levels.
+
+So once we accept that synchronization objects (locks) are an
+unavoidable fact of life, and once we accept that multi-task userspace
+apps have a very fair expectation of being able to use locks, we've got
+to think about how to offer the option of a deterministic locking
+implementation to user-space.
+
+Most of the technical counter-arguments against doing priority
+inheritance only apply to kernel-space locks. But user-space locks are
+different, there we cannot disable interrupts or make the task
+non-preemptible in a critical section, so the 'use spinlocks' argument
+does not apply (user-space spinlocks have the same priority inversion
+problems as other user-space locking constructs). Fact is, pretty much
+the only technique that currently enables good determinism for userspace
+locks (such as futex-based pthread mutexes) is priority inheritance:
+
+Currently (without PI), if a high-prio and a low-prio task shares a lock
+[this is a quite common scenario for most non-trivial RT applications],
+even if all critical sections are coded carefully to be deterministic
+(i.e. all critical sections are short in duration and only execute a
+limited number of instructions), the kernel cannot guarantee any
+deterministic execution of the high-prio task: any medium-priority task
+could preempt the low-prio task while it holds the shared lock and
+executes the critical section, and could delay it indefinitely.
+
+Implementation
+--------------
+
+As mentioned before, the userspace fastpath of PI-enabled pthread
+mutexes involves no kernel work at all - they behave quite similarly to
+normal futex-based locks: a 0 value means unlocked, and a value==TID
+means locked. (This is the same method as used by list-based robust
+futexes.) Userspace uses atomic ops to lock/unlock these mutexes without
+entering the kernel.
+
+To handle the slowpath, we have added two new futex ops:
+
+  - FUTEX_LOCK_PI
+  - FUTEX_UNLOCK_PI
+
+If the lock-acquire fastpath fails, [i.e. an atomic transition from 0 to
+TID fails], then FUTEX_LOCK_PI is called. The kernel does all the
+remaining work: if there is no futex-queue attached to the futex address
+yet then the code looks up the task that owns the futex [it has put its
+own TID into the futex value], and attaches a 'PI state' structure to
+the futex-queue. The pi_state includes an rt-mutex, which is a PI-aware,
+kernel-based synchronization object. The 'other' task is made the owner
+of the rt-mutex, and the FUTEX_WAITERS bit is atomically set in the
+futex value. Then this task tries to lock the rt-mutex, on which it
+blocks. Once it returns, it has the mutex acquired, and it sets the
+futex value to its own TID and returns. Userspace has no other work to
+perform - it now owns the lock, and futex value contains
+FUTEX_WAITERS|TID.
+
+If the unlock side fastpath succeeds, [i.e. userspace manages to do a
+TID -> 0 atomic transition of the futex value], then no kernel work is
+triggered.
+
+If the unlock fastpath fails (because the FUTEX_WAITERS bit is set),
+then FUTEX_UNLOCK_PI is called, and the kernel unlocks the futex on the
+behalf of userspace - and it also unlocks the attached
+pi_state->rt_mutex and thus wakes up any potential waiters.
+
+Note that under this approach, contrary to previous PI-futex approaches,
+there is no prior 'registration' of a PI-futex. [which is not quite
+possible anyway, due to existing ABI properties of pthread mutexes.]
+
+Also, under this scheme, 'robustness' and 'PI' are two orthogonal
+properties of futexes, and all four combinations are possible: futex,
+robust-futex, PI-futex, robust+PI-futex.
+
+More details about priority inheritance can be found in
+Documentation/locking/rt-mutex.rst.
diff --git a/Documentation/locking/preempt-locking.rst b/Documentation/locking/preempt-locking.rst
new file mode 100644
index 000000000..dce336134
--- /dev/null
+++ b/Documentation/locking/preempt-locking.rst
@@ -0,0 +1,144 @@
+===========================================================================
+Proper Locking Under a Preemptible Kernel: Keeping Kernel Code Preempt-Safe
+===========================================================================
+
+:Author: Robert Love <rml@tech9.net>
+
+
+Introduction
+============
+
+
+A preemptible kernel creates new locking issues.  The issues are the same as
+those under SMP: concurrency and reentrancy.  Thankfully, the Linux preemptible
+kernel model leverages existing SMP locking mechanisms.  Thus, the kernel
+requires explicit additional locking for very few additional situations.
+
+This document is for all kernel hackers.  Developing code in the kernel
+requires protecting these situations.
+ 
+
+RULE #1: Per-CPU data structures need explicit protection
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+
+Two similar problems arise. An example code snippet::
+
+	struct this_needs_locking tux[NR_CPUS];
+	tux[smp_processor_id()] = some_value;
+	/* task is preempted here... */
+	something = tux[smp_processor_id()];
+
+First, since the data is per-CPU, it may not have explicit SMP locking, but
+require it otherwise.  Second, when a preempted task is finally rescheduled,
+the previous value of smp_processor_id may not equal the current.  You must
+protect these situations by disabling preemption around them.
+
+You can also use put_cpu() and get_cpu(), which will disable preemption.
+
+
+RULE #2: CPU state must be protected.
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+
+Under preemption, the state of the CPU must be protected.  This is arch-
+dependent, but includes CPU structures and state not preserved over a context
+switch.  For example, on x86, entering and exiting FPU mode is now a critical
+section that must occur while preemption is disabled.  Think what would happen
+if the kernel is executing a floating-point instruction and is then preempted.
+Remember, the kernel does not save FPU state except for user tasks.  Therefore,
+upon preemption, the FPU registers will be sold to the lowest bidder.  Thus,
+preemption must be disabled around such regions.
+
+Note, some FPU functions are already explicitly preempt safe.  For example,
+kernel_fpu_begin and kernel_fpu_end will disable and enable preemption.
+
+
+RULE #3: Lock acquire and release must be performed by same task
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+
+A lock acquired in one task must be released by the same task.  This
+means you can't do oddball things like acquire a lock and go off to
+play while another task releases it.  If you want to do something
+like this, acquire and release the task in the same code path and
+have the caller wait on an event by the other task.
+
+
+Solution
+========
+
+
+Data protection under preemption is achieved by disabling preemption for the
+duration of the critical region.
+
+::
+
+  preempt_enable()		decrement the preempt counter
+  preempt_disable()		increment the preempt counter
+  preempt_enable_no_resched()	decrement, but do not immediately preempt
+  preempt_check_resched()	if needed, reschedule
+  preempt_count()		return the preempt counter
+
+The functions are nestable.  In other words, you can call preempt_disable
+n-times in a code path, and preemption will not be reenabled until the n-th
+call to preempt_enable.  The preempt statements define to nothing if
+preemption is not enabled.
+
+Note that you do not need to explicitly prevent preemption if you are holding
+any locks or interrupts are disabled, since preemption is implicitly disabled
+in those cases.
+
+But keep in mind that 'irqs disabled' is a fundamentally unsafe way of
+disabling preemption - any cond_resched() or cond_resched_lock() might trigger
+a reschedule if the preempt count is 0. A simple printk() might trigger a
+reschedule. So use this implicit preemption-disabling property only if you
+know that the affected codepath does not do any of this. Best policy is to use
+this only for small, atomic code that you wrote and which calls no complex
+functions.
+
+Example::
+
+	cpucache_t *cc; /* this is per-CPU */
+	preempt_disable();
+	cc = cc_data(searchp);
+	if (cc && cc->avail) {
+		__free_block(searchp, cc_entry(cc), cc->avail);
+		cc->avail = 0;
+	}
+	preempt_enable();
+	return 0;
+
+Notice how the preemption statements must encompass every reference of the
+critical variables.  Another example::
+
+	int buf[NR_CPUS];
+	set_cpu_val(buf);
+	if (buf[smp_processor_id()] == -1) printf(KERN_INFO "wee!\n");
+	spin_lock(&buf_lock);
+	/* ... */
+
+This code is not preempt-safe, but see how easily we can fix it by simply
+moving the spin_lock up two lines.
+
+
+Preventing preemption using interrupt disabling
+===============================================
+
+
+It is possible to prevent a preemption event using local_irq_disable and
+local_irq_save.  Note, when doing so, you must be very careful to not cause
+an event that would set need_resched and result in a preemption check.  When
+in doubt, rely on locking or explicit preemption disabling.
+
+Note in 2.5 interrupt disabling is now only per-CPU (e.g. local).
+
+An additional concern is proper usage of local_irq_disable and local_irq_save.
+These may be used to protect from preemption, however, on exit, if preemption
+may be enabled, a test to see if preemption is required should be done.  If
+these are called from the spin_lock and read/write lock macros, the right thing
+is done.  They may also be called within a spin-lock protected region, however,
+if they are ever called outside of this context, a test for preemption should
+be made. Do note that calls from interrupt context or bottom half/ tasklets
+are also protected by preemption locks and so may use the versions which do
+not check preemption.
diff --git a/Documentation/locking/robust-futex-ABI.rst b/Documentation/locking/robust-futex-ABI.rst
new file mode 100644
index 000000000..f24904f1c
--- /dev/null
+++ b/Documentation/locking/robust-futex-ABI.rst
@@ -0,0 +1,184 @@
+====================
+The robust futex ABI
+====================
+
+:Author: Started by Paul Jackson <pj@sgi.com>
+
+
+Robust_futexes provide a mechanism that is used in addition to normal
+futexes, for kernel assist of cleanup of held locks on task exit.
+
+The interesting data as to what futexes a thread is holding is kept on a
+linked list in user space, where it can be updated efficiently as locks
+are taken and dropped, without kernel intervention.  The only additional
+kernel intervention required for robust_futexes above and beyond what is
+required for futexes is:
+
+ 1) a one time call, per thread, to tell the kernel where its list of
+    held robust_futexes begins, and
+ 2) internal kernel code at exit, to handle any listed locks held
+    by the exiting thread.
+
+The existing normal futexes already provide a "Fast Userspace Locking"
+mechanism, which handles uncontested locking without needing a system
+call, and handles contested locking by maintaining a list of waiting
+threads in the kernel.  Options on the sys_futex(2) system call support
+waiting on a particular futex, and waking up the next waiter on a
+particular futex.
+
+For robust_futexes to work, the user code (typically in a library such
+as glibc linked with the application) has to manage and place the
+necessary list elements exactly as the kernel expects them.  If it fails
+to do so, then improperly listed locks will not be cleaned up on exit,
+probably causing deadlock or other such failure of the other threads
+waiting on the same locks.
+
+A thread that anticipates possibly using robust_futexes should first
+issue the system call::
+
+    asmlinkage long
+    sys_set_robust_list(struct robust_list_head __user *head, size_t len);
+
+The pointer 'head' points to a structure in the threads address space
+consisting of three words.  Each word is 32 bits on 32 bit arch's, or 64
+bits on 64 bit arch's, and local byte order.  Each thread should have
+its own thread private 'head'.
+
+If a thread is running in 32 bit compatibility mode on a 64 native arch
+kernel, then it can actually have two such structures - one using 32 bit
+words for 32 bit compatibility mode, and one using 64 bit words for 64
+bit native mode.  The kernel, if it is a 64 bit kernel supporting 32 bit
+compatibility mode, will attempt to process both lists on each task
+exit, if the corresponding sys_set_robust_list() call has been made to
+setup that list.
+
+  The first word in the memory structure at 'head' contains a
+  pointer to a single linked list of 'lock entries', one per lock,
+  as described below.  If the list is empty, the pointer will point
+  to itself, 'head'.  The last 'lock entry' points back to the 'head'.
+
+  The second word, called 'offset', specifies the offset from the
+  address of the associated 'lock entry', plus or minus, of what will
+  be called the 'lock word', from that 'lock entry'.  The 'lock word'
+  is always a 32 bit word, unlike the other words above.  The 'lock
+  word' holds 2 flag bits in the upper 2 bits, and the thread id (TID)
+  of the thread holding the lock in the bottom 30 bits.  See further
+  below for a description of the flag bits.
+
+  The third word, called 'list_op_pending', contains transient copy of
+  the address of the 'lock entry', during list insertion and removal,
+  and is needed to correctly resolve races should a thread exit while
+  in the middle of a locking or unlocking operation.
+
+Each 'lock entry' on the single linked list starting at 'head' consists
+of just a single word, pointing to the next 'lock entry', or back to
+'head' if there are no more entries.  In addition, nearby to each 'lock
+entry', at an offset from the 'lock entry' specified by the 'offset'
+word, is one 'lock word'.
+
+The 'lock word' is always 32 bits, and is intended to be the same 32 bit
+lock variable used by the futex mechanism, in conjunction with
+robust_futexes.  The kernel will only be able to wakeup the next thread
+waiting for a lock on a threads exit if that next thread used the futex
+mechanism to register the address of that 'lock word' with the kernel.
+
+For each futex lock currently held by a thread, if it wants this
+robust_futex support for exit cleanup of that lock, it should have one
+'lock entry' on this list, with its associated 'lock word' at the
+specified 'offset'.  Should a thread die while holding any such locks,
+the kernel will walk this list, mark any such locks with a bit
+indicating their holder died, and wakeup the next thread waiting for
+that lock using the futex mechanism.
+
+When a thread has invoked the above system call to indicate it
+anticipates using robust_futexes, the kernel stores the passed in 'head'
+pointer for that task.  The task may retrieve that value later on by
+using the system call::
+
+    asmlinkage long
+    sys_get_robust_list(int pid, struct robust_list_head __user **head_ptr,
+                        size_t __user *len_ptr);
+
+It is anticipated that threads will use robust_futexes embedded in
+larger, user level locking structures, one per lock.  The kernel
+robust_futex mechanism doesn't care what else is in that structure, so
+long as the 'offset' to the 'lock word' is the same for all
+robust_futexes used by that thread.  The thread should link those locks
+it currently holds using the 'lock entry' pointers.  It may also have
+other links between the locks, such as the reverse side of a double
+linked list, but that doesn't matter to the kernel.
+
+By keeping its locks linked this way, on a list starting with a 'head'
+pointer known to the kernel, the kernel can provide to a thread the
+essential service available for robust_futexes, which is to help clean
+up locks held at the time of (a perhaps unexpectedly) exit.
+
+Actual locking and unlocking, during normal operations, is handled
+entirely by user level code in the contending threads, and by the
+existing futex mechanism to wait for, and wakeup, locks.  The kernels
+only essential involvement in robust_futexes is to remember where the
+list 'head' is, and to walk the list on thread exit, handling locks
+still held by the departing thread, as described below.
+
+There may exist thousands of futex lock structures in a threads shared
+memory, on various data structures, at a given point in time. Only those
+lock structures for locks currently held by that thread should be on
+that thread's robust_futex linked lock list a given time.
+
+A given futex lock structure in a user shared memory region may be held
+at different times by any of the threads with access to that region. The
+thread currently holding such a lock, if any, is marked with the threads
+TID in the lower 30 bits of the 'lock word'.
+
+When adding or removing a lock from its list of held locks, in order for
+the kernel to correctly handle lock cleanup regardless of when the task
+exits (perhaps it gets an unexpected signal 9 in the middle of
+manipulating this list), the user code must observe the following
+protocol on 'lock entry' insertion and removal:
+
+On insertion:
+
+ 1) set the 'list_op_pending' word to the address of the 'lock entry'
+    to be inserted,
+ 2) acquire the futex lock,
+ 3) add the lock entry, with its thread id (TID) in the bottom 30 bits
+    of the 'lock word', to the linked list starting at 'head', and
+ 4) clear the 'list_op_pending' word.
+
+On removal:
+
+ 1) set the 'list_op_pending' word to the address of the 'lock entry'
+    to be removed,
+ 2) remove the lock entry for this lock from the 'head' list,
+ 3) release the futex lock, and
+ 4) clear the 'lock_op_pending' word.
+
+On exit, the kernel will consider the address stored in
+'list_op_pending' and the address of each 'lock word' found by walking
+the list starting at 'head'.  For each such address, if the bottom 30
+bits of the 'lock word' at offset 'offset' from that address equals the
+exiting threads TID, then the kernel will do two things:
+
+ 1) if bit 31 (0x80000000) is set in that word, then attempt a futex
+    wakeup on that address, which will waken the next thread that has
+    used to the futex mechanism to wait on that address, and
+ 2) atomically set  bit 30 (0x40000000) in the 'lock word'.
+
+In the above, bit 31 was set by futex waiters on that lock to indicate
+they were waiting, and bit 30 is set by the kernel to indicate that the
+lock owner died holding the lock.
+
+The kernel exit code will silently stop scanning the list further if at
+any point:
+
+ 1) the 'head' pointer or an subsequent linked list pointer
+    is not a valid address of a user space word
+ 2) the calculated location of the 'lock word' (address plus
+    'offset') is not the valid address of a 32 bit user space
+    word
+ 3) if the list contains more than 1 million (subject to
+    future kernel configuration changes) elements.
+
+When the kernel sees a list entry whose 'lock word' doesn't have the
+current threads TID in the lower 30 bits, it does nothing with that
+entry, and goes on to the next entry.
diff --git a/Documentation/locking/robust-futexes.rst b/Documentation/locking/robust-futexes.rst
new file mode 100644
index 000000000..6361fb01c
--- /dev/null
+++ b/Documentation/locking/robust-futexes.rst
@@ -0,0 +1,221 @@
+========================================
+A description of what robust futexes are
+========================================
+
+:Started by: Ingo Molnar <mingo@redhat.com>
+
+Background
+----------
+
+what are robust futexes? To answer that, we first need to understand
+what futexes are: normal futexes are special types of locks that in the
+noncontended case can be acquired/released from userspace without having
+to enter the kernel.
+
+A futex is in essence a user-space address, e.g. a 32-bit lock variable
+field. If userspace notices contention (the lock is already owned and
+someone else wants to grab it too) then the lock is marked with a value
+that says "there's a waiter pending", and the sys_futex(FUTEX_WAIT)
+syscall is used to wait for the other guy to release it. The kernel
+creates a 'futex queue' internally, so that it can later on match up the
+waiter with the waker - without them having to know about each other.
+When the owner thread releases the futex, it notices (via the variable
+value) that there were waiter(s) pending, and does the
+sys_futex(FUTEX_WAKE) syscall to wake them up.  Once all waiters have
+taken and released the lock, the futex is again back to 'uncontended'
+state, and there's no in-kernel state associated with it. The kernel
+completely forgets that there ever was a futex at that address. This
+method makes futexes very lightweight and scalable.
+
+"Robustness" is about dealing with crashes while holding a lock: if a
+process exits prematurely while holding a pthread_mutex_t lock that is
+also shared with some other process (e.g. yum segfaults while holding a
+pthread_mutex_t, or yum is kill -9-ed), then waiters for that lock need
+to be notified that the last owner of the lock exited in some irregular
+way.
+
+To solve such types of problems, "robust mutex" userspace APIs were
+created: pthread_mutex_lock() returns an error value if the owner exits
+prematurely - and the new owner can decide whether the data protected by
+the lock can be recovered safely.
+
+There is a big conceptual problem with futex based mutexes though: it is
+the kernel that destroys the owner task (e.g. due to a SEGFAULT), but
+the kernel cannot help with the cleanup: if there is no 'futex queue'
+(and in most cases there is none, futexes being fast lightweight locks)
+then the kernel has no information to clean up after the held lock!
+Userspace has no chance to clean up after the lock either - userspace is
+the one that crashes, so it has no opportunity to clean up. Catch-22.
+
+In practice, when e.g. yum is kill -9-ed (or segfaults), a system reboot
+is needed to release that futex based lock. This is one of the leading
+bugreports against yum.
+
+To solve this problem, the traditional approach was to extend the vma
+(virtual memory area descriptor) concept to have a notion of 'pending
+robust futexes attached to this area'. This approach requires 3 new
+syscall variants to sys_futex(): FUTEX_REGISTER, FUTEX_DEREGISTER and
+FUTEX_RECOVER. At do_exit() time, all vmas are searched to see whether
+they have a robust_head set. This approach has two fundamental problems
+left:
+
+ - it has quite complex locking and race scenarios. The vma-based
+   approach had been pending for years, but they are still not completely
+   reliable.
+
+ - they have to scan _every_ vma at sys_exit() time, per thread!
+
+The second disadvantage is a real killer: pthread_exit() takes around 1
+microsecond on Linux, but with thousands (or tens of thousands) of vmas
+every pthread_exit() takes a millisecond or more, also totally
+destroying the CPU's L1 and L2 caches!
+
+This is very much noticeable even for normal process sys_exit_group()
+calls: the kernel has to do the vma scanning unconditionally! (this is
+because the kernel has no knowledge about how many robust futexes there
+are to be cleaned up, because a robust futex might have been registered
+in another task, and the futex variable might have been simply mmap()-ed
+into this process's address space).
+
+This huge overhead forced the creation of CONFIG_FUTEX_ROBUST so that
+normal kernels can turn it off, but worse than that: the overhead makes
+robust futexes impractical for any type of generic Linux distribution.
+
+So something had to be done.
+
+New approach to robust futexes
+------------------------------
+
+At the heart of this new approach there is a per-thread private list of
+robust locks that userspace is holding (maintained by glibc) - which
+userspace list is registered with the kernel via a new syscall [this
+registration happens at most once per thread lifetime]. At do_exit()
+time, the kernel checks this user-space list: are there any robust futex
+locks to be cleaned up?
+
+In the common case, at do_exit() time, there is no list registered, so
+the cost of robust futexes is just a simple current->robust_list != NULL
+comparison. If the thread has registered a list, then normally the list
+is empty. If the thread/process crashed or terminated in some incorrect
+way then the list might be non-empty: in this case the kernel carefully
+walks the list [not trusting it], and marks all locks that are owned by
+this thread with the FUTEX_OWNER_DIED bit, and wakes up one waiter (if
+any).
+
+The list is guaranteed to be private and per-thread at do_exit() time,
+so it can be accessed by the kernel in a lockless way.
+
+There is one race possible though: since adding to and removing from the
+list is done after the futex is acquired by glibc, there is a few
+instructions window for the thread (or process) to die there, leaving
+the futex hung. To protect against this possibility, userspace (glibc)
+also maintains a simple per-thread 'list_op_pending' field, to allow the
+kernel to clean up if the thread dies after acquiring the lock, but just
+before it could have added itself to the list. Glibc sets this
+list_op_pending field before it tries to acquire the futex, and clears
+it after the list-add (or list-remove) has finished.
+
+That's all that is needed - all the rest of robust-futex cleanup is done
+in userspace [just like with the previous patches].
+
+Ulrich Drepper has implemented the necessary glibc support for this new
+mechanism, which fully enables robust mutexes.
+
+Key differences of this userspace-list based approach, compared to the
+vma based method:
+
+ - it's much, much faster: at thread exit time, there's no need to loop
+   over every vma (!), which the VM-based method has to do. Only a very
+   simple 'is the list empty' op is done.
+
+ - no VM changes are needed - 'struct address_space' is left alone.
+
+ - no registration of individual locks is needed: robust mutexes don't
+   need any extra per-lock syscalls. Robust mutexes thus become a very
+   lightweight primitive - so they don't force the application designer
+   to do a hard choice between performance and robustness - robust
+   mutexes are just as fast.
+
+ - no per-lock kernel allocation happens.
+
+ - no resource limits are needed.
+
+ - no kernel-space recovery call (FUTEX_RECOVER) is needed.
+
+ - the implementation and the locking is "obvious", and there are no
+   interactions with the VM.
+
+Performance
+-----------
+
+I have benchmarked the time needed for the kernel to process a list of 1
+million (!) held locks, using the new method [on a 2GHz CPU]:
+
+ - with FUTEX_WAIT set [contended mutex]: 130 msecs
+ - without FUTEX_WAIT set [uncontended mutex]: 30 msecs
+
+I have also measured an approach where glibc does the lock notification
+[which it currently does for !pshared robust mutexes], and that took 256
+msecs - clearly slower, due to the 1 million FUTEX_WAKE syscalls
+userspace had to do.
+
+(1 million held locks are unheard of - we expect at most a handful of
+locks to be held at a time. Nevertheless it's nice to know that this
+approach scales nicely.)
+
+Implementation details
+----------------------
+
+The patch adds two new syscalls: one to register the userspace list, and
+one to query the registered list pointer::
+
+ asmlinkage long
+ sys_set_robust_list(struct robust_list_head __user *head,
+                     size_t len);
+
+ asmlinkage long
+ sys_get_robust_list(int pid, struct robust_list_head __user **head_ptr,
+                     size_t __user *len_ptr);
+
+List registration is very fast: the pointer is simply stored in
+current->robust_list. [Note that in the future, if robust futexes become
+widespread, we could extend sys_clone() to register a robust-list head
+for new threads, without the need of another syscall.]
+
+So there is virtually zero overhead for tasks not using robust futexes,
+and even for robust futex users, there is only one extra syscall per
+thread lifetime, and the cleanup operation, if it happens, is fast and
+straightforward. The kernel doesn't have any internal distinction between
+robust and normal futexes.
+
+If a futex is found to be held at exit time, the kernel sets the
+following bit of the futex word::
+
+	#define FUTEX_OWNER_DIED        0x40000000
+
+and wakes up the next futex waiter (if any). User-space does the rest of
+the cleanup.
+
+Otherwise, robust futexes are acquired by glibc by putting the TID into
+the futex field atomically. Waiters set the FUTEX_WAITERS bit::
+
+	#define FUTEX_WAITERS           0x80000000
+
+and the remaining bits are for the TID.
+
+Testing, architecture support
+-----------------------------
+
+I've tested the new syscalls on x86 and x86_64, and have made sure the
+parsing of the userspace list is robust [ ;-) ] even if the list is
+deliberately corrupted.
+
+i386 and x86_64 syscalls are wired up at the moment, and Ulrich has
+tested the new glibc code (on x86_64 and i386), and it works for his
+robust-mutex testcases.
+
+All other architectures should build just fine too - but they won't have
+the new syscalls yet.
+
+Architectures need to implement the new futex_atomic_cmpxchg_inatomic()
+inline function before writing up the syscalls.
diff --git a/Documentation/locking/rt-mutex-design.rst b/Documentation/locking/rt-mutex-design.rst
new file mode 100644
index 000000000..59c2a64ef
--- /dev/null
+++ b/Documentation/locking/rt-mutex-design.rst
@@ -0,0 +1,574 @@
+==============================
+RT-mutex implementation design
+==============================
+
+Copyright (c) 2006 Steven Rostedt
+
+Licensed under the GNU Free Documentation License, Version 1.2
+
+
+This document tries to describe the design of the rtmutex.c implementation.
+It doesn't describe the reasons why rtmutex.c exists. For that please see
+Documentation/locking/rt-mutex.rst.  Although this document does explain problems
+that happen without this code, but that is in the concept to understand
+what the code actually is doing.
+
+The goal of this document is to help others understand the priority
+inheritance (PI) algorithm that is used, as well as reasons for the
+decisions that were made to implement PI in the manner that was done.
+
+
+Unbounded Priority Inversion
+----------------------------
+
+Priority inversion is when a lower priority process executes while a higher
+priority process wants to run.  This happens for several reasons, and
+most of the time it can't be helped.  Anytime a high priority process wants
+to use a resource that a lower priority process has (a mutex for example),
+the high priority process must wait until the lower priority process is done
+with the resource.  This is a priority inversion.  What we want to prevent
+is something called unbounded priority inversion.  That is when the high
+priority process is prevented from running by a lower priority process for
+an undetermined amount of time.
+
+The classic example of unbounded priority inversion is where you have three
+processes, let's call them processes A, B, and C, where A is the highest
+priority process, C is the lowest, and B is in between. A tries to grab a lock
+that C owns and must wait and lets C run to release the lock. But in the
+meantime, B executes, and since B is of a higher priority than C, it preempts C,
+but by doing so, it is in fact preempting A which is a higher priority process.
+Now there's no way of knowing how long A will be sleeping waiting for C
+to release the lock, because for all we know, B is a CPU hog and will
+never give C a chance to release the lock.  This is called unbounded priority
+inversion.
+
+Here's a little ASCII art to show the problem::
+
+     grab lock L1 (owned by C)
+       |
+  A ---+
+          C preempted by B
+            |
+  C    +----+
+
+  B         +-------->
+                  B now keeps A from running.
+
+
+Priority Inheritance (PI)
+-------------------------
+
+There are several ways to solve this issue, but other ways are out of scope
+for this document.  Here we only discuss PI.
+
+PI is where a process inherits the priority of another process if the other
+process blocks on a lock owned by the current process.  To make this easier
+to understand, let's use the previous example, with processes A, B, and C again.
+
+This time, when A blocks on the lock owned by C, C would inherit the priority
+of A.  So now if B becomes runnable, it would not preempt C, since C now has
+the high priority of A.  As soon as C releases the lock, it loses its
+inherited priority, and A then can continue with the resource that C had.
+
+Terminology
+-----------
+
+Here I explain some terminology that is used in this document to help describe
+the design that is used to implement PI.
+
+PI chain
+         - The PI chain is an ordered series of locks and processes that cause
+           processes to inherit priorities from a previous process that is
+           blocked on one of its locks.  This is described in more detail
+           later in this document.
+
+mutex
+         - In this document, to differentiate from locks that implement
+           PI and spin locks that are used in the PI code, from now on
+           the PI locks will be called a mutex.
+
+lock
+         - In this document from now on, I will use the term lock when
+           referring to spin locks that are used to protect parts of the PI
+           algorithm.  These locks disable preemption for UP (when
+           CONFIG_PREEMPT is enabled) and on SMP prevents multiple CPUs from
+           entering critical sections simultaneously.
+
+spin lock
+         - Same as lock above.
+
+waiter
+         - A waiter is a struct that is stored on the stack of a blocked
+           process.  Since the scope of the waiter is within the code for
+           a process being blocked on the mutex, it is fine to allocate
+           the waiter on the process's stack (local variable).  This
+           structure holds a pointer to the task, as well as the mutex that
+           the task is blocked on.  It also has rbtree node structures to
+           place the task in the waiters rbtree of a mutex as well as the
+           pi_waiters rbtree of a mutex owner task (described below).
+
+           waiter is sometimes used in reference to the task that is waiting
+           on a mutex. This is the same as waiter->task.
+
+waiters
+         - A list of processes that are blocked on a mutex.
+
+top waiter
+         - The highest priority process waiting on a specific mutex.
+
+top pi waiter
+              - The highest priority process waiting on one of the mutexes
+                that a specific process owns.
+
+Note:
+       task and process are used interchangeably in this document, mostly to
+       differentiate between two processes that are being described together.
+
+
+PI chain
+--------
+
+The PI chain is a list of processes and mutexes that may cause priority
+inheritance to take place.  Multiple chains may converge, but a chain
+would never diverge, since a process can't be blocked on more than one
+mutex at a time.
+
+Example::
+
+   Process:  A, B, C, D, E
+   Mutexes:  L1, L2, L3, L4
+
+   A owns: L1
+           B blocked on L1
+           B owns L2
+                  C blocked on L2
+                  C owns L3
+                         D blocked on L3
+                         D owns L4
+                                E blocked on L4
+
+The chain would be::
+
+   E->L4->D->L3->C->L2->B->L1->A
+
+To show where two chains merge, we could add another process F and
+another mutex L5 where B owns L5 and F is blocked on mutex L5.
+
+The chain for F would be::
+
+   F->L5->B->L1->A
+
+Since a process may own more than one mutex, but never be blocked on more than
+one, the chains merge.
+
+Here we show both chains::
+
+   E->L4->D->L3->C->L2-+
+                       |
+                       +->B->L1->A
+                       |
+                 F->L5-+
+
+For PI to work, the processes at the right end of these chains (or we may
+also call it the Top of the chain) must be equal to or higher in priority
+than the processes to the left or below in the chain.
+
+Also since a mutex may have more than one process blocked on it, we can
+have multiple chains merge at mutexes.  If we add another process G that is
+blocked on mutex L2::
+
+  G->L2->B->L1->A
+
+And once again, to show how this can grow I will show the merging chains
+again::
+
+   E->L4->D->L3->C-+
+                   +->L2-+
+                   |     |
+                 G-+     +->B->L1->A
+                         |
+                   F->L5-+
+
+If process G has the highest priority in the chain, then all the tasks up
+the chain (A and B in this example), must have their priorities increased
+to that of G.
+
+Mutex Waiters Tree
+------------------
+
+Every mutex keeps track of all the waiters that are blocked on itself. The
+mutex has a rbtree to store these waiters by priority.  This tree is protected
+by a spin lock that is located in the struct of the mutex. This lock is called
+wait_lock.
+
+
+Task PI Tree
+------------
+
+To keep track of the PI chains, each process has its own PI rbtree.  This is
+a tree of all top waiters of the mutexes that are owned by the process.
+Note that this tree only holds the top waiters and not all waiters that are
+blocked on mutexes owned by the process.
+
+The top of the task's PI tree is always the highest priority task that
+is waiting on a mutex that is owned by the task.  So if the task has
+inherited a priority, it will always be the priority of the task that is
+at the top of this tree.
+
+This tree is stored in the task structure of a process as a rbtree called
+pi_waiters.  It is protected by a spin lock also in the task structure,
+called pi_lock.  This lock may also be taken in interrupt context, so when
+locking the pi_lock, interrupts must be disabled.
+
+
+Depth of the PI Chain
+---------------------
+
+The maximum depth of the PI chain is not dynamic, and could actually be
+defined.  But is very complex to figure it out, since it depends on all
+the nesting of mutexes.  Let's look at the example where we have 3 mutexes,
+L1, L2, and L3, and four separate functions func1, func2, func3 and func4.
+The following shows a locking order of L1->L2->L3, but may not actually
+be directly nested that way::
+
+  void func1(void)
+  {
+	mutex_lock(L1);
+
+	/* do anything */
+
+	mutex_unlock(L1);
+  }
+
+  void func2(void)
+  {
+	mutex_lock(L1);
+	mutex_lock(L2);
+
+	/* do something */
+
+	mutex_unlock(L2);
+	mutex_unlock(L1);
+  }
+
+  void func3(void)
+  {
+	mutex_lock(L2);
+	mutex_lock(L3);
+
+	/* do something else */
+
+	mutex_unlock(L3);
+	mutex_unlock(L2);
+  }
+
+  void func4(void)
+  {
+	mutex_lock(L3);
+
+	/* do something again */
+
+	mutex_unlock(L3);
+  }
+
+Now we add 4 processes that run each of these functions separately.
+Processes A, B, C, and D which run functions func1, func2, func3 and func4
+respectively, and such that D runs first and A last.  With D being preempted
+in func4 in the "do something again" area, we have a locking that follows::
+
+  D owns L3
+         C blocked on L3
+         C owns L2
+                B blocked on L2
+                B owns L1
+                       A blocked on L1
+
+  And thus we have the chain A->L1->B->L2->C->L3->D.
+
+This gives us a PI depth of 4 (four processes), but looking at any of the
+functions individually, it seems as though they only have at most a locking
+depth of two.  So, although the locking depth is defined at compile time,
+it still is very difficult to find the possibilities of that depth.
+
+Now since mutexes can be defined by user-land applications, we don't want a DOS
+type of application that nests large amounts of mutexes to create a large
+PI chain, and have the code holding spin locks while looking at a large
+amount of data.  So to prevent this, the implementation not only implements
+a maximum lock depth, but also only holds at most two different locks at a
+time, as it walks the PI chain.  More about this below.
+
+
+Mutex owner and flags
+---------------------
+
+The mutex structure contains a pointer to the owner of the mutex.  If the
+mutex is not owned, this owner is set to NULL.  Since all architectures
+have the task structure on at least a two byte alignment (and if this is
+not true, the rtmutex.c code will be broken!), this allows for the least
+significant bit to be used as a flag.  Bit 0 is used as the "Has Waiters"
+flag. It's set whenever there are waiters on a mutex.
+
+See Documentation/locking/rt-mutex.rst for further details.
+
+cmpxchg Tricks
+--------------
+
+Some architectures implement an atomic cmpxchg (Compare and Exchange).  This
+is used (when applicable) to keep the fast path of grabbing and releasing
+mutexes short.
+
+cmpxchg is basically the following function performed atomically::
+
+  unsigned long _cmpxchg(unsigned long *A, unsigned long *B, unsigned long *C)
+  {
+	unsigned long T = *A;
+	if (*A == *B) {
+		*A = *C;
+	}
+	return T;
+  }
+  #define cmpxchg(a,b,c) _cmpxchg(&a,&b,&c)
+
+This is really nice to have, since it allows you to only update a variable
+if the variable is what you expect it to be.  You know if it succeeded if
+the return value (the old value of A) is equal to B.
+
+The macro rt_mutex_cmpxchg is used to try to lock and unlock mutexes. If
+the architecture does not support CMPXCHG, then this macro is simply set
+to fail every time.  But if CMPXCHG is supported, then this will
+help out extremely to keep the fast path short.
+
+The use of rt_mutex_cmpxchg with the flags in the owner field help optimize
+the system for architectures that support it.  This will also be explained
+later in this document.
+
+
+Priority adjustments
+--------------------
+
+The implementation of the PI code in rtmutex.c has several places that a
+process must adjust its priority.  With the help of the pi_waiters of a
+process this is rather easy to know what needs to be adjusted.
+
+The functions implementing the task adjustments are rt_mutex_adjust_prio
+and rt_mutex_setprio. rt_mutex_setprio is only used in rt_mutex_adjust_prio.
+
+rt_mutex_adjust_prio examines the priority of the task, and the highest
+priority process that is waiting any of mutexes owned by the task. Since
+the pi_waiters of a task holds an order by priority of all the top waiters
+of all the mutexes that the task owns, we simply need to compare the top
+pi waiter to its own normal/deadline priority and take the higher one.
+Then rt_mutex_setprio is called to adjust the priority of the task to the
+new priority. Note that rt_mutex_setprio is defined in kernel/sched/core.c
+to implement the actual change in priority.
+
+Note:
+	For the "prio" field in task_struct, the lower the number, the
+	higher the priority. A "prio" of 5 is of higher priority than a
+	"prio" of 10.
+
+It is interesting to note that rt_mutex_adjust_prio can either increase
+or decrease the priority of the task.  In the case that a higher priority
+process has just blocked on a mutex owned by the task, rt_mutex_adjust_prio
+would increase/boost the task's priority.  But if a higher priority task
+were for some reason to leave the mutex (timeout or signal), this same function
+would decrease/unboost the priority of the task.  That is because the pi_waiters
+always contains the highest priority task that is waiting on a mutex owned
+by the task, so we only need to compare the priority of that top pi waiter
+to the normal priority of the given task.
+
+
+High level overview of the PI chain walk
+----------------------------------------
+
+The PI chain walk is implemented by the function rt_mutex_adjust_prio_chain.
+
+The implementation has gone through several iterations, and has ended up
+with what we believe is the best.  It walks the PI chain by only grabbing
+at most two locks at a time, and is very efficient.
+
+The rt_mutex_adjust_prio_chain can be used either to boost or lower process
+priorities.
+
+rt_mutex_adjust_prio_chain is called with a task to be checked for PI
+(de)boosting (the owner of a mutex that a process is blocking on), a flag to
+check for deadlocking, the mutex that the task owns, a pointer to a waiter
+that is the process's waiter struct that is blocked on the mutex (although this
+parameter may be NULL for deboosting), a pointer to the mutex on which the task
+is blocked, and a top_task as the top waiter of the mutex.
+
+For this explanation, I will not mention deadlock detection. This explanation
+will try to stay at a high level.
+
+When this function is called, there are no locks held.  That also means
+that the state of the owner and lock can change when entered into this function.
+
+Before this function is called, the task has already had rt_mutex_adjust_prio
+performed on it.  This means that the task is set to the priority that it
+should be at, but the rbtree nodes of the task's waiter have not been updated
+with the new priorities, and this task may not be in the proper locations
+in the pi_waiters and waiters trees that the task is blocked on. This function
+solves all that.
+
+The main operation of this function is summarized by Thomas Gleixner in
+rtmutex.c. See the 'Chain walk basics and protection scope' comment for further
+details.
+
+Taking of a mutex (The walk through)
+------------------------------------
+
+OK, now let's take a look at the detailed walk through of what happens when
+taking a mutex.
+
+The first thing that is tried is the fast taking of the mutex.  This is
+done when we have CMPXCHG enabled (otherwise the fast taking automatically
+fails).  Only when the owner field of the mutex is NULL can the lock be
+taken with the CMPXCHG and nothing else needs to be done.
+
+If there is contention on the lock, we go about the slow path
+(rt_mutex_slowlock).
+
+The slow path function is where the task's waiter structure is created on
+the stack.  This is because the waiter structure is only needed for the
+scope of this function.  The waiter structure holds the nodes to store
+the task on the waiters tree of the mutex, and if need be, the pi_waiters
+tree of the owner.
+
+The wait_lock of the mutex is taken since the slow path of unlocking the
+mutex also takes this lock.
+
+We then call try_to_take_rt_mutex.  This is where the architecture that
+does not implement CMPXCHG would always grab the lock (if there's no
+contention).
+
+try_to_take_rt_mutex is used every time the task tries to grab a mutex in the
+slow path.  The first thing that is done here is an atomic setting of
+the "Has Waiters" flag of the mutex's owner field. By setting this flag
+now, the current owner of the mutex being contended for can't release the mutex
+without going into the slow unlock path, and it would then need to grab the
+wait_lock, which this code currently holds. So setting the "Has Waiters" flag
+forces the current owner to synchronize with this code.
+
+The lock is taken if the following are true:
+
+   1) The lock has no owner
+   2) The current task is the highest priority against all other
+      waiters of the lock
+
+If the task succeeds to acquire the lock, then the task is set as the
+owner of the lock, and if the lock still has waiters, the top_waiter
+(highest priority task waiting on the lock) is added to this task's
+pi_waiters tree.
+
+If the lock is not taken by try_to_take_rt_mutex(), then the
+task_blocks_on_rt_mutex() function is called. This will add the task to
+the lock's waiter tree and propagate the pi chain of the lock as well
+as the lock's owner's pi_waiters tree. This is described in the next
+section.
+
+Task blocks on mutex
+--------------------
+
+The accounting of a mutex and process is done with the waiter structure of
+the process.  The "task" field is set to the process, and the "lock" field
+to the mutex.  The rbtree node of waiter are initialized to the processes
+current priority.
+
+Since the wait_lock was taken at the entry of the slow lock, we can safely
+add the waiter to the task waiter tree.  If the current process is the
+highest priority process currently waiting on this mutex, then we remove the
+previous top waiter process (if it exists) from the pi_waiters of the owner,
+and add the current process to that tree.  Since the pi_waiter of the owner
+has changed, we call rt_mutex_adjust_prio on the owner to see if the owner
+should adjust its priority accordingly.
+
+If the owner is also blocked on a lock, and had its pi_waiters changed
+(or deadlock checking is on), we unlock the wait_lock of the mutex and go ahead
+and run rt_mutex_adjust_prio_chain on the owner, as described earlier.
+
+Now all locks are released, and if the current process is still blocked on a
+mutex (waiter "task" field is not NULL), then we go to sleep (call schedule).
+
+Waking up in the loop
+---------------------
+
+The task can then wake up for a couple of reasons:
+  1) The previous lock owner released the lock, and the task now is top_waiter
+  2) we received a signal or timeout
+
+In both cases, the task will try again to acquire the lock. If it
+does, then it will take itself off the waiters tree and set itself back
+to the TASK_RUNNING state.
+
+In first case, if the lock was acquired by another task before this task
+could get the lock, then it will go back to sleep and wait to be woken again.
+
+The second case is only applicable for tasks that are grabbing a mutex
+that can wake up before getting the lock, either due to a signal or
+a timeout (i.e. rt_mutex_timed_futex_lock()). When woken, it will try to
+take the lock again, if it succeeds, then the task will return with the
+lock held, otherwise it will return with -EINTR if the task was woken
+by a signal, or -ETIMEDOUT if it timed out.
+
+
+Unlocking the Mutex
+-------------------
+
+The unlocking of a mutex also has a fast path for those architectures with
+CMPXCHG.  Since the taking of a mutex on contention always sets the
+"Has Waiters" flag of the mutex's owner, we use this to know if we need to
+take the slow path when unlocking the mutex.  If the mutex doesn't have any
+waiters, the owner field of the mutex would equal the current process and
+the mutex can be unlocked by just replacing the owner field with NULL.
+
+If the owner field has the "Has Waiters" bit set (or CMPXCHG is not available),
+the slow unlock path is taken.
+
+The first thing done in the slow unlock path is to take the wait_lock of the
+mutex.  This synchronizes the locking and unlocking of the mutex.
+
+A check is made to see if the mutex has waiters or not.  On architectures that
+do not have CMPXCHG, this is the location that the owner of the mutex will
+determine if a waiter needs to be awoken or not.  On architectures that
+do have CMPXCHG, that check is done in the fast path, but it is still needed
+in the slow path too.  If a waiter of a mutex woke up because of a signal
+or timeout between the time the owner failed the fast path CMPXCHG check and
+the grabbing of the wait_lock, the mutex may not have any waiters, thus the
+owner still needs to make this check. If there are no waiters then the mutex
+owner field is set to NULL, the wait_lock is released and nothing more is
+needed.
+
+If there are waiters, then we need to wake one up.
+
+On the wake up code, the pi_lock of the current owner is taken.  The top
+waiter of the lock is found and removed from the waiters tree of the mutex
+as well as the pi_waiters tree of the current owner. The "Has Waiters" bit is
+marked to prevent lower priority tasks from stealing the lock.
+
+Finally we unlock the pi_lock of the pending owner and wake it up.
+
+
+Contact
+-------
+
+For updates on this document, please email Steven Rostedt <rostedt@goodmis.org>
+
+
+Credits
+-------
+
+Author:  Steven Rostedt <rostedt@goodmis.org>
+
+Updated: Alex Shi <alex.shi@linaro.org>	- 7/6/2017
+
+Original Reviewers:
+		     Ingo Molnar, Thomas Gleixner, Thomas Duetsch, and
+		     Randy Dunlap
+
+Update (7/6/2017) Reviewers: Steven Rostedt and Sebastian Siewior
+
+Updates
+-------
+
+This document was originally written for 2.6.17-rc3-mm1
+was updated on 4.12
diff --git a/Documentation/locking/rt-mutex.rst b/Documentation/locking/rt-mutex.rst
new file mode 100644
index 000000000..3b5097a38
--- /dev/null
+++ b/Documentation/locking/rt-mutex.rst
@@ -0,0 +1,77 @@
+==================================
+RT-mutex subsystem with PI support
+==================================
+
+RT-mutexes with priority inheritance are used to support PI-futexes,
+which enable pthread_mutex_t priority inheritance attributes
+(PTHREAD_PRIO_INHERIT). [See Documentation/locking/pi-futex.rst for more details
+about PI-futexes.]
+
+This technology was developed in the -rt tree and streamlined for
+pthread_mutex support.
+
+Basic principles:
+-----------------
+
+RT-mutexes extend the semantics of simple mutexes by the priority
+inheritance protocol.
+
+A low priority owner of a rt-mutex inherits the priority of a higher
+priority waiter until the rt-mutex is released. If the temporarily
+boosted owner blocks on a rt-mutex itself it propagates the priority
+boosting to the owner of the other rt_mutex it gets blocked on. The
+priority boosting is immediately removed once the rt_mutex has been
+unlocked.
+
+This approach allows us to shorten the block of high-prio tasks on
+mutexes which protect shared resources. Priority inheritance is not a
+magic bullet for poorly designed applications, but it allows
+well-designed applications to use userspace locks in critical parts of
+an high priority thread, without losing determinism.
+
+The enqueueing of the waiters into the rtmutex waiter tree is done in
+priority order. For same priorities FIFO order is chosen. For each
+rtmutex, only the top priority waiter is enqueued into the owner's
+priority waiters tree. This tree too queues in priority order. Whenever
+the top priority waiter of a task changes (for example it timed out or
+got a signal), the priority of the owner task is readjusted. The
+priority enqueueing is handled by "pi_waiters".
+
+RT-mutexes are optimized for fastpath operations and have no internal
+locking overhead when locking an uncontended mutex or unlocking a mutex
+without waiters. The optimized fastpath operations require cmpxchg
+support. [If that is not available then the rt-mutex internal spinlock
+is used]
+
+The state of the rt-mutex is tracked via the owner field of the rt-mutex
+structure:
+
+lock->owner holds the task_struct pointer of the owner. Bit 0 is used to
+keep track of the "lock has waiters" state:
+
+ ============ ======= ================================================
+ owner        bit0    Notes
+ ============ ======= ================================================
+ NULL         0       lock is free (fast acquire possible)
+ NULL         1       lock is free and has waiters and the top waiter
+		      is going to take the lock [1]_
+ taskpointer  0       lock is held (fast release possible)
+ taskpointer  1       lock is held and has waiters [2]_
+ ============ ======= ================================================
+
+The fast atomic compare exchange based acquire and release is only
+possible when bit 0 of lock->owner is 0.
+
+.. [1] It also can be a transitional state when grabbing the lock
+       with ->wait_lock is held. To prevent any fast path cmpxchg to the lock,
+       we need to set the bit0 before looking at the lock, and the owner may
+       be NULL in this small time, hence this can be a transitional state.
+
+.. [2] There is a small time when bit 0 is set but there are no
+       waiters. This can happen when grabbing the lock in the slow path.
+       To prevent a cmpxchg of the owner releasing the lock, we need to
+       set this bit before looking at the lock.
+
+BTW, there is still technically a "Pending Owner", it's just not called
+that anymore. The pending owner happens to be the top_waiter of a lock
+that has no owner and has been woken up to grab the lock.
diff --git a/Documentation/locking/seqlock.rst b/Documentation/locking/seqlock.rst
new file mode 100644
index 000000000..bfda1a5fe
--- /dev/null
+++ b/Documentation/locking/seqlock.rst
@@ -0,0 +1,239 @@
+======================================
+Sequence counters and sequential locks
+======================================
+
+Introduction
+============
+
+Sequence counters are a reader-writer consistency mechanism with
+lockless readers (read-only retry loops), and no writer starvation. They
+are used for data that's rarely written to (e.g. system time), where the
+reader wants a consistent set of information and is willing to retry if
+that information changes.
+
+A data set is consistent when the sequence count at the beginning of the
+read side critical section is even and the same sequence count value is
+read again at the end of the critical section. The data in the set must
+be copied out inside the read side critical section. If the sequence
+count has changed between the start and the end of the critical section,
+the reader must retry.
+
+Writers increment the sequence count at the start and the end of their
+critical section. After starting the critical section the sequence count
+is odd and indicates to the readers that an update is in progress. At
+the end of the write side critical section the sequence count becomes
+even again which lets readers make progress.
+
+A sequence counter write side critical section must never be preempted
+or interrupted by read side sections. Otherwise the reader will spin for
+the entire scheduler tick due to the odd sequence count value and the
+interrupted writer. If that reader belongs to a real-time scheduling
+class, it can spin forever and the kernel will livelock.
+
+This mechanism cannot be used if the protected data contains pointers,
+as the writer can invalidate a pointer that the reader is following.
+
+
+.. _seqcount_t:
+
+Sequence counters (``seqcount_t``)
+==================================
+
+This is the raw counting mechanism, which does not protect against
+multiple writers.  Write side critical sections must thus be serialized
+by an external lock.
+
+If the write serialization primitive is not implicitly disabling
+preemption, preemption must be explicitly disabled before entering the
+write side section. If the read section can be invoked from hardirq or
+softirq contexts, interrupts or bottom halves must also be respectively
+disabled before entering the write section.
+
+If it's desired to automatically handle the sequence counter
+requirements of writer serialization and non-preemptibility, use
+:ref:`seqlock_t` instead.
+
+Initialization::
+
+	/* dynamic */
+	seqcount_t foo_seqcount;
+	seqcount_init(&foo_seqcount);
+
+	/* static */
+	static seqcount_t foo_seqcount = SEQCNT_ZERO(foo_seqcount);
+
+	/* C99 struct init */
+	struct {
+		.seq   = SEQCNT_ZERO(foo.seq),
+	} foo;
+
+Write path::
+
+	/* Serialized context with disabled preemption */
+
+	write_seqcount_begin(&foo_seqcount);
+
+	/* ... [[write-side critical section]] ... */
+
+	write_seqcount_end(&foo_seqcount);
+
+Read path::
+
+	do {
+		seq = read_seqcount_begin(&foo_seqcount);
+
+		/* ... [[read-side critical section]] ... */
+
+	} while (read_seqcount_retry(&foo_seqcount, seq));
+
+
+.. _seqcount_locktype_t:
+
+Sequence counters with associated locks (``seqcount_LOCKNAME_t``)
+-----------------------------------------------------------------
+
+As discussed at :ref:`seqcount_t`, sequence count write side critical
+sections must be serialized and non-preemptible. This variant of
+sequence counters associate the lock used for writer serialization at
+initialization time, which enables lockdep to validate that the write
+side critical sections are properly serialized.
+
+This lock association is a NOOP if lockdep is disabled and has neither
+storage nor runtime overhead. If lockdep is enabled, the lock pointer is
+stored in struct seqcount and lockdep's "lock is held" assertions are
+injected at the beginning of the write side critical section to validate
+that it is properly protected.
+
+For lock types which do not implicitly disable preemption, preemption
+protection is enforced in the write side function.
+
+The following sequence counters with associated locks are defined:
+
+  - ``seqcount_spinlock_t``
+  - ``seqcount_raw_spinlock_t``
+  - ``seqcount_rwlock_t``
+  - ``seqcount_mutex_t``
+  - ``seqcount_ww_mutex_t``
+
+The sequence counter read and write APIs can take either a plain
+seqcount_t or any of the seqcount_LOCKNAME_t variants above.
+
+Initialization (replace "LOCKNAME" with one of the supported locks)::
+
+	/* dynamic */
+	seqcount_LOCKNAME_t foo_seqcount;
+	seqcount_LOCKNAME_init(&foo_seqcount, &lock);
+
+	/* static */
+	static seqcount_LOCKNAME_t foo_seqcount =
+		SEQCNT_LOCKNAME_ZERO(foo_seqcount, &lock);
+
+	/* C99 struct init */
+	struct {
+		.seq   = SEQCNT_LOCKNAME_ZERO(foo.seq, &lock),
+	} foo;
+
+Write path: same as in :ref:`seqcount_t`, while running from a context
+with the associated write serialization lock acquired.
+
+Read path: same as in :ref:`seqcount_t`.
+
+
+.. _seqcount_latch_t:
+
+Latch sequence counters (``seqcount_latch_t``)
+----------------------------------------------
+
+Latch sequence counters are a multiversion concurrency control mechanism
+where the embedded seqcount_t counter even/odd value is used to switch
+between two copies of protected data. This allows the sequence counter
+read path to safely interrupt its own write side critical section.
+
+Use seqcount_latch_t when the write side sections cannot be protected
+from interruption by readers. This is typically the case when the read
+side can be invoked from NMI handlers.
+
+Check `raw_write_seqcount_latch()` for more information.
+
+
+.. _seqlock_t:
+
+Sequential locks (``seqlock_t``)
+================================
+
+This contains the :ref:`seqcount_t` mechanism earlier discussed, plus an
+embedded spinlock for writer serialization and non-preemptibility.
+
+If the read side section can be invoked from hardirq or softirq context,
+use the write side function variants which disable interrupts or bottom
+halves respectively.
+
+Initialization::
+
+	/* dynamic */
+	seqlock_t foo_seqlock;
+	seqlock_init(&foo_seqlock);
+
+	/* static */
+	static DEFINE_SEQLOCK(foo_seqlock);
+
+	/* C99 struct init */
+	struct {
+		.seql   = __SEQLOCK_UNLOCKED(foo.seql)
+	} foo;
+
+Write path::
+
+	write_seqlock(&foo_seqlock);
+
+	/* ... [[write-side critical section]] ... */
+
+	write_sequnlock(&foo_seqlock);
+
+Read path, three categories:
+
+1. Normal Sequence readers which never block a writer but they must
+   retry if a writer is in progress by detecting change in the sequence
+   number.  Writers do not wait for a sequence reader::
+
+	do {
+		seq = read_seqbegin(&foo_seqlock);
+
+		/* ... [[read-side critical section]] ... */
+
+	} while (read_seqretry(&foo_seqlock, seq));
+
+2. Locking readers which will wait if a writer or another locking reader
+   is in progress. A locking reader in progress will also block a writer
+   from entering its critical section. This read lock is
+   exclusive. Unlike rwlock_t, only one locking reader can acquire it::
+
+	read_seqlock_excl(&foo_seqlock);
+
+	/* ... [[read-side critical section]] ... */
+
+	read_sequnlock_excl(&foo_seqlock);
+
+3. Conditional lockless reader (as in 1), or locking reader (as in 2),
+   according to a passed marker. This is used to avoid lockless readers
+   starvation (too much retry loops) in case of a sharp spike in write
+   activity. First, a lockless read is tried (even marker passed). If
+   that trial fails (odd sequence counter is returned, which is used as
+   the next iteration marker), the lockless read is transformed to a
+   full locking read and no retry loop is necessary::
+
+	/* marker; even initialization */
+	int seq = 0;
+	do {
+		read_seqbegin_or_lock(&foo_seqlock, &seq);
+
+		/* ... [[read-side critical section]] ... */
+
+	} while (need_seqretry(&foo_seqlock, seq));
+	done_seqretry(&foo_seqlock, seq);
+
+
+API documentation
+=================
+
+.. kernel-doc:: include/linux/seqlock.h
diff --git a/Documentation/locking/spinlocks.rst b/Documentation/locking/spinlocks.rst
new file mode 100644
index 000000000..bec96f7a9
--- /dev/null
+++ b/Documentation/locking/spinlocks.rst
@@ -0,0 +1,165 @@
+===============
+Locking lessons
+===============
+
+Lesson 1: Spin locks
+====================
+
+The most basic primitive for locking is spinlock::
+
+  static DEFINE_SPINLOCK(xxx_lock);
+
+	unsigned long flags;
+
+	spin_lock_irqsave(&xxx_lock, flags);
+	... critical section here ..
+	spin_unlock_irqrestore(&xxx_lock, flags);
+
+The above is always safe. It will disable interrupts _locally_, but the
+spinlock itself will guarantee the global lock, so it will guarantee that
+there is only one thread-of-control within the region(s) protected by that
+lock. This works well even under UP also, so the code does _not_ need to
+worry about UP vs SMP issues: the spinlocks work correctly under both.
+
+   NOTE! Implications of spin_locks for memory are further described in:
+
+     Documentation/memory-barriers.txt
+
+       (5) ACQUIRE operations.
+
+       (6) RELEASE operations.
+
+The above is usually pretty simple (you usually need and want only one
+spinlock for most things - using more than one spinlock can make things a
+lot more complex and even slower and is usually worth it only for
+sequences that you **know** need to be split up: avoid it at all cost if you
+aren't sure).
+
+This is really the only really hard part about spinlocks: once you start
+using spinlocks they tend to expand to areas you might not have noticed
+before, because you have to make sure the spinlocks correctly protect the
+shared data structures **everywhere** they are used. The spinlocks are most
+easily added to places that are completely independent of other code (for
+example, internal driver data structures that nobody else ever touches).
+
+   NOTE! The spin-lock is safe only when you **also** use the lock itself
+   to do locking across CPU's, which implies that EVERYTHING that
+   touches a shared variable has to agree about the spinlock they want
+   to use.
+
+----
+
+Lesson 2: reader-writer spinlocks.
+==================================
+
+If your data accesses have a very natural pattern where you usually tend
+to mostly read from the shared variables, the reader-writer locks
+(rw_lock) versions of the spinlocks are sometimes useful. They allow multiple
+readers to be in the same critical region at once, but if somebody wants
+to change the variables it has to get an exclusive write lock.
+
+   NOTE! reader-writer locks require more atomic memory operations than
+   simple spinlocks.  Unless the reader critical section is long, you
+   are better off just using spinlocks.
+
+The routines look the same as above::
+
+   rwlock_t xxx_lock = __RW_LOCK_UNLOCKED(xxx_lock);
+
+	unsigned long flags;
+
+	read_lock_irqsave(&xxx_lock, flags);
+	.. critical section that only reads the info ...
+	read_unlock_irqrestore(&xxx_lock, flags);
+
+	write_lock_irqsave(&xxx_lock, flags);
+	.. read and write exclusive access to the info ...
+	write_unlock_irqrestore(&xxx_lock, flags);
+
+The above kind of lock may be useful for complex data structures like
+linked lists, especially searching for entries without changing the list
+itself.  The read lock allows many concurrent readers.  Anything that
+**changes** the list will have to get the write lock.
+
+   NOTE! RCU is better for list traversal, but requires careful
+   attention to design detail (see Documentation/RCU/listRCU.rst).
+
+Also, you cannot "upgrade" a read-lock to a write-lock, so if you at _any_
+time need to do any changes (even if you don't do it every time), you have
+to get the write-lock at the very beginning.
+
+   NOTE! We are working hard to remove reader-writer spinlocks in most
+   cases, so please don't add a new one without consensus.  (Instead, see
+   Documentation/RCU/rcu.rst for complete information.)
+
+----
+
+Lesson 3: spinlocks revisited.
+==============================
+
+The single spin-lock primitives above are by no means the only ones. They
+are the most safe ones, and the ones that work under all circumstances,
+but partly **because** they are safe they are also fairly slow. They are slower
+than they'd need to be, because they do have to disable interrupts
+(which is just a single instruction on a x86, but it's an expensive one -
+and on other architectures it can be worse).
+
+If you have a case where you have to protect a data structure across
+several CPU's and you want to use spinlocks you can potentially use
+cheaper versions of the spinlocks. IFF you know that the spinlocks are
+never used in interrupt handlers, you can use the non-irq versions::
+
+	spin_lock(&lock);
+	...
+	spin_unlock(&lock);
+
+(and the equivalent read-write versions too, of course). The spinlock will
+guarantee the same kind of exclusive access, and it will be much faster.
+This is useful if you know that the data in question is only ever
+manipulated from a "process context", ie no interrupts involved.
+
+The reasons you mustn't use these versions if you have interrupts that
+play with the spinlock is that you can get deadlocks::
+
+	spin_lock(&lock);
+	...
+		<- interrupt comes in:
+			spin_lock(&lock);
+
+where an interrupt tries to lock an already locked variable. This is ok if
+the other interrupt happens on another CPU, but it is _not_ ok if the
+interrupt happens on the same CPU that already holds the lock, because the
+lock will obviously never be released (because the interrupt is waiting
+for the lock, and the lock-holder is interrupted by the interrupt and will
+not continue until the interrupt has been processed).
+
+(This is also the reason why the irq-versions of the spinlocks only need
+to disable the _local_ interrupts - it's ok to use spinlocks in interrupts
+on other CPU's, because an interrupt on another CPU doesn't interrupt the
+CPU that holds the lock, so the lock-holder can continue and eventually
+releases the lock).
+
+		Linus
+
+----
+
+Reference information:
+======================
+
+For dynamic initialization, use spin_lock_init() or rwlock_init() as
+appropriate::
+
+   spinlock_t xxx_lock;
+   rwlock_t xxx_rw_lock;
+
+   static int __init xxx_init(void)
+   {
+	spin_lock_init(&xxx_lock);
+	rwlock_init(&xxx_rw_lock);
+	...
+   }
+
+   module_init(xxx_init);
+
+For static initialization, use DEFINE_SPINLOCK() / DEFINE_RWLOCK() or
+__SPIN_LOCK_UNLOCKED() / __RW_LOCK_UNLOCKED() as appropriate.
diff --git a/Documentation/locking/ww-mutex-design.rst b/Documentation/locking/ww-mutex-design.rst
new file mode 100644
index 000000000..6a8f8beb9
--- /dev/null
+++ b/Documentation/locking/ww-mutex-design.rst
@@ -0,0 +1,393 @@
+======================================
+Wound/Wait Deadlock-Proof Mutex Design
+======================================
+
+Please read mutex-design.rst first, as it applies to wait/wound mutexes too.
+
+Motivation for WW-Mutexes
+-------------------------
+
+GPU's do operations that commonly involve many buffers.  Those buffers
+can be shared across contexts/processes, exist in different memory
+domains (for example VRAM vs system memory), and so on.  And with
+PRIME / dmabuf, they can even be shared across devices.  So there are
+a handful of situations where the driver needs to wait for buffers to
+become ready.  If you think about this in terms of waiting on a buffer
+mutex for it to become available, this presents a problem because
+there is no way to guarantee that buffers appear in a execbuf/batch in
+the same order in all contexts.  That is directly under control of
+userspace, and a result of the sequence of GL calls that an application
+makes.	Which results in the potential for deadlock.  The problem gets
+more complex when you consider that the kernel may need to migrate the
+buffer(s) into VRAM before the GPU operates on the buffer(s), which
+may in turn require evicting some other buffers (and you don't want to
+evict other buffers which are already queued up to the GPU), but for a
+simplified understanding of the problem you can ignore this.
+
+The algorithm that the TTM graphics subsystem came up with for dealing with
+this problem is quite simple.  For each group of buffers (execbuf) that need
+to be locked, the caller would be assigned a unique reservation id/ticket,
+from a global counter.  In case of deadlock while locking all the buffers
+associated with a execbuf, the one with the lowest reservation ticket (i.e.
+the oldest task) wins, and the one with the higher reservation id (i.e. the
+younger task) unlocks all of the buffers that it has already locked, and then
+tries again.
+
+In the RDBMS literature, a reservation ticket is associated with a transaction.
+and the deadlock handling approach is called Wait-Die. The name is based on
+the actions of a locking thread when it encounters an already locked mutex.
+If the transaction holding the lock is younger, the locking transaction waits.
+If the transaction holding the lock is older, the locking transaction backs off
+and dies. Hence Wait-Die.
+There is also another algorithm called Wound-Wait:
+If the transaction holding the lock is younger, the locking transaction
+wounds the transaction holding the lock, requesting it to die.
+If the transaction holding the lock is older, it waits for the other
+transaction. Hence Wound-Wait.
+The two algorithms are both fair in that a transaction will eventually succeed.
+However, the Wound-Wait algorithm is typically stated to generate fewer backoffs
+compared to Wait-Die, but is, on the other hand, associated with more work than
+Wait-Die when recovering from a backoff. Wound-Wait is also a preemptive
+algorithm in that transactions are wounded by other transactions, and that
+requires a reliable way to pick up the wounded condition and preempt the
+running transaction. Note that this is not the same as process preemption. A
+Wound-Wait transaction is considered preempted when it dies (returning
+-EDEADLK) following a wound.
+
+Concepts
+--------
+
+Compared to normal mutexes two additional concepts/objects show up in the lock
+interface for w/w mutexes:
+
+Acquire context: To ensure eventual forward progress it is important that a task
+trying to acquire locks doesn't grab a new reservation id, but keeps the one it
+acquired when starting the lock acquisition. This ticket is stored in the
+acquire context. Furthermore the acquire context keeps track of debugging state
+to catch w/w mutex interface abuse. An acquire context is representing a
+transaction.
+
+W/w class: In contrast to normal mutexes the lock class needs to be explicit for
+w/w mutexes, since it is required to initialize the acquire context. The lock
+class also specifies what algorithm to use, Wound-Wait or Wait-Die.
+
+Furthermore there are three different class of w/w lock acquire functions:
+
+* Normal lock acquisition with a context, using ww_mutex_lock.
+
+* Slowpath lock acquisition on the contending lock, used by the task that just
+  killed its transaction after having dropped all already acquired locks.
+  These functions have the _slow postfix.
+
+  From a simple semantics point-of-view the _slow functions are not strictly
+  required, since simply calling the normal ww_mutex_lock functions on the
+  contending lock (after having dropped all other already acquired locks) will
+  work correctly. After all if no other ww mutex has been acquired yet there's
+  no deadlock potential and hence the ww_mutex_lock call will block and not
+  prematurely return -EDEADLK. The advantage of the _slow functions is in
+  interface safety:
+
+  - ww_mutex_lock has a __must_check int return type, whereas ww_mutex_lock_slow
+    has a void return type. Note that since ww mutex code needs loops/retries
+    anyway the __must_check doesn't result in spurious warnings, even though the
+    very first lock operation can never fail.
+  - When full debugging is enabled ww_mutex_lock_slow checks that all acquired
+    ww mutex have been released (preventing deadlocks) and makes sure that we
+    block on the contending lock (preventing spinning through the -EDEADLK
+    slowpath until the contended lock can be acquired).
+
+* Functions to only acquire a single w/w mutex, which results in the exact same
+  semantics as a normal mutex. This is done by calling ww_mutex_lock with a NULL
+  context.
+
+  Again this is not strictly required. But often you only want to acquire a
+  single lock in which case it's pointless to set up an acquire context (and so
+  better to avoid grabbing a deadlock avoidance ticket).
+
+Of course, all the usual variants for handling wake-ups due to signals are also
+provided.
+
+Usage
+-----
+
+The algorithm (Wait-Die vs Wound-Wait) is chosen by using either
+DEFINE_WW_CLASS() (Wound-Wait) or DEFINE_WD_CLASS() (Wait-Die)
+As a rough rule of thumb, use Wound-Wait iff you
+expect the number of simultaneous competing transactions to be typically small,
+and you want to reduce the number of rollbacks.
+
+Three different ways to acquire locks within the same w/w class. Common
+definitions for methods #1 and #2::
+
+  static DEFINE_WW_CLASS(ww_class);
+
+  struct obj {
+	struct ww_mutex lock;
+	/* obj data */
+  };
+
+  struct obj_entry {
+	struct list_head head;
+	struct obj *obj;
+  };
+
+Method 1, using a list in execbuf->buffers that's not allowed to be reordered.
+This is useful if a list of required objects is already tracked somewhere.
+Furthermore the lock helper can use propagate the -EALREADY return code back to
+the caller as a signal that an object is twice on the list. This is useful if
+the list is constructed from userspace input and the ABI requires userspace to
+not have duplicate entries (e.g. for a gpu commandbuffer submission ioctl)::
+
+  int lock_objs(struct list_head *list, struct ww_acquire_ctx *ctx)
+  {
+	struct obj *res_obj = NULL;
+	struct obj_entry *contended_entry = NULL;
+	struct obj_entry *entry;
+
+	ww_acquire_init(ctx, &ww_class);
+
+  retry:
+	list_for_each_entry (entry, list, head) {
+		if (entry->obj == res_obj) {
+			res_obj = NULL;
+			continue;
+		}
+		ret = ww_mutex_lock(&entry->obj->lock, ctx);
+		if (ret < 0) {
+			contended_entry = entry;
+			goto err;
+		}
+	}
+
+	ww_acquire_done(ctx);
+	return 0;
+
+  err:
+	list_for_each_entry_continue_reverse (entry, list, head)
+		ww_mutex_unlock(&entry->obj->lock);
+
+	if (res_obj)
+		ww_mutex_unlock(&res_obj->lock);
+
+	if (ret == -EDEADLK) {
+		/* we lost out in a seqno race, lock and retry.. */
+		ww_mutex_lock_slow(&contended_entry->obj->lock, ctx);
+		res_obj = contended_entry->obj;
+		goto retry;
+	}
+	ww_acquire_fini(ctx);
+
+	return ret;
+  }
+
+Method 2, using a list in execbuf->buffers that can be reordered. Same semantics
+of duplicate entry detection using -EALREADY as method 1 above. But the
+list-reordering allows for a bit more idiomatic code::
+
+  int lock_objs(struct list_head *list, struct ww_acquire_ctx *ctx)
+  {
+	struct obj_entry *entry, *entry2;
+
+	ww_acquire_init(ctx, &ww_class);
+
+	list_for_each_entry (entry, list, head) {
+		ret = ww_mutex_lock(&entry->obj->lock, ctx);
+		if (ret < 0) {
+			entry2 = entry;
+
+			list_for_each_entry_continue_reverse (entry2, list, head)
+				ww_mutex_unlock(&entry2->obj->lock);
+
+			if (ret != -EDEADLK) {
+				ww_acquire_fini(ctx);
+				return ret;
+			}
+
+			/* we lost out in a seqno race, lock and retry.. */
+			ww_mutex_lock_slow(&entry->obj->lock, ctx);
+
+			/*
+			 * Move buf to head of the list, this will point
+			 * buf->next to the first unlocked entry,
+			 * restarting the for loop.
+			 */
+			list_del(&entry->head);
+			list_add(&entry->head, list);
+		}
+	}
+
+	ww_acquire_done(ctx);
+	return 0;
+  }
+
+Unlocking works the same way for both methods #1 and #2::
+
+  void unlock_objs(struct list_head *list, struct ww_acquire_ctx *ctx)
+  {
+	struct obj_entry *entry;
+
+	list_for_each_entry (entry, list, head)
+		ww_mutex_unlock(&entry->obj->lock);
+
+	ww_acquire_fini(ctx);
+  }
+
+Method 3 is useful if the list of objects is constructed ad-hoc and not upfront,
+e.g. when adjusting edges in a graph where each node has its own ww_mutex lock,
+and edges can only be changed when holding the locks of all involved nodes. w/w
+mutexes are a natural fit for such a case for two reasons:
+
+- They can handle lock-acquisition in any order which allows us to start walking
+  a graph from a starting point and then iteratively discovering new edges and
+  locking down the nodes those edges connect to.
+- Due to the -EALREADY return code signalling that a given objects is already
+  held there's no need for additional book-keeping to break cycles in the graph
+  or keep track off which looks are already held (when using more than one node
+  as a starting point).
+
+Note that this approach differs in two important ways from the above methods:
+
+- Since the list of objects is dynamically constructed (and might very well be
+  different when retrying due to hitting the -EDEADLK die condition) there's
+  no need to keep any object on a persistent list when it's not locked. We can
+  therefore move the list_head into the object itself.
+- On the other hand the dynamic object list construction also means that the -EALREADY return
+  code can't be propagated.
+
+Note also that methods #1 and #2 and method #3 can be combined, e.g. to first lock a
+list of starting nodes (passed in from userspace) using one of the above
+methods. And then lock any additional objects affected by the operations using
+method #3 below. The backoff/retry procedure will be a bit more involved, since
+when the dynamic locking step hits -EDEADLK we also need to unlock all the
+objects acquired with the fixed list. But the w/w mutex debug checks will catch
+any interface misuse for these cases.
+
+Also, method 3 can't fail the lock acquisition step since it doesn't return
+-EALREADY. Of course this would be different when using the _interruptible
+variants, but that's outside of the scope of these examples here::
+
+  struct obj {
+	struct ww_mutex ww_mutex;
+	struct list_head locked_list;
+  };
+
+  static DEFINE_WW_CLASS(ww_class);
+
+  void __unlock_objs(struct list_head *list)
+  {
+	struct obj *entry, *temp;
+
+	list_for_each_entry_safe (entry, temp, list, locked_list) {
+		/* need to do that before unlocking, since only the current lock holder is
+		allowed to use object */
+		list_del(&entry->locked_list);
+		ww_mutex_unlock(entry->ww_mutex)
+	}
+  }
+
+  void lock_objs(struct list_head *list, struct ww_acquire_ctx *ctx)
+  {
+	struct obj *obj;
+
+	ww_acquire_init(ctx, &ww_class);
+
+  retry:
+	/* re-init loop start state */
+	loop {
+		/* magic code which walks over a graph and decides which objects
+		 * to lock */
+
+		ret = ww_mutex_lock(obj->ww_mutex, ctx);
+		if (ret == -EALREADY) {
+			/* we have that one already, get to the next object */
+			continue;
+		}
+		if (ret == -EDEADLK) {
+			__unlock_objs(list);
+
+			ww_mutex_lock_slow(obj, ctx);
+			list_add(&entry->locked_list, list);
+			goto retry;
+		}
+
+		/* locked a new object, add it to the list */
+		list_add_tail(&entry->locked_list, list);
+	}
+
+	ww_acquire_done(ctx);
+	return 0;
+  }
+
+  void unlock_objs(struct list_head *list, struct ww_acquire_ctx *ctx)
+  {
+	__unlock_objs(list);
+	ww_acquire_fini(ctx);
+  }
+
+Method 4: Only lock one single objects. In that case deadlock detection and
+prevention is obviously overkill, since with grabbing just one lock you can't
+produce a deadlock within just one class. To simplify this case the w/w mutex
+api can be used with a NULL context.
+
+Implementation Details
+----------------------
+
+Design:
+^^^^^^^
+
+  ww_mutex currently encapsulates a struct mutex, this means no extra overhead for
+  normal mutex locks, which are far more common. As such there is only a small
+  increase in code size if wait/wound mutexes are not used.
+
+  We maintain the following invariants for the wait list:
+
+  (1) Waiters with an acquire context are sorted by stamp order; waiters
+      without an acquire context are interspersed in FIFO order.
+  (2) For Wait-Die, among waiters with contexts, only the first one can have
+      other locks acquired already (ctx->acquired > 0). Note that this waiter
+      may come after other waiters without contexts in the list.
+
+  The Wound-Wait preemption is implemented with a lazy-preemption scheme:
+  The wounded status of the transaction is checked only when there is
+  contention for a new lock and hence a true chance of deadlock. In that
+  situation, if the transaction is wounded, it backs off, clears the
+  wounded status and retries. A great benefit of implementing preemption in
+  this way is that the wounded transaction can identify a contending lock to
+  wait for before restarting the transaction. Just blindly restarting the
+  transaction would likely make the transaction end up in a situation where
+  it would have to back off again.
+
+  In general, not much contention is expected. The locks are typically used to
+  serialize access to resources for devices, and optimization focus should
+  therefore be directed towards the uncontended cases.
+
+Lockdep:
+^^^^^^^^
+
+  Special care has been taken to warn for as many cases of api abuse
+  as possible. Some common api abuses will be caught with
+  CONFIG_DEBUG_MUTEXES, but CONFIG_PROVE_LOCKING is recommended.
+
+  Some of the errors which will be warned about:
+   - Forgetting to call ww_acquire_fini or ww_acquire_init.
+   - Attempting to lock more mutexes after ww_acquire_done.
+   - Attempting to lock the wrong mutex after -EDEADLK and
+     unlocking all mutexes.
+   - Attempting to lock the right mutex after -EDEADLK,
+     before unlocking all mutexes.
+
+   - Calling ww_mutex_lock_slow before -EDEADLK was returned.
+
+   - Unlocking mutexes with the wrong unlock function.
+   - Calling one of the ww_acquire_* twice on the same context.
+   - Using a different ww_class for the mutex than for the ww_acquire_ctx.
+   - Normal lockdep errors that can result in deadlocks.
+
+  Some of the lockdep errors that can result in deadlocks:
+   - Calling ww_acquire_init to initialize a second ww_acquire_ctx before
+     having called ww_acquire_fini on the first.
+   - 'normal' deadlocks that can occur.
+
+FIXME:
+  Update this section once we have the TASK_DEADLOCK task state flag magic
+  implemented.