1 files changed, 427 insertions, 0 deletions
diff --git a/debian/patches-rt/0144-rtmutex-add-rwsem-implementation-based-on-rtmutex.patch b/debian/patches-rt/0144-rtmutex-add-rwsem-implementation-based-on-rtmutex.patch
new file mode 100644
index 000000000..b05b5ef45
--- /dev/null
+++ b/debian/patches-rt/0144-rtmutex-add-rwsem-implementation-based-on-rtmutex.patch
@@ -0,0 +1,427 @@
+From d5648fc940b9d669cad909970b95064ee3634612 Mon Sep 17 00:00:00 2001
+From: Thomas Gleixner <tglx@linutronix.de>
+Date: Thu, 12 Oct 2017 17:28:34 +0200
+Subject: [PATCH 144/347] rtmutex: add rwsem implementation based on rtmutex
+Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/4.19/older/patches-4.19.246-rt110.tar.xz
+
+The RT specific R/W semaphore implementation restricts the number of readers
+to one because a writer cannot block on multiple readers and inherit its
+priority or budget.
+
+The single reader restricting is painful in various ways:
+
+ - Performance bottleneck for multi-threaded applications in the page fault
+   path (mmap sem)
+
+ - Progress blocker for drivers which are carefully crafted to avoid the
+   potential reader/writer deadlock in mainline.
+
+The analysis of the writer code pathes shows, that properly written RT tasks
+should not take them. Syscalls like mmap(), file access which take mmap sem
+write locked have unbound latencies which are completely unrelated to mmap
+sem. Other R/W sem users like graphics drivers are not suitable for RT tasks
+either.
+
+So there is little risk to hurt RT tasks when the RT rwsem implementation is
+changed in the following way:
+
+ - Allow concurrent readers
+
+ - Make writers block until the last reader left the critical section. This
+   blocking is not subject to priority/budget inheritance.
+
+ - Readers blocked on a writer inherit their priority/budget in the normal
+   way.
+
+There is a drawback with this scheme. R/W semaphores become writer unfair
+though the applications which have triggered writer starvation (mostly on
+mmap_sem) in the past are not really the typical workloads running on a RT
+system. So while it's unlikely to hit writer starvation, it's possible. If
+there are unexpected workloads on RT systems triggering it, we need to rethink
+the approach.
+
+Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
+Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
+---
+ include/linux/rwsem_rt.h  |  68 +++++++++
+ kernel/locking/rwsem-rt.c | 293 ++++++++++++++++++++++++++++++++++++++
+ 2 files changed, 361 insertions(+)
+ create mode 100644 include/linux/rwsem_rt.h
+ create mode 100644 kernel/locking/rwsem-rt.c
+
+diff --git a/include/linux/rwsem_rt.h b/include/linux/rwsem_rt.h
+new file mode 100644
+index 000000000000..2018ff77904a
+--- /dev/null
++++ b/include/linux/rwsem_rt.h
+@@ -0,0 +1,68 @@
++#ifndef _LINUX_RWSEM_RT_H
++#define _LINUX_RWSEM_RT_H
++
++#ifndef _LINUX_RWSEM_H
++#error "Include rwsem.h"
++#endif
++
++#include <linux/rtmutex.h>
++#include <linux/swait.h>
++
++#define READER_BIAS		(1U << 31)
++#define WRITER_BIAS		(1U << 30)
++
++struct rw_semaphore {
++	atomic_t		readers;
++	struct rt_mutex		rtmutex;
++#ifdef CONFIG_DEBUG_LOCK_ALLOC
++	struct lockdep_map	dep_map;
++#endif
++};
++
++#define __RWSEM_INITIALIZER(name)				\
++{								\
++	.readers = ATOMIC_INIT(READER_BIAS),			\
++	.rtmutex = __RT_MUTEX_INITIALIZER(name.rtmutex),	\
++	RW_DEP_MAP_INIT(name)					\
++}
++
++#define DECLARE_RWSEM(lockname) \
++	struct rw_semaphore lockname = __RWSEM_INITIALIZER(lockname)
++
++extern void  __rwsem_init(struct rw_semaphore *rwsem, const char *name,
++			  struct lock_class_key *key);
++
++#define __init_rwsem(sem, name, key)			\
++do {							\
++		rt_mutex_init(&(sem)->rtmutex);		\
++		__rwsem_init((sem), (name), (key));	\
++} while (0)
++
++#define init_rwsem(sem)					\
++do {							\
++	static struct lock_class_key __key;		\
++							\
++	__init_rwsem((sem), #sem, &__key);		\
++} while (0)
++
++static inline int rwsem_is_locked(struct rw_semaphore *sem)
++{
++	return atomic_read(&sem->readers) != READER_BIAS;
++}
++
++static inline int rwsem_is_contended(struct rw_semaphore *sem)
++{
++	return atomic_read(&sem->readers) > 0;
++}
++
++extern void __down_read(struct rw_semaphore *sem);
++extern int __down_read_killable(struct rw_semaphore *sem);
++extern int __down_read_trylock(struct rw_semaphore *sem);
++extern void __down_write(struct rw_semaphore *sem);
++extern int __must_check __down_write_killable(struct rw_semaphore *sem);
++extern int __down_write_trylock(struct rw_semaphore *sem);
++extern void __up_read(struct rw_semaphore *sem);
++extern void __up_write(struct rw_semaphore *sem);
++extern void __downgrade_write(struct rw_semaphore *sem);
++
++#endif
+diff --git a/kernel/locking/rwsem-rt.c b/kernel/locking/rwsem-rt.c
+new file mode 100644
+index 000000000000..7d3c5cf3d23d
+--- /dev/null
++++ b/kernel/locking/rwsem-rt.c
+@@ -0,0 +1,293 @@
++/*
++ */
++#include <linux/rwsem.h>
++#include <linux/sched/debug.h>
++#include <linux/sched/signal.h>
++#include <linux/export.h>
++
++#include "rtmutex_common.h"
++
++/*
++ * RT-specific reader/writer semaphores
++ *
++ * down_write()
++ *  1) Lock sem->rtmutex
++ *  2) Remove the reader BIAS to force readers into the slow path
++ *  3) Wait until all readers have left the critical region
++ *  4) Mark it write locked
++ *
++ * up_write()
++ *  1) Remove the write locked marker
++ *  2) Set the reader BIAS so readers can use the fast path again
++ *  3) Unlock sem->rtmutex to release blocked readers
++ *
++ * down_read()
++ *  1) Try fast path acquisition (reader BIAS is set)
++ *  2) Take sem->rtmutex.wait_lock which protects the writelocked flag
++ *  3) If !writelocked, acquire it for read
++ *  4) If writelocked, block on sem->rtmutex
++ *  5) unlock sem->rtmutex, goto 1)
++ *
++ * up_read()
++ *  1) Try fast path release (reader count != 1)
++ *  2) Wake the writer waiting in down_write()#3
++ *
++ * down_read()#3 has the consequence, that rw semaphores on RT are not writer
++ * fair, but writers, which should be avoided in RT tasks (think mmap_sem),
++ * are subject to the rtmutex priority/DL inheritance mechanism.
++ *
++ * It's possible to make the rw semaphores writer fair by keeping a list of
++ * active readers. A blocked writer would force all newly incoming readers to
++ * block on the rtmutex, but the rtmutex would have to be proxy locked for one
++ * reader after the other. We can't use multi-reader inheritance because there
++ * is no way to support that with SCHED_DEADLINE. Implementing the one by one
++ * reader boosting/handover mechanism is a major surgery for a very dubious
++ * value.
++ *
++ * The risk of writer starvation is there, but the pathological use cases
++ * which trigger it are not necessarily the typical RT workloads.
++ */
++
++void __rwsem_init(struct rw_semaphore *sem, const char *name,
++		  struct lock_class_key *key)
++{
++#ifdef CONFIG_DEBUG_LOCK_ALLOC
++	/*
++	 * Make sure we are not reinitializing a held semaphore:
++	 */
++	debug_check_no_locks_freed((void *)sem, sizeof(*sem));
++	lockdep_init_map(&sem->dep_map, name, key, 0);
++#endif
++	atomic_set(&sem->readers, READER_BIAS);
++}
++EXPORT_SYMBOL(__rwsem_init);
++
++int __down_read_trylock(struct rw_semaphore *sem)
++{
++	int r, old;
++
++	/*
++	 * Increment reader count, if sem->readers < 0, i.e. READER_BIAS is
++	 * set.
++	 */
++	for (r = atomic_read(&sem->readers); r < 0;) {
++		old = atomic_cmpxchg(&sem->readers, r, r + 1);
++		if (likely(old == r))
++			return 1;
++		r = old;
++	}
++	return 0;
++}
++
++static int __sched __down_read_common(struct rw_semaphore *sem, int state)
++{
++	struct rt_mutex *m = &sem->rtmutex;
++	struct rt_mutex_waiter waiter;
++	int ret;
++
++	if (__down_read_trylock(sem))
++		return 0;
++
++	might_sleep();
++	raw_spin_lock_irq(&m->wait_lock);
++	/*
++	 * Allow readers as long as the writer has not completely
++	 * acquired the semaphore for write.
++	 */
++	if (atomic_read(&sem->readers) != WRITER_BIAS) {
++		atomic_inc(&sem->readers);
++		raw_spin_unlock_irq(&m->wait_lock);
++		return 0;
++	}
++
++	/*
++	 * Call into the slow lock path with the rtmutex->wait_lock
++	 * held, so this can't result in the following race:
++	 *
++	 * Reader1		Reader2		Writer
++	 *			down_read()
++	 *					down_write()
++	 *					rtmutex_lock(m)
++	 *					swait()
++	 * down_read()
++	 * unlock(m->wait_lock)
++	 *			up_read()
++	 *			swake()
++	 *					lock(m->wait_lock)
++	 *					sem->writelocked=true
++	 *					unlock(m->wait_lock)
++	 *
++	 *					up_write()
++	 *					sem->writelocked=false
++	 *					rtmutex_unlock(m)
++	 *			down_read()
++	 *					down_write()
++	 *					rtmutex_lock(m)
++	 *					swait()
++	 * rtmutex_lock(m)
++	 *
++	 * That would put Reader1 behind the writer waiting on
++	 * Reader2 to call up_read() which might be unbound.
++	 */
++	rt_mutex_init_waiter(&waiter, false);
++	ret = rt_mutex_slowlock_locked(m, state, NULL, RT_MUTEX_MIN_CHAINWALK,
++				       &waiter);
++	/*
++	 * The slowlock() above is guaranteed to return with the rtmutex (for
++	 * ret = 0) is now held, so there can't be a writer active. Increment
++	 * the reader count and immediately drop the rtmutex again.
++	 * For ret != 0 we don't hold the rtmutex and need unlock the wait_lock.
++	 * We don't own the lock then.
++	 */
++	if (!ret)
++		atomic_inc(&sem->readers);
++	raw_spin_unlock_irq(&m->wait_lock);
++	if (!ret)
++		__rt_mutex_unlock(m);
++
++	debug_rt_mutex_free_waiter(&waiter);
++	return ret;
++}
++
++void __down_read(struct rw_semaphore *sem)
++{
++	int ret;
++
++	ret = __down_read_common(sem, TASK_UNINTERRUPTIBLE);
++	WARN_ON_ONCE(ret);
++}
++
++int __down_read_killable(struct rw_semaphore *sem)
++{
++	int ret;
++
++	ret = __down_read_common(sem, TASK_KILLABLE);
++	if (likely(!ret))
++		return ret;
++	WARN_ONCE(ret != -EINTR, "Unexpected state: %d\n", ret);
++	return -EINTR;
++}
++
++void __up_read(struct rw_semaphore *sem)
++{
++	struct rt_mutex *m = &sem->rtmutex;
++	struct task_struct *tsk;
++
++	/*
++	 * sem->readers can only hit 0 when a writer is waiting for the
++	 * active readers to leave the critical region.
++	 */
++	if (!atomic_dec_and_test(&sem->readers))
++		return;
++
++	might_sleep();
++	raw_spin_lock_irq(&m->wait_lock);
++	/*
++	 * Wake the writer, i.e. the rtmutex owner. It might release the
++	 * rtmutex concurrently in the fast path (due to a signal), but to
++	 * clean up the rwsem it needs to acquire m->wait_lock. The worst
++	 * case which can happen is a spurious wakeup.
++	 */
++	tsk = rt_mutex_owner(m);
++	if (tsk)
++		wake_up_process(tsk);
++
++	raw_spin_unlock_irq(&m->wait_lock);
++}
++
++static void __up_write_unlock(struct rw_semaphore *sem, int bias,
++			      unsigned long flags)
++{
++	struct rt_mutex *m = &sem->rtmutex;
++
++	atomic_add(READER_BIAS - bias, &sem->readers);
++	raw_spin_unlock_irqrestore(&m->wait_lock, flags);
++	__rt_mutex_unlock(m);
++}
++
++static int __sched __down_write_common(struct rw_semaphore *sem, int state)
++{
++	struct rt_mutex *m = &sem->rtmutex;
++	unsigned long flags;
++
++	/* Take the rtmutex as a first step */
++	if (__rt_mutex_lock_state(m, state))
++		return -EINTR;
++
++	/* Force readers into slow path */
++	atomic_sub(READER_BIAS, &sem->readers);
++	might_sleep();
++
++	set_current_state(state);
++	for (;;) {
++		raw_spin_lock_irqsave(&m->wait_lock, flags);
++		/* Have all readers left the critical region? */
++		if (!atomic_read(&sem->readers)) {
++			atomic_set(&sem->readers, WRITER_BIAS);
++			__set_current_state(TASK_RUNNING);
++			raw_spin_unlock_irqrestore(&m->wait_lock, flags);
++			return 0;
++		}
++
++		if (signal_pending_state(state, current)) {
++			__set_current_state(TASK_RUNNING);
++			__up_write_unlock(sem, 0, flags);
++			return -EINTR;
++		}
++		raw_spin_unlock_irqrestore(&m->wait_lock, flags);
++
++		if (atomic_read(&sem->readers) != 0) {
++			schedule();
++			set_current_state(state);
++		}
++	}
++}
++
++void __sched __down_write(struct rw_semaphore *sem)
++{
++	__down_write_common(sem, TASK_UNINTERRUPTIBLE);
++}
++
++int __sched __down_write_killable(struct rw_semaphore *sem)
++{
++	return __down_write_common(sem, TASK_KILLABLE);
++}
++
++int __down_write_trylock(struct rw_semaphore *sem)
++{
++	struct rt_mutex *m = &sem->rtmutex;
++	unsigned long flags;
++
++	if (!__rt_mutex_trylock(m))
++		return 0;
++
++	atomic_sub(READER_BIAS, &sem->readers);
++
++	raw_spin_lock_irqsave(&m->wait_lock, flags);
++	if (!atomic_read(&sem->readers)) {
++		atomic_set(&sem->readers, WRITER_BIAS);
++		raw_spin_unlock_irqrestore(&m->wait_lock, flags);
++		return 1;
++	}
++	__up_write_unlock(sem, 0, flags);
++	return 0;
++}
++
++void __up_write(struct rw_semaphore *sem)
++{
++	struct rt_mutex *m = &sem->rtmutex;
++	unsigned long flags;
++
++	raw_spin_lock_irqsave(&m->wait_lock, flags);
++	__up_write_unlock(sem, WRITER_BIAS, flags);
++}
++
++void __downgrade_write(struct rw_semaphore *sem)
++{
++	struct rt_mutex *m = &sem->rtmutex;
++	unsigned long flags;
++
++	raw_spin_lock_irqsave(&m->wait_lock, flags);
++	/* Release it and account current as reader */
++	__up_write_unlock(sem, WRITER_BIAS - 1, flags);
++}
+-- 
+2.36.1
+