Adding upstream version 6.6.15.upstream/6.6.15

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

1 files changed, 1233 insertions, 0 deletions

diff --git a/kernel/futex/pi.c b/kernel/futex/pi.c
new file mode 100644
index 0000000000..ce2889f123
--- /dev/null
+++ b/kernel/futex/pi.c
@@ -0,0 +1,1233 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include <linux/slab.h>
+#include <linux/sched/task.h>
+
+#include "futex.h"
+#include "../locking/rtmutex_common.h"
+
+/*
+ * PI code:
+ */
+int refill_pi_state_cache(void)
+{
+	struct futex_pi_state *pi_state;
+
+	if (likely(current->pi_state_cache))
+		return 0;
+
+	pi_state = kzalloc(sizeof(*pi_state), GFP_KERNEL);
+
+	if (!pi_state)
+		return -ENOMEM;
+
+	INIT_LIST_HEAD(&pi_state->list);
+	/* pi_mutex gets initialized later */
+	pi_state->owner = NULL;
+	refcount_set(&pi_state->refcount, 1);
+	pi_state->key = FUTEX_KEY_INIT;
+
+	current->pi_state_cache = pi_state;
+
+	return 0;
+}
+
+static struct futex_pi_state *alloc_pi_state(void)
+{
+	struct futex_pi_state *pi_state = current->pi_state_cache;
+
+	WARN_ON(!pi_state);
+	current->pi_state_cache = NULL;
+
+	return pi_state;
+}
+
+static void pi_state_update_owner(struct futex_pi_state *pi_state,
+				  struct task_struct *new_owner)
+{
+	struct task_struct *old_owner = pi_state->owner;
+
+	lockdep_assert_held(&pi_state->pi_mutex.wait_lock);
+
+	if (old_owner) {
+		raw_spin_lock(&old_owner->pi_lock);
+		WARN_ON(list_empty(&pi_state->list));
+		list_del_init(&pi_state->list);
+		raw_spin_unlock(&old_owner->pi_lock);
+	}
+
+	if (new_owner) {
+		raw_spin_lock(&new_owner->pi_lock);
+		WARN_ON(!list_empty(&pi_state->list));
+		list_add(&pi_state->list, &new_owner->pi_state_list);
+		pi_state->owner = new_owner;
+		raw_spin_unlock(&new_owner->pi_lock);
+	}
+}
+
+void get_pi_state(struct futex_pi_state *pi_state)
+{
+	WARN_ON_ONCE(!refcount_inc_not_zero(&pi_state->refcount));
+}
+
+/*
+ * Drops a reference to the pi_state object and frees or caches it
+ * when the last reference is gone.
+ */
+void put_pi_state(struct futex_pi_state *pi_state)
+{
+	if (!pi_state)
+		return;
+
+	if (!refcount_dec_and_test(&pi_state->refcount))
+		return;
+
+	/*
+	 * If pi_state->owner is NULL, the owner is most probably dying
+	 * and has cleaned up the pi_state already
+	 */
+	if (pi_state->owner) {
+		unsigned long flags;
+
+		raw_spin_lock_irqsave(&pi_state->pi_mutex.wait_lock, flags);
+		pi_state_update_owner(pi_state, NULL);
+		rt_mutex_proxy_unlock(&pi_state->pi_mutex);
+		raw_spin_unlock_irqrestore(&pi_state->pi_mutex.wait_lock, flags);
+	}
+
+	if (current->pi_state_cache) {
+		kfree(pi_state);
+	} else {
+		/*
+		 * pi_state->list is already empty.
+		 * clear pi_state->owner.
+		 * refcount is at 0 - put it back to 1.
+		 */
+		pi_state->owner = NULL;
+		refcount_set(&pi_state->refcount, 1);
+		current->pi_state_cache = pi_state;
+	}
+}
+
+/*
+ * We need to check the following states:
+ *
+ *      Waiter | pi_state | pi->owner | uTID      | uODIED | ?
+ *
+ * [1]  NULL   | ---      | ---       | 0         | 0/1    | Valid
+ * [2]  NULL   | ---      | ---       | >0        | 0/1    | Valid
+ *
+ * [3]  Found  | NULL     | --        | Any       | 0/1    | Invalid
+ *
+ * [4]  Found  | Found    | NULL      | 0         | 1      | Valid
+ * [5]  Found  | Found    | NULL      | >0        | 1      | Invalid
+ *
+ * [6]  Found  | Found    | task      | 0         | 1      | Valid
+ *
+ * [7]  Found  | Found    | NULL      | Any       | 0      | Invalid
+ *
+ * [8]  Found  | Found    | task      | ==taskTID | 0/1    | Valid
+ * [9]  Found  | Found    | task      | 0         | 0      | Invalid
+ * [10] Found  | Found    | task      | !=taskTID | 0/1    | Invalid
+ *
+ * [1]	Indicates that the kernel can acquire the futex atomically. We
+ *	came here due to a stale FUTEX_WAITERS/FUTEX_OWNER_DIED bit.
+ *
+ * [2]	Valid, if TID does not belong to a kernel thread. If no matching
+ *      thread is found then it indicates that the owner TID has died.
+ *
+ * [3]	Invalid. The waiter is queued on a non PI futex
+ *
+ * [4]	Valid state after exit_robust_list(), which sets the user space
+ *	value to FUTEX_WAITERS | FUTEX_OWNER_DIED.
+ *
+ * [5]	The user space value got manipulated between exit_robust_list()
+ *	and exit_pi_state_list()
+ *
+ * [6]	Valid state after exit_pi_state_list() which sets the new owner in
+ *	the pi_state but cannot access the user space value.
+ *
+ * [7]	pi_state->owner can only be NULL when the OWNER_DIED bit is set.
+ *
+ * [8]	Owner and user space value match
+ *
+ * [9]	There is no transient state which sets the user space TID to 0
+ *	except exit_robust_list(), but this is indicated by the
+ *	FUTEX_OWNER_DIED bit. See [4]
+ *
+ * [10] There is no transient state which leaves owner and user space
+ *	TID out of sync. Except one error case where the kernel is denied
+ *	write access to the user address, see fixup_pi_state_owner().
+ *
+ *
+ * Serialization and lifetime rules:
+ *
+ * hb->lock:
+ *
+ *	hb -> futex_q, relation
+ *	futex_q -> pi_state, relation
+ *
+ *	(cannot be raw because hb can contain arbitrary amount
+ *	 of futex_q's)
+ *
+ * pi_mutex->wait_lock:
+ *
+ *	{uval, pi_state}
+ *
+ *	(and pi_mutex 'obviously')
+ *
+ * p->pi_lock:
+ *
+ *	p->pi_state_list -> pi_state->list, relation
+ *	pi_mutex->owner -> pi_state->owner, relation
+ *
+ * pi_state->refcount:
+ *
+ *	pi_state lifetime
+ *
+ *
+ * Lock order:
+ *
+ *   hb->lock
+ *     pi_mutex->wait_lock
+ *       p->pi_lock
+ *
+ */
+
+/*
+ * Validate that the existing waiter has a pi_state and sanity check
+ * the pi_state against the user space value. If correct, attach to
+ * it.
+ */
+static int attach_to_pi_state(u32 __user *uaddr, u32 uval,
+			      struct futex_pi_state *pi_state,
+			      struct futex_pi_state **ps)
+{
+	pid_t pid = uval & FUTEX_TID_MASK;
+	u32 uval2;
+	int ret;
+
+	/*
+	 * Userspace might have messed up non-PI and PI futexes [3]
+	 */
+	if (unlikely(!pi_state))
+		return -EINVAL;
+
+	/*
+	 * We get here with hb->lock held, and having found a
+	 * futex_top_waiter(). This means that futex_lock_pi() of said futex_q
+	 * has dropped the hb->lock in between futex_queue() and futex_unqueue_pi(),
+	 * which in turn means that futex_lock_pi() still has a reference on
+	 * our pi_state.
+	 *
+	 * The waiter holding a reference on @pi_state also protects against
+	 * the unlocked put_pi_state() in futex_unlock_pi(), futex_lock_pi()
+	 * and futex_wait_requeue_pi() as it cannot go to 0 and consequently
+	 * free pi_state before we can take a reference ourselves.
+	 */
+	WARN_ON(!refcount_read(&pi_state->refcount));
+
+	/*
+	 * Now that we have a pi_state, we can acquire wait_lock
+	 * and do the state validation.
+	 */
+	raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
+
+	/*
+	 * Since {uval, pi_state} is serialized by wait_lock, and our current
+	 * uval was read without holding it, it can have changed. Verify it
+	 * still is what we expect it to be, otherwise retry the entire
+	 * operation.
+	 */
+	if (futex_get_value_locked(&uval2, uaddr))
+		goto out_efault;
+
+	if (uval != uval2)
+		goto out_eagain;
+
+	/*
+	 * Handle the owner died case:
+	 */
+	if (uval & FUTEX_OWNER_DIED) {
+		/*
+		 * exit_pi_state_list sets owner to NULL and wakes the
+		 * topmost waiter. The task which acquires the
+		 * pi_state->rt_mutex will fixup owner.
+		 */
+		if (!pi_state->owner) {
+			/*
+			 * No pi state owner, but the user space TID
+			 * is not 0. Inconsistent state. [5]
+			 */
+			if (pid)
+				goto out_einval;
+			/*
+			 * Take a ref on the state and return success. [4]
+			 */
+			goto out_attach;
+		}
+
+		/*
+		 * If TID is 0, then either the dying owner has not
+		 * yet executed exit_pi_state_list() or some waiter
+		 * acquired the rtmutex in the pi state, but did not
+		 * yet fixup the TID in user space.
+		 *
+		 * Take a ref on the state and return success. [6]
+		 */
+		if (!pid)
+			goto out_attach;
+	} else {
+		/*
+		 * If the owner died bit is not set, then the pi_state
+		 * must have an owner. [7]
+		 */
+		if (!pi_state->owner)
+			goto out_einval;
+	}
+
+	/*
+	 * Bail out if user space manipulated the futex value. If pi
+	 * state exists then the owner TID must be the same as the
+	 * user space TID. [9/10]
+	 */
+	if (pid != task_pid_vnr(pi_state->owner))
+		goto out_einval;
+
+out_attach:
+	get_pi_state(pi_state);
+	raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
+	*ps = pi_state;
+	return 0;
+
+out_einval:
+	ret = -EINVAL;
+	goto out_error;
+
+out_eagain:
+	ret = -EAGAIN;
+	goto out_error;
+
+out_efault:
+	ret = -EFAULT;
+	goto out_error;
+
+out_error:
+	raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
+	return ret;
+}
+
+static int handle_exit_race(u32 __user *uaddr, u32 uval,
+			    struct task_struct *tsk)
+{
+	u32 uval2;
+
+	/*
+	 * If the futex exit state is not yet FUTEX_STATE_DEAD, tell the
+	 * caller that the alleged owner is busy.
+	 */
+	if (tsk && tsk->futex_state != FUTEX_STATE_DEAD)
+		return -EBUSY;
+
+	/*
+	 * Reread the user space value to handle the following situation:
+	 *
+	 * CPU0				CPU1
+	 *
+	 * sys_exit()			sys_futex()
+	 *  do_exit()			 futex_lock_pi()
+	 *                                futex_lock_pi_atomic()
+	 *   exit_signals(tsk)		    No waiters:
+	 *    tsk->flags |= PF_EXITING;	    *uaddr == 0x00000PID
+	 *  mm_release(tsk)		    Set waiter bit
+	 *   exit_robust_list(tsk) {	    *uaddr = 0x80000PID;
+	 *      Set owner died		    attach_to_pi_owner() {
+	 *    *uaddr = 0xC0000000;	     tsk = get_task(PID);
+	 *   }				     if (!tsk->flags & PF_EXITING) {
+	 *  ...				       attach();
+	 *  tsk->futex_state =               } else {
+	 *	FUTEX_STATE_DEAD;              if (tsk->futex_state !=
+	 *					  FUTEX_STATE_DEAD)
+	 *				         return -EAGAIN;
+	 *				       return -ESRCH; <--- FAIL
+	 *				     }
+	 *
+	 * Returning ESRCH unconditionally is wrong here because the
+	 * user space value has been changed by the exiting task.
+	 *
+	 * The same logic applies to the case where the exiting task is
+	 * already gone.
+	 */
+	if (futex_get_value_locked(&uval2, uaddr))
+		return -EFAULT;
+
+	/* If the user space value has changed, try again. */
+	if (uval2 != uval)
+		return -EAGAIN;
+
+	/*
+	 * The exiting task did not have a robust list, the robust list was
+	 * corrupted or the user space value in *uaddr is simply bogus.
+	 * Give up and tell user space.
+	 */
+	return -ESRCH;
+}
+
+static void __attach_to_pi_owner(struct task_struct *p, union futex_key *key,
+				 struct futex_pi_state **ps)
+{
+	/*
+	 * No existing pi state. First waiter. [2]
+	 *
+	 * This creates pi_state, we have hb->lock held, this means nothing can
+	 * observe this state, wait_lock is irrelevant.
+	 */
+	struct futex_pi_state *pi_state = alloc_pi_state();
+
+	/*
+	 * Initialize the pi_mutex in locked state and make @p
+	 * the owner of it:
+	 */
+	rt_mutex_init_proxy_locked(&pi_state->pi_mutex, p);
+
+	/* Store the key for possible exit cleanups: */
+	pi_state->key = *key;
+
+	WARN_ON(!list_empty(&pi_state->list));
+	list_add(&pi_state->list, &p->pi_state_list);
+	/*
+	 * Assignment without holding pi_state->pi_mutex.wait_lock is safe
+	 * because there is no concurrency as the object is not published yet.
+	 */
+	pi_state->owner = p;
+
+	*ps = pi_state;
+}
+/*
+ * Lookup the task for the TID provided from user space and attach to
+ * it after doing proper sanity checks.
+ */
+static int attach_to_pi_owner(u32 __user *uaddr, u32 uval, union futex_key *key,
+			      struct futex_pi_state **ps,
+			      struct task_struct **exiting)
+{
+	pid_t pid = uval & FUTEX_TID_MASK;
+	struct task_struct *p;
+
+	/*
+	 * We are the first waiter - try to look up the real owner and attach
+	 * the new pi_state to it, but bail out when TID = 0 [1]
+	 *
+	 * The !pid check is paranoid. None of the call sites should end up
+	 * with pid == 0, but better safe than sorry. Let the caller retry
+	 */
+	if (!pid)
+		return -EAGAIN;
+	p = find_get_task_by_vpid(pid);
+	if (!p)
+		return handle_exit_race(uaddr, uval, NULL);
+
+	if (unlikely(p->flags & PF_KTHREAD)) {
+		put_task_struct(p);
+		return -EPERM;
+	}
+
+	/*
+	 * We need to look at the task state to figure out, whether the
+	 * task is exiting. To protect against the change of the task state
+	 * in futex_exit_release(), we do this protected by p->pi_lock:
+	 */
+	raw_spin_lock_irq(&p->pi_lock);
+	if (unlikely(p->futex_state != FUTEX_STATE_OK)) {
+		/*
+		 * The task is on the way out. When the futex state is
+		 * FUTEX_STATE_DEAD, we know that the task has finished
+		 * the cleanup:
+		 */
+		int ret = handle_exit_race(uaddr, uval, p);
+
+		raw_spin_unlock_irq(&p->pi_lock);
+		/*
+		 * If the owner task is between FUTEX_STATE_EXITING and
+		 * FUTEX_STATE_DEAD then store the task pointer and keep
+		 * the reference on the task struct. The calling code will
+		 * drop all locks, wait for the task to reach
+		 * FUTEX_STATE_DEAD and then drop the refcount. This is
+		 * required to prevent a live lock when the current task
+		 * preempted the exiting task between the two states.
+		 */
+		if (ret == -EBUSY)
+			*exiting = p;
+		else
+			put_task_struct(p);
+		return ret;
+	}
+
+	__attach_to_pi_owner(p, key, ps);
+	raw_spin_unlock_irq(&p->pi_lock);
+
+	put_task_struct(p);
+
+	return 0;
+}
+
+static int lock_pi_update_atomic(u32 __user *uaddr, u32 uval, u32 newval)
+{
+	int err;
+	u32 curval;
+
+	if (unlikely(should_fail_futex(true)))
+		return -EFAULT;
+
+	err = futex_cmpxchg_value_locked(&curval, uaddr, uval, newval);
+	if (unlikely(err))
+		return err;
+
+	/* If user space value changed, let the caller retry */
+	return curval != uval ? -EAGAIN : 0;
+}
+
+/**
+ * futex_lock_pi_atomic() - Atomic work required to acquire a pi aware futex
+ * @uaddr:		the pi futex user address
+ * @hb:			the pi futex hash bucket
+ * @key:		the futex key associated with uaddr and hb
+ * @ps:			the pi_state pointer where we store the result of the
+ *			lookup
+ * @task:		the task to perform the atomic lock work for.  This will
+ *			be "current" except in the case of requeue pi.
+ * @exiting:		Pointer to store the task pointer of the owner task
+ *			which is in the middle of exiting
+ * @set_waiters:	force setting the FUTEX_WAITERS bit (1) or not (0)
+ *
+ * Return:
+ *  -  0 - ready to wait;
+ *  -  1 - acquired the lock;
+ *  - <0 - error
+ *
+ * The hb->lock must be held by the caller.
+ *
+ * @exiting is only set when the return value is -EBUSY. If so, this holds
+ * a refcount on the exiting task on return and the caller needs to drop it
+ * after waiting for the exit to complete.
+ */
+int futex_lock_pi_atomic(u32 __user *uaddr, struct futex_hash_bucket *hb,
+			 union futex_key *key,
+			 struct futex_pi_state **ps,
+			 struct task_struct *task,
+			 struct task_struct **exiting,
+			 int set_waiters)
+{
+	u32 uval, newval, vpid = task_pid_vnr(task);
+	struct futex_q *top_waiter;
+	int ret;
+
+	/*
+	 * Read the user space value first so we can validate a few
+	 * things before proceeding further.
+	 */
+	if (futex_get_value_locked(&uval, uaddr))
+		return -EFAULT;
+
+	if (unlikely(should_fail_futex(true)))
+		return -EFAULT;
+
+	/*
+	 * Detect deadlocks.
+	 */
+	if ((unlikely((uval & FUTEX_TID_MASK) == vpid)))
+		return -EDEADLK;
+
+	if ((unlikely(should_fail_futex(true))))
+		return -EDEADLK;
+
+	/*
+	 * Lookup existing state first. If it exists, try to attach to
+	 * its pi_state.
+	 */
+	top_waiter = futex_top_waiter(hb, key);
+	if (top_waiter)
+		return attach_to_pi_state(uaddr, uval, top_waiter->pi_state, ps);
+
+	/*
+	 * No waiter and user TID is 0. We are here because the
+	 * waiters or the owner died bit is set or called from
+	 * requeue_cmp_pi or for whatever reason something took the
+	 * syscall.
+	 */
+	if (!(uval & FUTEX_TID_MASK)) {
+		/*
+		 * We take over the futex. No other waiters and the user space
+		 * TID is 0. We preserve the owner died bit.
+		 */
+		newval = uval & FUTEX_OWNER_DIED;
+		newval |= vpid;
+
+		/* The futex requeue_pi code can enforce the waiters bit */
+		if (set_waiters)
+			newval |= FUTEX_WAITERS;
+
+		ret = lock_pi_update_atomic(uaddr, uval, newval);
+		if (ret)
+			return ret;
+
+		/*
+		 * If the waiter bit was requested the caller also needs PI
+		 * state attached to the new owner of the user space futex.
+		 *
+		 * @task is guaranteed to be alive and it cannot be exiting
+		 * because it is either sleeping or waiting in
+		 * futex_requeue_pi_wakeup_sync().
+		 *
+		 * No need to do the full attach_to_pi_owner() exercise
+		 * because @task is known and valid.
+		 */
+		if (set_waiters) {
+			raw_spin_lock_irq(&task->pi_lock);
+			__attach_to_pi_owner(task, key, ps);
+			raw_spin_unlock_irq(&task->pi_lock);
+		}
+		return 1;
+	}
+
+	/*
+	 * First waiter. Set the waiters bit before attaching ourself to
+	 * the owner. If owner tries to unlock, it will be forced into
+	 * the kernel and blocked on hb->lock.
+	 */
+	newval = uval | FUTEX_WAITERS;
+	ret = lock_pi_update_atomic(uaddr, uval, newval);
+	if (ret)
+		return ret;
+	/*
+	 * If the update of the user space value succeeded, we try to
+	 * attach to the owner. If that fails, no harm done, we only
+	 * set the FUTEX_WAITERS bit in the user space variable.
+	 */
+	return attach_to_pi_owner(uaddr, newval, key, ps, exiting);
+}
+
+/*
+ * Caller must hold a reference on @pi_state.
+ */
+static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_pi_state *pi_state)
+{
+	struct rt_mutex_waiter *top_waiter;
+	struct task_struct *new_owner;
+	bool postunlock = false;
+	DEFINE_RT_WAKE_Q(wqh);
+	u32 curval, newval;
+	int ret = 0;
+
+	top_waiter = rt_mutex_top_waiter(&pi_state->pi_mutex);
+	if (WARN_ON_ONCE(!top_waiter)) {
+		/*
+		 * As per the comment in futex_unlock_pi() this should not happen.
+		 *
+		 * When this happens, give up our locks and try again, giving
+		 * the futex_lock_pi() instance time to complete, either by
+		 * waiting on the rtmutex or removing itself from the futex
+		 * queue.
+		 */
+		ret = -EAGAIN;
+		goto out_unlock;
+	}
+
+	new_owner = top_waiter->task;
+
+	/*
+	 * We pass it to the next owner. The WAITERS bit is always kept
+	 * enabled while there is PI state around. We cleanup the owner
+	 * died bit, because we are the owner.
+	 */
+	newval = FUTEX_WAITERS | task_pid_vnr(new_owner);
+
+	if (unlikely(should_fail_futex(true))) {
+		ret = -EFAULT;
+		goto out_unlock;
+	}
+
+	ret = futex_cmpxchg_value_locked(&curval, uaddr, uval, newval);
+	if (!ret && (curval != uval)) {
+		/*
+		 * If a unconditional UNLOCK_PI operation (user space did not
+		 * try the TID->0 transition) raced with a waiter setting the
+		 * FUTEX_WAITERS flag between get_user() and locking the hash
+		 * bucket lock, retry the operation.
+		 */
+		if ((FUTEX_TID_MASK & curval) == uval)
+			ret = -EAGAIN;
+		else
+			ret = -EINVAL;
+	}
+
+	if (!ret) {
+		/*
+		 * This is a point of no return; once we modified the uval
+		 * there is no going back and subsequent operations must
+		 * not fail.
+		 */
+		pi_state_update_owner(pi_state, new_owner);
+		postunlock = __rt_mutex_futex_unlock(&pi_state->pi_mutex, &wqh);
+	}
+
+out_unlock:
+	raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
+
+	if (postunlock)
+		rt_mutex_postunlock(&wqh);
+
+	return ret;
+}
+
+static int __fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
+				  struct task_struct *argowner)
+{
+	struct futex_pi_state *pi_state = q->pi_state;
+	struct task_struct *oldowner, *newowner;
+	u32 uval, curval, newval, newtid;
+	int err = 0;
+
+	oldowner = pi_state->owner;
+
+	/*
+	 * We are here because either:
+	 *
+	 *  - we stole the lock and pi_state->owner needs updating to reflect
+	 *    that (@argowner == current),
+	 *
+	 * or:
+	 *
+	 *  - someone stole our lock and we need to fix things to point to the
+	 *    new owner (@argowner == NULL).
+	 *
+	 * Either way, we have to replace the TID in the user space variable.
+	 * This must be atomic as we have to preserve the owner died bit here.
+	 *
+	 * Note: We write the user space value _before_ changing the pi_state
+	 * because we can fault here. Imagine swapped out pages or a fork
+	 * that marked all the anonymous memory readonly for cow.
+	 *
+	 * Modifying pi_state _before_ the user space value would leave the
+	 * pi_state in an inconsistent state when we fault here, because we
+	 * need to drop the locks to handle the fault. This might be observed
+	 * in the PID checks when attaching to PI state .
+	 */
+retry:
+	if (!argowner) {
+		if (oldowner != current) {
+			/*
+			 * We raced against a concurrent self; things are
+			 * already fixed up. Nothing to do.
+			 */
+			return 0;
+		}
+
+		if (__rt_mutex_futex_trylock(&pi_state->pi_mutex)) {
+			/* We got the lock. pi_state is correct. Tell caller. */
+			return 1;
+		}
+
+		/*
+		 * The trylock just failed, so either there is an owner or
+		 * there is a higher priority waiter than this one.
+		 */
+		newowner = rt_mutex_owner(&pi_state->pi_mutex);
+		/*
+		 * If the higher priority waiter has not yet taken over the
+		 * rtmutex then newowner is NULL. We can't return here with
+		 * that state because it's inconsistent vs. the user space
+		 * state. So drop the locks and try again. It's a valid
+		 * situation and not any different from the other retry
+		 * conditions.
+		 */
+		if (unlikely(!newowner)) {
+			err = -EAGAIN;
+			goto handle_err;
+		}
+	} else {
+		WARN_ON_ONCE(argowner != current);
+		if (oldowner == current) {
+			/*
+			 * We raced against a concurrent self; things are
+			 * already fixed up. Nothing to do.
+			 */
+			return 1;
+		}
+		newowner = argowner;
+	}
+
+	newtid = task_pid_vnr(newowner) | FUTEX_WAITERS;
+	/* Owner died? */
+	if (!pi_state->owner)
+		newtid |= FUTEX_OWNER_DIED;
+
+	err = futex_get_value_locked(&uval, uaddr);
+	if (err)
+		goto handle_err;
+
+	for (;;) {
+		newval = (uval & FUTEX_OWNER_DIED) | newtid;
+
+		err = futex_cmpxchg_value_locked(&curval, uaddr, uval, newval);
+		if (err)
+			goto handle_err;
+
+		if (curval == uval)
+			break;
+		uval = curval;
+	}
+
+	/*
+	 * We fixed up user space. Now we need to fix the pi_state
+	 * itself.
+	 */
+	pi_state_update_owner(pi_state, newowner);
+
+	return argowner == current;
+
+	/*
+	 * In order to reschedule or handle a page fault, we need to drop the
+	 * locks here. In the case of a fault, this gives the other task
+	 * (either the highest priority waiter itself or the task which stole
+	 * the rtmutex) the chance to try the fixup of the pi_state. So once we
+	 * are back from handling the fault we need to check the pi_state after
+	 * reacquiring the locks and before trying to do another fixup. When
+	 * the fixup has been done already we simply return.
+	 *
+	 * Note: we hold both hb->lock and pi_mutex->wait_lock. We can safely
+	 * drop hb->lock since the caller owns the hb -> futex_q relation.
+	 * Dropping the pi_mutex->wait_lock requires the state revalidate.
+	 */
+handle_err:
+	raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
+	spin_unlock(q->lock_ptr);
+
+	switch (err) {
+	case -EFAULT:
+		err = fault_in_user_writeable(uaddr);
+		break;
+
+	case -EAGAIN:
+		cond_resched();
+		err = 0;
+		break;
+
+	default:
+		WARN_ON_ONCE(1);
+		break;
+	}
+
+	spin_lock(q->lock_ptr);
+	raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
+
+	/*
+	 * Check if someone else fixed it for us:
+	 */
+	if (pi_state->owner != oldowner)
+		return argowner == current;
+
+	/* Retry if err was -EAGAIN or the fault in succeeded */
+	if (!err)
+		goto retry;
+
+	/*
+	 * fault_in_user_writeable() failed so user state is immutable. At
+	 * best we can make the kernel state consistent but user state will
+	 * be most likely hosed and any subsequent unlock operation will be
+	 * rejected due to PI futex rule [10].
+	 *
+	 * Ensure that the rtmutex owner is also the pi_state owner despite
+	 * the user space value claiming something different. There is no
+	 * point in unlocking the rtmutex if current is the owner as it
+	 * would need to wait until the next waiter has taken the rtmutex
+	 * to guarantee consistent state. Keep it simple. Userspace asked
+	 * for this wreckaged state.
+	 *
+	 * The rtmutex has an owner - either current or some other
+	 * task. See the EAGAIN loop above.
+	 */
+	pi_state_update_owner(pi_state, rt_mutex_owner(&pi_state->pi_mutex));
+
+	return err;
+}
+
+static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
+				struct task_struct *argowner)
+{
+	struct futex_pi_state *pi_state = q->pi_state;
+	int ret;
+
+	lockdep_assert_held(q->lock_ptr);
+
+	raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
+	ret = __fixup_pi_state_owner(uaddr, q, argowner);
+	raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
+	return ret;
+}
+
+/**
+ * fixup_pi_owner() - Post lock pi_state and corner case management
+ * @uaddr:	user address of the futex
+ * @q:		futex_q (contains pi_state and access to the rt_mutex)
+ * @locked:	if the attempt to take the rt_mutex succeeded (1) or not (0)
+ *
+ * After attempting to lock an rt_mutex, this function is called to cleanup
+ * the pi_state owner as well as handle race conditions that may allow us to
+ * acquire the lock. Must be called with the hb lock held.
+ *
+ * Return:
+ *  -  1 - success, lock taken;
+ *  -  0 - success, lock not taken;
+ *  - <0 - on error (-EFAULT)
+ */
+int fixup_pi_owner(u32 __user *uaddr, struct futex_q *q, int locked)
+{
+	if (locked) {
+		/*
+		 * Got the lock. We might not be the anticipated owner if we
+		 * did a lock-steal - fix up the PI-state in that case:
+		 *
+		 * Speculative pi_state->owner read (we don't hold wait_lock);
+		 * since we own the lock pi_state->owner == current is the
+		 * stable state, anything else needs more attention.
+		 */
+		if (q->pi_state->owner != current)
+			return fixup_pi_state_owner(uaddr, q, current);
+		return 1;
+	}
+
+	/*
+	 * If we didn't get the lock; check if anybody stole it from us. In
+	 * that case, we need to fix up the uval to point to them instead of
+	 * us, otherwise bad things happen. [10]
+	 *
+	 * Another speculative read; pi_state->owner == current is unstable
+	 * but needs our attention.
+	 */
+	if (q->pi_state->owner == current)
+		return fixup_pi_state_owner(uaddr, q, NULL);
+
+	/*
+	 * Paranoia check. If we did not take the lock, then we should not be
+	 * the owner of the rt_mutex. Warn and establish consistent state.
+	 */
+	if (WARN_ON_ONCE(rt_mutex_owner(&q->pi_state->pi_mutex) == current))
+		return fixup_pi_state_owner(uaddr, q, current);
+
+	return 0;
+}
+
+/*
+ * Userspace tried a 0 -> TID atomic transition of the futex value
+ * and failed. The kernel side here does the whole locking operation:
+ * if there are waiters then it will block as a consequence of relying
+ * on rt-mutexes, it does PI, etc. (Due to races the kernel might see
+ * a 0 value of the futex too.).
+ *
+ * Also serves as futex trylock_pi()'ing, and due semantics.
+ */
+int futex_lock_pi(u32 __user *uaddr, unsigned int flags, ktime_t *time, int trylock)
+{
+	struct hrtimer_sleeper timeout, *to;
+	struct task_struct *exiting = NULL;
+	struct rt_mutex_waiter rt_waiter;
+	struct futex_hash_bucket *hb;
+	struct futex_q q = futex_q_init;
+	int res, ret;
+
+	if (!IS_ENABLED(CONFIG_FUTEX_PI))
+		return -ENOSYS;
+
+	if (refill_pi_state_cache())
+		return -ENOMEM;
+
+	to = futex_setup_timer(time, &timeout, flags, 0);
+
+retry:
+	ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &q.key, FUTEX_WRITE);
+	if (unlikely(ret != 0))
+		goto out;
+
+retry_private:
+	hb = futex_q_lock(&q);
+
+	ret = futex_lock_pi_atomic(uaddr, hb, &q.key, &q.pi_state, current,
+				   &exiting, 0);
+	if (unlikely(ret)) {
+		/*
+		 * Atomic work succeeded and we got the lock,
+		 * or failed. Either way, we do _not_ block.
+		 */
+		switch (ret) {
+		case 1:
+			/* We got the lock. */
+			ret = 0;
+			goto out_unlock_put_key;
+		case -EFAULT:
+			goto uaddr_faulted;
+		case -EBUSY:
+		case -EAGAIN:
+			/*
+			 * Two reasons for this:
+			 * - EBUSY: Task is exiting and we just wait for the
+			 *   exit to complete.
+			 * - EAGAIN: The user space value changed.
+			 */
+			futex_q_unlock(hb);
+			/*
+			 * Handle the case where the owner is in the middle of
+			 * exiting. Wait for the exit to complete otherwise
+			 * this task might loop forever, aka. live lock.
+			 */
+			wait_for_owner_exiting(ret, exiting);
+			cond_resched();
+			goto retry;
+		default:
+			goto out_unlock_put_key;
+		}
+	}
+
+	WARN_ON(!q.pi_state);
+
+	/*
+	 * Only actually queue now that the atomic ops are done:
+	 */
+	__futex_queue(&q, hb);
+
+	if (trylock) {
+		ret = rt_mutex_futex_trylock(&q.pi_state->pi_mutex);
+		/* Fixup the trylock return value: */
+		ret = ret ? 0 : -EWOULDBLOCK;
+		goto no_block;
+	}
+
+	rt_mutex_init_waiter(&rt_waiter);
+
+	/*
+	 * On PREEMPT_RT, when hb->lock becomes an rt_mutex, we must not
+	 * hold it while doing rt_mutex_start_proxy(), because then it will
+	 * include hb->lock in the blocking chain, even through we'll not in
+	 * fact hold it while blocking. This will lead it to report -EDEADLK
+	 * and BUG when futex_unlock_pi() interleaves with this.
+	 *
+	 * Therefore acquire wait_lock while holding hb->lock, but drop the
+	 * latter before calling __rt_mutex_start_proxy_lock(). This
+	 * interleaves with futex_unlock_pi() -- which does a similar lock
+	 * handoff -- such that the latter can observe the futex_q::pi_state
+	 * before __rt_mutex_start_proxy_lock() is done.
+	 */
+	raw_spin_lock_irq(&q.pi_state->pi_mutex.wait_lock);
+	spin_unlock(q.lock_ptr);
+	/*
+	 * __rt_mutex_start_proxy_lock() unconditionally enqueues the @rt_waiter
+	 * such that futex_unlock_pi() is guaranteed to observe the waiter when
+	 * it sees the futex_q::pi_state.
+	 */
+	ret = __rt_mutex_start_proxy_lock(&q.pi_state->pi_mutex, &rt_waiter, current);
+	raw_spin_unlock_irq(&q.pi_state->pi_mutex.wait_lock);
+
+	if (ret) {
+		if (ret == 1)
+			ret = 0;
+		goto cleanup;
+	}
+
+	if (unlikely(to))
+		hrtimer_sleeper_start_expires(to, HRTIMER_MODE_ABS);
+
+	ret = rt_mutex_wait_proxy_lock(&q.pi_state->pi_mutex, to, &rt_waiter);
+
+cleanup:
+	spin_lock(q.lock_ptr);
+	/*
+	 * If we failed to acquire the lock (deadlock/signal/timeout), we must
+	 * first acquire the hb->lock before removing the lock from the
+	 * rt_mutex waitqueue, such that we can keep the hb and rt_mutex wait
+	 * lists consistent.
+	 *
+	 * In particular; it is important that futex_unlock_pi() can not
+	 * observe this inconsistency.
+	 */
+	if (ret && !rt_mutex_cleanup_proxy_lock(&q.pi_state->pi_mutex, &rt_waiter))
+		ret = 0;
+
+no_block:
+	/*
+	 * Fixup the pi_state owner and possibly acquire the lock if we
+	 * haven't already.
+	 */
+	res = fixup_pi_owner(uaddr, &q, !ret);
+	/*
+	 * If fixup_pi_owner() returned an error, propagate that.  If it acquired
+	 * the lock, clear our -ETIMEDOUT or -EINTR.
+	 */
+	if (res)
+		ret = (res < 0) ? res : 0;
+
+	futex_unqueue_pi(&q);
+	spin_unlock(q.lock_ptr);
+	goto out;
+
+out_unlock_put_key:
+	futex_q_unlock(hb);
+
+out:
+	if (to) {
+		hrtimer_cancel(&to->timer);
+		destroy_hrtimer_on_stack(&to->timer);
+	}
+	return ret != -EINTR ? ret : -ERESTARTNOINTR;
+
+uaddr_faulted:
+	futex_q_unlock(hb);
+
+	ret = fault_in_user_writeable(uaddr);
+	if (ret)
+		goto out;
+
+	if (!(flags & FLAGS_SHARED))
+		goto retry_private;
+
+	goto retry;
+}
+
+/*
+ * Userspace attempted a TID -> 0 atomic transition, and failed.
+ * This is the in-kernel slowpath: we look up the PI state (if any),
+ * and do the rt-mutex unlock.
+ */
+int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
+{
+	u32 curval, uval, vpid = task_pid_vnr(current);
+	union futex_key key = FUTEX_KEY_INIT;
+	struct futex_hash_bucket *hb;
+	struct futex_q *top_waiter;
+	int ret;
+
+	if (!IS_ENABLED(CONFIG_FUTEX_PI))
+		return -ENOSYS;
+
+retry:
+	if (get_user(uval, uaddr))
+		return -EFAULT;
+	/*
+	 * We release only a lock we actually own:
+	 */
+	if ((uval & FUTEX_TID_MASK) != vpid)
+		return -EPERM;
+
+	ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &key, FUTEX_WRITE);
+	if (ret)
+		return ret;
+
+	hb = futex_hash(&key);
+	spin_lock(&hb->lock);
+
+	/*
+	 * Check waiters first. We do not trust user space values at
+	 * all and we at least want to know if user space fiddled
+	 * with the futex value instead of blindly unlocking.
+	 */
+	top_waiter = futex_top_waiter(hb, &key);
+	if (top_waiter) {
+		struct futex_pi_state *pi_state = top_waiter->pi_state;
+
+		ret = -EINVAL;
+		if (!pi_state)
+			goto out_unlock;
+
+		/*
+		 * If current does not own the pi_state then the futex is
+		 * inconsistent and user space fiddled with the futex value.
+		 */
+		if (pi_state->owner != current)
+			goto out_unlock;
+
+		get_pi_state(pi_state);
+		/*
+		 * By taking wait_lock while still holding hb->lock, we ensure
+		 * there is no point where we hold neither; and therefore
+		 * wake_futex_p() must observe a state consistent with what we
+		 * observed.
+		 *
+		 * In particular; this forces __rt_mutex_start_proxy() to
+		 * complete such that we're guaranteed to observe the
+		 * rt_waiter. Also see the WARN in wake_futex_pi().
+		 */
+		raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
+		spin_unlock(&hb->lock);
+
+		/* drops pi_state->pi_mutex.wait_lock */
+		ret = wake_futex_pi(uaddr, uval, pi_state);
+
+		put_pi_state(pi_state);
+
+		/*
+		 * Success, we're done! No tricky corner cases.
+		 */
+		if (!ret)
+			return ret;
+		/*
+		 * The atomic access to the futex value generated a
+		 * pagefault, so retry the user-access and the wakeup:
+		 */
+		if (ret == -EFAULT)
+			goto pi_faulted;
+		/*
+		 * A unconditional UNLOCK_PI op raced against a waiter
+		 * setting the FUTEX_WAITERS bit. Try again.
+		 */
+		if (ret == -EAGAIN)
+			goto pi_retry;
+		/*
+		 * wake_futex_pi has detected invalid state. Tell user
+		 * space.
+		 */
+		return ret;
+	}
+
+	/*
+	 * We have no kernel internal state, i.e. no waiters in the
+	 * kernel. Waiters which are about to queue themselves are stuck
+	 * on hb->lock. So we can safely ignore them. We do neither
+	 * preserve the WAITERS bit not the OWNER_DIED one. We are the
+	 * owner.
+	 */
+	if ((ret = futex_cmpxchg_value_locked(&curval, uaddr, uval, 0))) {
+		spin_unlock(&hb->lock);
+		switch (ret) {
+		case -EFAULT:
+			goto pi_faulted;
+
+		case -EAGAIN:
+			goto pi_retry;
+
+		default:
+			WARN_ON_ONCE(1);
+			return ret;
+		}
+	}
+
+	/*
+	 * If uval has changed, let user space handle it.
+	 */
+	ret = (curval == uval) ? 0 : -EAGAIN;
+
+out_unlock:
+	spin_unlock(&hb->lock);
+	return ret;
+
+pi_retry:
+	cond_resched();
+	goto retry;
+
+pi_faulted:
+
+	ret = fault_in_user_writeable(uaddr);
+	if (!ret)
+		goto retry;
+
+	return ret;
+}
+