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Commit 25f13b40 authored by Nicolai Hähnle's avatar Nicolai Hähnle Committed by Ingo Molnar
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locking/ww_mutex: Re-check ww->ctx in the inner optimistic spin loop 



In the following scenario, thread #1 should back off its attempt to lock
ww1 and unlock ww2 (assuming the acquire context stamps are ordered
accordingly).

    Thread #0               Thread #1
    ---------               ---------
                            successfully lock ww2
    set ww1->base.owner
                            attempt to lock ww1
                            confirm ww1->ctx == NULL
                            enter mutex_spin_on_owner
    set ww1->ctx

What was likely to happen previously is:

    attempt to lock ww2
    refuse to spin because
      ww2->ctx != NULL
    schedule()
                            detect thread #0 is off CPU
                            stop optimistic spin
                            return -EDEADLK
                            unlock ww2
                            wakeup thread #0
    lock ww2

Now, we are more likely to see:

                            detect ww1->ctx != NULL
                            stop optimistic spin
                            return -EDEADLK
                            unlock ww2
    successfully lock ww2

... because thread #1 will stop its optimistic spin as soon as possible.

The whole scenario is quite unlikely, since it requires thread #1 to get
between thread #0 setting the owner and setting the ctx. But since we're
idling here anyway, the additional check is basically free.

Found by inspection.

Signed-off-by: default avatarNicolai Hähnle <Nicolai.Haehnle@amd.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Maarten Lankhorst <dev@mblankhorst.nl>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dri-devel@lists.freedesktop.org
Link: http://lkml.kernel.org/r/1482346000-9927-10-git-send-email-nhaehnle@gmail.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 427b1820

kernel/locking/mutex.c

+29 −20
Original line number Diff line number Diff line
@@ -372,11 +372,14 @@ ww_mutex_set_context_slowpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx) 


#ifdef CONFIG_MUTEX_SPIN_ON_OWNER

/*

 * Look out! "owner" is an entirely speculative pointer

 * access and not reliable.

 * Look out! "owner" is an entirely speculative pointer access and not

 * reliable.

 *

 * "noinline" so that this function shows up on perf profiles.

 */

static noinline

bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)

bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner,

			 struct ww_acquire_ctx *ww_ctx)

{

	bool ret = true;
@@ -399,6 +402,28 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner) 
			break;

		}



		if (ww_ctx && ww_ctx->acquired > 0) {

			struct ww_mutex *ww;



			ww = container_of(lock, struct ww_mutex, base);



			/*

			 * If ww->ctx is set the contents are undefined, only

			 * by acquiring wait_lock there is a guarantee that

			 * they are not invalid when reading.

			 *

			 * As such, when deadlock detection needs to be

			 * performed the optimistic spinning cannot be done.

			 *

			 * Check this in every inner iteration because we may

			 * be racing against another thread's ww_mutex_lock.

			 */

			if (READ_ONCE(ww->ctx)) {

				ret = false;

				break;

			}

		}



		cpu_relax();

	}

	rcu_read_unlock();
@@ -484,22 +509,6 @@ mutex_optimistic_spin(struct mutex *lock, struct ww_acquire_ctx *ww_ctx, 
	for (;;) {

		struct task_struct *owner;



		if (use_ww_ctx && ww_ctx && ww_ctx->acquired > 0) {

			struct ww_mutex *ww;



			ww = container_of(lock, struct ww_mutex, base);

			/*

			 * If ww->ctx is set the contents are undefined, only

			 * by acquiring wait_lock there is a guarantee that

			 * they are not invalid when reading.

			 *

			 * As such, when deadlock detection needs to be

			 * performed the optimistic spinning cannot be done.

			 */

			if (READ_ONCE(ww->ctx))

				goto fail_unlock;

		}



		/* Try to acquire the mutex... */

		owner = __mutex_trylock_or_owner(lock);

		if (!owner)
@@ -509,7 +518,7 @@ mutex_optimistic_spin(struct mutex *lock, struct ww_acquire_ctx *ww_ctx, 
		 * There's an owner, wait for it to either

		 * release the lock or go to sleep.

		 */

		if (!mutex_spin_on_owner(lock, owner))

		if (!mutex_spin_on_owner(lock, owner, ww_ctx))

			goto fail_unlock;



		/*