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Commit 200b1874 authored by Nicolai Hähnle's avatar Nicolai Hähnle Committed by Ingo Molnar
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locking/ww_mutex: Notify waiters that have to back off while adding tasks to wait list 



While adding our task as a waiter, detect if another task should back off
because of us.

With this patch, we establish the invariant that the wait list contains
at most one (sleeping) waiter with ww_ctx->acquired > 0, and this waiter
will be the first waiter with a context.

Since only waiters with ww_ctx->acquired > 0 have to back off, this allows
us to be much more economical with wakeups.

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/1482346000-9927-8-git-send-email-nhaehnle@gmail.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 6baa5c60

kernel/locking/mutex.c

+30 −10
Original line number Diff line number Diff line
@@ -596,15 +596,29 @@ void __sched ww_mutex_unlock(struct ww_mutex *lock) 
EXPORT_SYMBOL(ww_mutex_unlock);



static inline int __sched

__ww_mutex_lock_check_stamp(struct mutex *lock, struct ww_acquire_ctx *ctx)

__ww_mutex_lock_check_stamp(struct mutex *lock, struct mutex_waiter *waiter,

			    struct ww_acquire_ctx *ctx)

{

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

	struct ww_acquire_ctx *hold_ctx = READ_ONCE(ww->ctx);

	struct mutex_waiter *cur;



	if (hold_ctx && __ww_ctx_stamp_after(ctx, hold_ctx))

		goto deadlock;



	/*

	 * If there is a waiter in front of us that has a context, then its

	 * stamp is earlier than ours and we must back off.

	 */

	cur = waiter;

	list_for_each_entry_continue_reverse(cur, &lock->wait_list, list) {

		if (cur->ww_ctx)

			goto deadlock;

	}



	if (!hold_ctx)

	return 0;



	if (__ww_ctx_stamp_after(ctx, hold_ctx)) {

deadlock:

#ifdef CONFIG_DEBUG_MUTEXES

	DEBUG_LOCKS_WARN_ON(ctx->contending_lock);

	ctx->contending_lock = ww;
@@ -612,9 +626,6 @@ __ww_mutex_lock_check_stamp(struct mutex *lock, struct ww_acquire_ctx *ctx) 
	return -EDEADLK;

}



	return 0;

}



static inline int __sched

__ww_mutex_add_waiter(struct mutex_waiter *waiter,

		      struct mutex *lock,
@@ -655,6 +666,15 @@ __ww_mutex_add_waiter(struct mutex_waiter *waiter, 
		}



		pos = &cur->list;



		/*

		 * Wake up the waiter so that it gets a chance to back

		 * off.

		 */

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

			debug_mutex_wake_waiter(lock, cur);

			wake_up_process(cur->task);

		}

	}



	list_add_tail(&waiter->list, pos);
@@ -746,7 +766,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, 
		}



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

			ret = __ww_mutex_lock_check_stamp(lock, ww_ctx);

			ret = __ww_mutex_lock_check_stamp(lock, &waiter, ww_ctx);

			if (ret)

				goto err;

		}