Commit 4c663cfc authored May 24, 2013 by Imre Deak Committed by Linus Torvalds May 24, 2013

wait: fix false timeouts when using wait_event_timeout()

Many callers of the wait_event_timeout() and
wait_event_interruptible_timeout() expect that the return value will be
positive if the specified condition becomes true before the timeout
elapses.  However, at the moment this isn't guaranteed.  If the wake-up
handler is delayed enough, the time remaining until timeout will be
calculated as 0 - and passed back as a return value - even if the
condition became true before the timeout has passed.

Fix this by returning at least 1 if the condition becomes true.  This
semantic is in line with what wait_for_condition_timeout() does; see
commit bb10ed09 ("sched: fix wait_for_completion_timeout() spurious
failure under heavy load").

Daniel said "We have 3 instances of this bug in drm/i915.  One case even
where we switch between the interruptible and not interruptible
wait_event_timeout variants, foolishly presuming they have the same
semantics.  I very much like this."

One such bug is reported at
  https://bugs.freedesktop.org/show_bug.cgi?id=64133



Signed-off-by: Imre Deak <imre.deak@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: David Howells <dhowells@redhat.com>
Acked-by: Jens Axboe <axboe@kernel.dk>
Cc: "Paul E.  McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Dave Jones <davej@redhat.com>
Cc: Lukas Czerner <lczerner@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 7b92d03c

include/linux/wait.h

+11 −5

Original line number	Diff line number	Diff line
		@@ -217,6 +217,8 @@ do { \
		if (!ret) \
		break; \
		} \
		if (!ret && (condition)) \
		ret = 1; \
		finish_wait(&wq, &__wait); \
		} while (0)

		@@ -233,8 +235,9 @@ do { \
		* wake_up() has to be called after changing any variable that could
		* change the result of the wait condition.
		*
		* The function returns 0 if the @timeout elapsed, and the remaining
		* jiffies if the condition evaluated to true before the timeout elapsed.
		* The function returns 0 if the @timeout elapsed, or the remaining
		* jiffies (at least 1) if the @condition evaluated to %true before
		* the @timeout elapsed.
		*/
		#define wait_event_timeout(wq, condition, timeout) \
		({ \
		@@ -302,6 +305,8 @@ do { \
		ret = -ERESTARTSYS; \
		break; \
		} \
		if (!ret && (condition)) \
		ret = 1; \
		finish_wait(&wq, &__wait); \
		} while (0)

		@@ -318,9 +323,10 @@ do { \
		* wake_up() has to be called after changing any variable that could
		* change the result of the wait condition.
		*
		* The function returns 0 if the @timeout elapsed, -ERESTARTSYS if it
		* was interrupted by a signal, and the remaining jiffies otherwise
		* if the condition evaluated to true before the timeout elapsed.
		* Returns:
		* 0 if the @timeout elapsed, -%ERESTARTSYS if it was interrupted by
		* a signal, or the remaining jiffies (at least 1) if the @condition
		* evaluated to %true before the @timeout elapsed.
		*/
		#define wait_event_interruptible_timeout(wq, condition, timeout) \
		({ \