linux-kernel - [60/94] wait: fix false timeouts when using wait_event

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Message-ID: <lsq.1369712993.310594029@decadent.org.uk>
Date:	Tue, 28 May 2013 04:49:53 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Jens Axboe" <axboe@...nel.dk>, "Imre Deak" <imre.deak@...el.com>,
	"Dave Jones" <davej@...hat.com>,
	"Lukas Czerner" <lczerner@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"David Howells" <dhowells@...hat.com>,
	"Daniel Vetter" <daniel.vetter@...ll.ch>
Subject: [60/94] wait: fix false timeouts when using wait_event_timeout()

3.2.46-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Imre Deak <imre.deak@...el.com>

commit 4c663cfc523a88d97a8309b04a089c27dc57fd7e upstream.

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@...el.com>
Acked-by: Daniel Vetter <daniel.vetter@...ll.ch>
Acked-by: David Howells <dhowells@...hat.com>
Acked-by: Jens Axboe <axboe@...nel.dk>
Cc: "Paul E.  McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: Dave Jones <davej@...hat.com>
Cc: Lukas Czerner <lczerner@...hat.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 include/linux/wait.h |   16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

--- a/include/linux/wait.h
+++ b/include/linux/wait.h
@@ -233,6 +233,8 @@ do {									\
 		if (!ret)						\
 			break;						\
 	}								\
+	if (!ret && (condition))					\
+		ret = 1;						\
 	finish_wait(&wq, &__wait);					\
 } while (0)
 
@@ -249,8 +251,9 @@ 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)			\
 ({									\
@@ -318,6 +321,8 @@ do {									\
 		ret = -ERESTARTSYS;					\
 		break;							\
 	}								\
+	if (!ret && (condition))					\
+		ret = 1;						\
 	finish_wait(&wq, &__wait);					\
 } while (0)
 
@@ -334,9 +339,10 @@ 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)	\
 ({									\

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