[027/141] wait: fix false timeouts when using wait_event_timeout()

Steven Rostedt
2013-07-03T18:53:42Z

[-- Attachment #1: 0027-wait-fix-false-timeouts-when-using-wait_event_timeou.patch --]
[-- Type: text/plain, Size: 3549 bytes --]

3.6.11.6 stable review patch.
If anyone has any objections, please let me know.

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

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

[ Upstream commit 4c663cfc523a88d97a8309b04a089c27dc57fd7e ]

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>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
include/linux/wait.h | 16 +++++++++++-----
1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/include/linux/wait.h b/include/linux/wait.h
index 1dee81c..6c6c20e 100644
--- 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) \
({ \
--
1.7.10.4