[PATCH] wait: fix false timeouts when using wait_event_timeout()

[Imre Deak]

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".

Signed-off-by: Imre Deak <imre.deak@xxxxxxxxx>
---
 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 7cb64d4..5336842 100644
--- a/include/linux/wait.h
+++ b/include/linux/wait.h
@@ -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)	\
 ({									\
-- 
1.7.10.4

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