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

Just noticed this commit...

commit 4c663cfc523a88d97a8309b04a089c27dc57fd7e
Author: Imre Deak <imre.deak@intel.com>
Date: Fri May 24 15:55:09 2013 -0700

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.

OK, agreed.

--- 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)

Well, this evaluates "condition" twice, perhaps it would be more
clean to do, say,

#define __wait_event_timeout(wq, condition, ret) \
do { \
DEFINE_WAIT(__wait); \
\
for (;;) { \
prepare_to_wait(&wq, &__wait, TASK_UNINTERRUPTIBLE); \
if (condition) { \
if (!ret) \
ret = 1; \
break; \
} else if (!ret) \
break; \
ret = schedule_timeout(ret); \
} \
finish_wait(&wq, &__wait); \
} while (0)

but this is minor.

@@ -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.

This is still not true if timeout == 0.

Shouldn't we also change wait_event_timeout() ? Say,

#define wait_event_timeout(wq, condition, timeout) \
({ \
long __ret = timeout; \
if (!(condition)) \
__wait_event_timeout(wq, condition, __ret); \
else if (!__ret) \
__ret = 1; \
__ret; \
})

Or wait_event_timeout(timeout => 0) is not legal in a non-void context?

To me the code like

long wait_for_something(bool nonblock)
{
timeout = nonblock ? 0 : DEFAULT_TIMEOUT;
return wait_event_timeout(..., timeout);
}

looks reasonable and correct. But it is not?

Oleg.


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