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

[Imre Deak <imre.deak@intel.com>]

On Tue, 2013-06-04 at 21:28 +0200, Oleg Nesterov wrote:
> 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?

I don't see why it would be not legal. Note though that in the above
form wait_event_timeout(cond, 0) would still schedule() if cond is
false, which is not what I'd expect from a non-blocking function.

--Imre