On Tue, 2013-06-04 at 21:28 +0200, Oleg Nesterov wrote: > Hello, > > Just noticed this commit... > > commit 4c663cfc523a88d97a8309b04a089c27dc57fd7e > Author: Imre Deak <[email protected]> > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
