linux-kernel - Re: [PATCH] wait: fix false timeouts when using wait_event

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Message-ID: <20130605163702.GA26135@redhat.com>
Date:	Wed, 5 Jun 2013 18:37:02 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Imre Deak <imre.deak@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Dave Jones <davej@...hat.com>,
	David Howells <dhowells@...hat.com>,
	Jens Axboe <axboe@...nel.dk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Lukas Czerner <lczerner@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] wait: fix false timeouts when using
	wait_event_timeout()

On 06/05, Imre Deak wrote:
>
> On Wed, 2013-06-05 at 00:35 +0300, Imre Deak wrote:
> > On Tue, 2013-06-04 at 21:28 +0200, Oleg Nesterov wrote:
> > >
> > > 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.

Yes, if false. But what if it is true?

> Ah sorry, if you also rewrite __wait_event_timeout() then timeout=>0
> wouldn't schedule(), so things would work as expected.

Can't understand... probably you missed my point. Let me try again.

I think that wait_eveint_timeout(wq, COND, 0) should return !!(COND).
But it doesn't, for example wait_event_timeout(wq, true, 0) == 0, this
doesn'tlook right to me.

And, this is off-topic, but wait_eveint_timeout/__wait_eveint_timeout
do not match wait_event/__wait_event. IOW, you can't use
__wait_eveint_timeout() if you do not need the fast-path check.

So. How about

	#define __wait_event_timeout(wq, condition, timeout)			\
	({									\
		DEFINE_WAIT(__wait);						\
		long __ret = 0, __to = timeout;					\
										\
		for (;;) {							\
			prepare_to_wait(&wq, &__wait, TASK_UNINTERRUPTIBLE);	\
			if (condition) {					\
				__ret = __to ?: 1;				\
				break;						\
			}							\
			if (!__to)						\
				break;						\
			__to = schedule_timeout(__to);				\
		}								\
		finish_wait(&wq, &__wait);					\
		__ret;								\
	})

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

?

Othwerwise we should document the fact that the caller should alvays verify
timeout != 0 if it checks the result of wait_event_timeout().

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