linux-kernel - Re: [PATCH 0/6] sched, wait: Collapse __wait

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Date:	Mon, 30 Sep 2013 20:13:58 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/6] sched, wait: Collapse __wait_event macros -v4

On 09/30, Peter Zijlstra wrote:
>
> On Mon, Sep 30, 2013 at 07:40:54PM +0200, Oleg Nesterov wrote:
> > Once again, of course I do not blame this series, but
> > wait_event_timeout(wq, true, 0) still returns 0.
>
> So we have:
>
> [...snip...]

> So wait_event_timeout(wq, true, 0) turns into:

Not really, because of fast-path check,

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

we do not even call __wait_event_timeout() if "condition" is already
true at the start.

> >  	___wait_event(wq, ___wait_cond_timeout(condition),		\
> >  		      TASK_INTERRUPTIBLE, 0, ret,	      		\
> >  		      spin_unlock_irq(&lock);				\
> > -		      __ret = schedule_timeout(__ret);			\
> > +		      ___wait_schedule_timeout(__ret);			\
> >  		      spin_lock_irq(&lock));
>
> You can't do that; you'll break/return without the lock held.

Yes, see another email, already noticed this.


And let me repeat just in case, I think this series is fine, just
wait.h needs another minor/orthogonal fix.

Oleg.

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