linux-kernel - Re: [RFC v5] wait: prevent exclusive waiter starvation

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Date:	Tue, 27 Jan 2009 04:23:59 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Matthew Wilcox <matthew@....cx>,
	Chuck Lever <cel@...i.umich.edu>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC v5] wait: prevent exclusive waiter starvation

On 01/26, Johannes Weiner wrote:
>
> Another iteration.  I didn't use a general finish_wait_exclusive() but
> a version of this function that just returns whether we were woken
> through the queue or not.

But if your helper (finish_wait_woken) returns true, we always need
to wakeup the next waiter, or we don't need to use it. So why not
place the wakeup in the helper itself?

> --- a/include/linux/wait.h
> +++ b/include/linux/wait.h
> @@ -333,16 +333,20 @@ do {									\
>  	for (;;) {							\
>  		prepare_to_wait_exclusive(&wq, &__wait,			\
>  					TASK_INTERRUPTIBLE);		\
> -		if (condition)						\
> +		if (condition) {					\
> +			finish_wait(&wq, &__wait);			\
>  			break;						\
> +		}							\
>  		if (!signal_pending(current)) {				\
>  			schedule();					\
>  			continue;					\
>  		}							\
>  		ret = -ERESTARTSYS;					\
> +		if (finish_wait_woken(&wq, &__wait))			\
> +			__wake_up_common(&wq, TASK_INTERRUPTIBLE,	\

No, we can't use __wake_up_common() without wq->lock.

Oleg.

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