linux-kernel - Re: [RFC v4] wait: prevent waiter starvation in __wait_on_bit

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Message-ID: <20090123004702.GA18362@redhat.com>
Date:	Fri, 23 Jan 2009 01:47:02 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Dmitry Adamushko <dmitry.adamushko@...il.com>
Cc:	Johannes Weiner <hannes@...xchg.org>,
	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 v4] wait: prevent waiter starvation in __wait_on_bit_lock

On 01/23, Dmitry Adamushko wrote:
>
> 2009/1/22 Oleg Nesterov <oleg@...hat.com>:
> >
> > I think this is correct, and (unfortunately ;) you are right:
> > we need rmb() even after finish_wait().
>
> Hum, I think it's actually not necessary in this particular case when
> (1) "the next contender is us" and (2) we are in the "ret != 0" path
> so that the only thing we really care about -- if we were exclusivly
> woken up, then wake up somebody else [*].
>
> "the next contender is us" implies that we were still on the 'wq'
> queue when __wake_up_bit() -> __wake_up() has been called, meaning
> that wq->lock has also been taken (in __wake_up()).
>
> Now, on our side, we are definitely on the 'wq' queue before calling
> finish_wait(), meaning that we also take the wq->lock.
>
> In short, wq->lock is a sync. mechanism in this case. The scheme is as follows:
>
> our side:
>
> [ finish_wait() ]
>
> lock(wq->lock);

But we can skip lock(wq->lock), afaics.

Without rmb(), test_bit() can be re-ordered with list_empty_careful()
in finish_wait() and even with __set_task_state(TASK_RUNNING).

> p.s. if the explanation is vague or heh even wrong, it's definitely
> due to the lack of sleep ;-))

The same on my side ;)

Oleg.

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