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

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Date:	Fri, 23 Jan 2009 11:07:48 +0100
From:	Dmitry Adamushko <dmitry.adamushko@...il.com>
To:	Oleg Nesterov <oleg@...hat.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

2009/1/23 Oleg Nesterov <oleg@...hat.com>:
> 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).

But taking into account the constraints of this special case, namely
(1), we can't skip lock(wq->lock).

(1) "the next contender is us"

In this particular situation, we are only interested in the case when
we were woken up by __wake_up_bit().

that means we are _on_ the 'wq' list when we do finish_wait() -> we do
take the 'wq->lock'.

Moreover, imagine the following case (roughly similar to finish_wait()):

if (LOAD(a) == 1) {
    // do something here
    mb();
}

LOAD(b);

Can LOAD(b) be reordered with LOAD(a)?

I'd imagine that it can be done with CPUs that do execute 2 branch
paths in advance but then LOAD(b) must be re-loaded if (a == 1) and we
hit mb().


>
> Oleg.
>

-- 
Best regards,
Dmitry Adamushko
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