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

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Message-ID: <b647ffbd0901230436x3408203bw10834d013beab16c@mail.gmail.com>
Date:	Fri, 23 Jan 2009 13:36:33 +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:
>>
>> 2009/1/23 Oleg Nesterov <oleg@...hat.com>:
>> > On 01/23, Dmitry Adamushko wrote:
>> >>
>> >> 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().
>
> Yes,
>
>> that means we are _on_ the 'wq' list when we do finish_wait() -> we do
>> take the 'wq->lock'.
>
> Hmm. No?
>
> We are doing exclusive wait, and we use autoremove_wake_function().
> If we were woken, we are removed from ->task_list.

Argh, right, somehow I've made wrong assumptions on the wake-up part :-/


>
> Oleg.
>

-- 
Best regards,
Dmitry Adamushko
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