linux-kernel - Re: [PATCH 06/12] rwsem: wake queued readers when other readers are active

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Message-ID: <u2p8d20b11a1005122241o12d5df8dl43c669a058665f40@mail.gmail.com>
Date:	Wed, 12 May 2010 22:41:20 -0700
From:	Michel Lespinasse <walken@...gle.com>
To:	David Howells <dhowells@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Waychison <mikew@...gle.com>,
	Suleiman Souhlal <suleiman@...gle.com>,
	Ying Han <yinghan@...gle.com>
Subject: Re: [PATCH 06/12] rwsem: wake queued readers when other readers are 
	active

On Wed, May 12, 2010 at 7:39 PM, Michel Lespinasse <walken@...gle.com> wrote:
> On Wed, May 12, 2010 at 5:22 AM, David Howells <dhowells@...hat.com> wrote:
>> Michel Lespinasse <walken@...gle.com> wrote:
>>
>>> In this situation, it would be perfectly fine to let threads B and C work
>>> in parallel as they each only want a read acquire on the rwsem. We can
>>> recognize this situation and let A wake B as long as there are no active
>>> writers on the rwsem.
>>
>> There can't be any active writers on the rwsem.  An active writer must have
>> just been upped and is in the process of waking the first sleeper up.
>
> Yes. My point is that by the point thread A (the writer that just got
> upped) gets around to waking B (a blocked reader), another reader C
> might have gotten active already. We don't want the nonzero active
> count (due to C) to prevent B from getting woken.

My bad - this is actually fine. C will notice there are still waiting
threads, so it will run rwsem_down_read_failed and queue itself. At
this point the active count will go back down to 0 and B and C will
both get woken.

I'll merge this back into change 7 since change 7 does require this in
order to work.

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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