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

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Date:	Wed, 12 May 2010 19:39:37 -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 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.

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