linux-kernel - Re: [PATCH for-3.3] mempool: clean up and document synchronization and memory barrier usage

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Message-ID: <20111221145714.GB25657@redhat.com>
Date:	Wed, 21 Dec 2011 15:57:14 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH for-3.3] mempool: clean up and document synchronization
	and memory barrier usage

On 12/21, Oleg Nesterov wrote:
>
> On 12/20, Tejun Heo wrote:
> >
> > Furthermore, mempool_alloc() is already holding pool->lock when it
> > decides that it needs to wait.  There is no reason to do unlock - add
> > waitqueue - test condition again.  It can simply add itself to
> > waitqueue while holding pool->lock and then unlock and sleep.
>
> Confused. I agree, we can hold pool->lock until schedule(). But, at
> the same time, why should we hold it?

Ah, I see.

> Or I missed the reason why we must not unlock before prepare_to_wait?

I didn't notice that this removes another "if (!pool->curr_nr)" check.

Oleg.

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