linux-kernel - Re: [PATCH] slow_work_execute() needs mb() before test_bit(SLOW_WORK

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Message-ID: <20090413201348.GA16661@redhat.com>
Date:	Mon, 13 Apr 2009 22:13:48 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	David Miller <davem@...emloft.net>
Cc:	Serge Hallyn <serue@...ibm.com>, Steve Dickson <steved@...hat.com>,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Daire Byrne <Daire.Byrne@...mestore.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] slow_work_execute() needs mb() before
	test_bit(SLOW_WORK_PENDING)

On 04/13, Oleg Nesterov wrote:
>
> slow_work_execute:
>
> 	clear_bit_unlock(SLOW_WORK_EXECUTING, &work->flags);
>
> 	if (test_bit(SLOW_WORK_PENDING, &work->flags) {
>
> clear_bit_unlock() implies release semantics, iow we have a one-way barrier
> before clear_bit(). But we need the mb() semantics after clear_bit(), before
> we test SLOW_WORK_PENDING. Otherwise we can miss SLOW_WORK_ENQ_DEFERRED if
> we race slow_work_enqueue().

However, given that both clear_bit() and set_bit() use the same word, perhaps
this is not possible.

But in that case I don't understand why do we need clear_bit_unlock(), not just
clear_bit(), and how "mb is not needeed" could be derived from documentation.

Oleg.

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