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Date: Mon, 13 Apr 2009 22:13:48 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Andrew Morton <akpm@linux-foundation.org>,
       David Howells <dhowells@redhat.com>, David Miller <davem@davemloft.net>
Cc: Serge Hallyn <serue@us.ibm.com>, Steve Dickson <steved@redhat.com>,
       Trond Myklebust <Trond.Myklebust@netapp.com>,
       Al Viro <viro@zeniv.linux.org.uk>,
       Daire Byrne <Daire.Byrne@framestore.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] slow_work_execute() needs mb() before
	test_bit(SLOW_WORK_PENDING)
Message-ID: <20090413201348.GA16661@redhat.com>
References: <20090413201306.GA16653@redhat.com>
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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.