linux-kernel - Re: [RFC,PATCH] workqueues: turn queue_work() into the "barrier" for work->func()

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Message-ID: <20081112115804.GA3444@redhat.com>
Date:	Wed, 12 Nov 2008 12:58:04 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	David Howells <dhowells@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Jiri Pirko <jpirko@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC,PATCH] workqueues: turn queue_work() into the "barrier"
	for work->func()

On 11/11, David Howells wrote:
>
> Oleg Nesterov <oleg@...hat.com> wrote:
>
> > I think the caller of queue_work() has all rights to expect that
> > the next invocation of work_func() must see "VAR == 1", but this
> > is not true if the work is already pending.
>
> As you said, queue_work() does test_and_set_bit() which implies smp_mb()
> either side of the function, so you're half way there, and run_workqueue()
> calls spin_unlock_irq() just before calling work_clear_pending()...  So might
> it make sense to move the work_clear_pending() into locked section?  Or would
> that require an smp_mb__before_clear_bit()?

This can't really help, afaics. We still need mb() between clear_bit(_PENDING)
and LOAD(VAR). Because unlock() is the "one way" barrier, LOAD(VAR) can leak
into the critical section, and it can be re-ordered with clear_bit() inside
the critical section.

Oleg.

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