linux-kernel - Re: [PATCH] poll: allow f

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Date:	Thu, 27 Nov 2008 21:13:58 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	Tejun Heo <htejun@...il.com>,
	Eric Van Hensbergen <ericvh@...il.com>,
	Ron Minnich <rminnich@...dia.gov>, Ingo Molnar <mingo@...e.hu>,
	Christoph Hellwig <hch@...radead.org>,
	Miklos Szeredi <mszeredi@...e.cz>,
	Brad Boyer <flar@...andria.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Roland McGrath <roland@...hat.com>,
	Mauro Carvalho Chehab <mchehab@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] poll: allow f_op->poll to sleep, take#5

On 11/26, Davide Libenzi wrote:
>
> w1) WR dev->events
> w2) WR triggered (1)
> w3) WMB
> w4) WR task->state (RUNNING)
> ...
> That is, an MB before w3 (triggered=1) and a set_mb(triggered,0) at
> s5+s6. The spinlock on the queue taken before entering pollwake() is not
> enough to guarantee the required ordering, since a LOCK is no guarantee
> that operations before it are visible after the LOCK.
> Without the MB at w2, it could happen [w3, s5, s7, w1] that will make us
> miss the event *and* sleep.

I think you are very right. Actually, this is just like

W:
	fill_data(&DATA);
	wmb();
	DATA_is_ready = 1;	// triggered
	wake_up(wq);

S:
	set_current_state(state);
	if (DATA_is_ready)
		ret = poll(&DATA);
	else
		schedule();

without wmb() above poll(&DATA) can obviously return the wrong value.

Oleg.

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