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

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Date:	Thu, 27 Nov 2008 18:18:49 +0900
From:	Tejun Heo <htejun@...il.com>
To:	Davide Libenzi <davidel@...ilserver.org>
CC:	Oleg Nesterov <oleg@...hat.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

Hello,

Davide Libenzi wrote:
> Hmmm, I just noticed that the set_current_state(TASK_INTERRUPTIBLE) at the 
> beginning of the ->poll() loop has been dropped (and it makes sense since 
> now ->poll() can sleep).

Yeah, that's exactly what the ->triggered condition replaces.

> w1) WR dev->events
> w2) MB
> w3) WR triggered (1)
> w4) WMB
> w5) WR task->state (RUNNING)
> 
> Poller side:
>  
> s1) WR task->state (TASK_INTERRUPTIBLE)
> s2) MB
> s3) RD triggered
> s4) IF0 => RD task->state (if !RUNNING -> sleep)
> s5) WR triggered (0)
> s6) MB
> s7) RD dev->events
> 
> 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.

Yeah, it seems we'll need something which is equivalent to smp_wmb()
in try_to_wake_up().  So, the original set_mb() should have stayed
there while just adding the latter one.  Will prep yet another take of
the patch.  Thanks for the detailed analysis.

-- 
tejun
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