linux-kernel - Re: [PATCH] timerfd/eventfd context lock doesn't protect against poll

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Message-Id: <20070517233722.8d4be064.akpm@linux-foundation.org>
Date:	Thu, 17 May 2007 23:37:22 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	Davi Arnaut <davi@...ent.com.br>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] timerfd/eventfd context lock doesn't protect against
 poll_wait

On Thu, 17 May 2007 23:20:05 -0700 (PDT) Davide Libenzi <davidel@...ilserver.org> wrote:

> On Fri, 18 May 2007, Davi Arnaut wrote:
> 
> > Hi,
> > 
> > poll_wait() callback may modify the waitqueue without holding the
> > context private lock.
> 
> Thx Davi, patch is correct. Nice catch. But at this point instead of 
> ending up getting two locks, we may look into using Andrew suggestion of 
> reusing the waitqueue lock. Is it universally considered a "legal" 
> operation?
> 

I think it's a reasonable thing to do in core kernel code.  It'd be more
worrisome if it was done way down in some rarely-visited device driver.

btw, the code at present doesn't explicitly take care of WQ_FLAG_EXCLUSIVE.
I guess as you want wake-all behaviour that's OK, but it might be worth a
mention somewhrre.

hm, fs/signalfd.c is "Copyright (C) 2003 Linus Torvalds".  He was before
his time, that lad.

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