linux-kernel - Re: [PATCH] robust futex thread exit race

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Message-ID: <alpine.LFD.0.9999.0709301750430.3142@localhost.localdomain>
Date:	Sun, 30 Sep 2007 17:53:24 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Martin Schwidefsky <schwidefsky@...ibm.com>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Ulrich Drepper <drepper@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] robust futex thread exit race


On Sun, 30 Sep 2007, Ingo Molnar wrote:
> * Martin Schwidefsky <schwidefsky@...ibm.com> wrote:
> 
> > Hi Ingo,
> > I finally found the bug that causes tst-robust8 from the glibc to fail
> > on s390x. Turned out to be a common code problem with the processing of
> > the robust futex list. The patch below fixes the bug for me.
> 
> good catch! A quick preliminary review of your patch indicates it's fine 
> - and it might be v2.6.23 material.
> 
> Acked-by: Ingo Molnar <mingo@...e.hu>

  Acked-by: Thomas Gleixner <tglx@...utronix.de>
 
> > Calling handle_futex_death in exit_robust_list for the different 
> > robust mutexes of a thread basically frees the mutex. Another thread 
> > might grab the lock immediately which updates the next pointer of the 
> > mutex. fetch_robust_entry over the next pointer might therefore branch 
> > into the robust mutex list of a different thread. This can cause two 
> > problems: 1) some mutexes held by the dead thread are not getting 
> > freed and 2) some mutexs held by a different thread are freed. The 
> > next point need to be read before calling handle_futex_death.
> 
> nasty race... Ulrich, Thomas, do you concur?

Yes. Where do they sell those brown paperbags again ?

	tglx

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