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

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Date:	Sun, 30 Sep 2007 17:18:27 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Ulrich Drepper <drepper@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] robust futex thread exit race


* 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>

> 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?

	Ingo
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