Re: [PATCH] robust futex thread exit race

From: Ingo Molnar
Date: Sun Sep 30 2007 - 11:19:10 EST

* Martin Schwidefsky <schwidefsky@xxxxxxxxxx> 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@xxxxxxx>

> 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?

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