From: Tony Breeds <tony@bakeyournoodle.com>
Date: Wed, 19 May 2010 05:46:36 +0000 (+1000)
Subject: mutex: Fix optimistic spinning vs. BKL
X-Git-Tag: v2.6.35-rc1~2^2
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=fd6be105b883244127a734ac9f14ae94a022dcc0;p=pandora-kernel.git

mutex: Fix optimistic spinning vs. BKL

Currently, we can hit a nasty case with optimistic
spinning on mutexes:

    CPU A tries to take a mutex, while holding the BKL

    CPU B tried to take the BLK while holding the mutex

This looks like a AB-BA scenario but in practice, is
allowed and happens due to the auto-release on
schedule() nature of the BKL.

In that case, the optimistic spinning code can get us
into a situation where instead of going to sleep, A
will spin waiting for B who is spinning waiting for
A, and the only way out of that loop is the
need_resched() test in mutex_spin_on_owner().

This patch fixes it by completely disabling spinning
if we own the BKL. This adds one more detail to the
extensive list of reasons why it's a bad idea for
kernel code to be holding the BKL.

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: <stable@kernel.org>
LKML-Reference: <20100519054636.GC12389@ozlabs.org>
[ added an unlikely() attribute to the branch ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---

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