| Date | Thu, 01 Jul 2010 10:30:40 -0700 | From | Greg KH <> | Subject | [patch 014/149] mutex: Fix optimistic spinning vs. BKL |
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2.6.32-stable review patch. If anyone has any objections, please let us know.
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From: Tony Breeds <tony@bakeyournoodle.com>
commit fd6be105b883244127a734ac9f14ae94a022dcc0 upstream.
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> LKML-Reference: <20100519054636.GC12389@ozlabs.org> [ added an unlikely() attribute to the branch ] Signed-off-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
--- kernel/mutex.c | 7 +++++++ 1 file changed, 7 insertions(+)
--- a/kernel/mutex.c +++ b/kernel/mutex.c @@ -172,6 +172,13 @@ __mutex_lock_common(struct mutex *lock, struct thread_info *owner; /* + * If we own the BKL, then don't spin. The owner of + * the mutex might be waiting on us to release the BKL. + */ + if (unlikely(current->lock_depth >= 0)) + break; + + /* * If there's an owner, wait for it to either * release the lock or go to sleep. */
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