Messages in this thread | | | Subject | Re: RFC: mutex: hung tasks on SMP platforms with asm-generic/mutex-xchg.h | From | Peter Zijlstra <> | Date | Mon, 13 Aug 2012 16:05:17 +0200 |
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On Mon, 2012-08-13 at 09:35 -0400, Nicolas Pitre wrote: > Date: Fri, 10 Aug 2012 15:22:09 +0100 > From: Will Deacon <will.deacon@arm.com> > Subject: [PATCH] mutex: place lock in contended state after fastpath_lock failure > > ARM recently moved to asm-generic/mutex-xchg.h for its mutex > implementation after the previous implementation was found to be missing > some crucial memory barriers. However, this has revealed some problems > running hackbench on SMP platforms due to the way in which the > MUTEX_SPIN_ON_OWNER code operates. > > The symptoms are that a bunch of hackbench tasks are left waiting on an > unlocked mutex and therefore never get woken up to claim it. This boils > down to the following sequence of events: > > Task A Task B Task C Lock value > 0 1 > 1 lock() 0 > 2 lock() 0 > 3 spin(A) 0 > 4 unlock() 1 > 5 lock() 0 > 6 cmpxchg(1,0) 0 > 7 contended() -1 > 8 lock() 0 > 9 spin(C) 0 > 10 unlock() 1 > 11 cmpxchg(1,0) 0 > 12 unlock() 1 > > At this point, the lock is unlocked, but Task B is in an uninterruptible > sleep with nobody to wake it up. > > This patch fixes the problem by ensuring we put the lock into the > contended state if we fail to acquire it on the fastpath, ensuring that > any blocked waiters are woken up when the mutex is released. > > Cc: Arnd Bergmann <arnd@arndb.de> > Cc: Thomas Gleixner <tglx@linutronix.de> > Cc: Chris Mason <chris.mason@fusionio.com> > Cc: Ingo Molnar <mingo@elte.hu> > Cc: <stable@vger.kernel.org> > Signed-off-by: Will Deacon <will.deacon@arm.com> > Reviewed-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Will you carry this through the ARM tree or do you want me/Ingo to take it?
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