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.dea...@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 <a...@arndb.de> > Cc: Thomas Gleixner <t...@linutronix.de> > Cc: Chris Mason <chris.ma...@fusionio.com> > Cc: Ingo Molnar <mi...@elte.hu> > Cc: <sta...@vger.kernel.org> > Signed-off-by: Will Deacon <will.dea...@arm.com> > Reviewed-by: Nicolas Pitre <n...@linaro.org>
Acked-by: Peter Zijlstra <a.p.zijls...@chello.nl> Will you carry this through the ARM tree or do you want me/Ingo to take it? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
