Messages in this thread | | | Date | Fri, 10 Aug 2012 12:10:42 -0400 (EDT) | From | Nicolas Pitre <> | Subject | Re: [PATCH] mutex: place lock in contended state after fastpath_lock failure |
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On Fri, 10 Aug 2012, Will Deacon wrote:
> 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: Nicolas Pitre <nico@fluxnic.net> > Cc: <stable@vger.kernel.org> > Signed-off-by: Will Deacon <will.deacon@arm.com>
Reviewed-by: Nicolas Pitre <nico@linaro.org>
> --- > > Nico: Can I add your S-o-B to this please? Also, preliminary benchmarks > are now showing a slight performance improvement on A15 if I use > the -dec variant rather than -xchg. I'll follow up with a patch > once I've got more numbers.
Good.
> > include/asm-generic/mutex-xchg.h | 11 +++++++++-- > 1 files changed, 9 insertions(+), 2 deletions(-) > > diff --git a/include/asm-generic/mutex-xchg.h b/include/asm-generic/mutex-xchg.h > index 580a6d3..c04e0db 100644 > --- a/include/asm-generic/mutex-xchg.h > +++ b/include/asm-generic/mutex-xchg.h > @@ -26,7 +26,13 @@ static inline void > __mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *)) > { > if (unlikely(atomic_xchg(count, 0) != 1)) > - fail_fn(count); > + /* > + * We failed to acquire the lock, so mark it contended > + * to ensure that any waiting tasks are woken up by the > + * unlock slow path. > + */ > + if (likely(atomic_xchg(count, -1) != 1)) > + fail_fn(count); > } > > /** > @@ -43,7 +49,8 @@ static inline int > __mutex_fastpath_lock_retval(atomic_t *count, int (*fail_fn)(atomic_t *)) > { > if (unlikely(atomic_xchg(count, 0) != 1)) > - return fail_fn(count); > + if (likely(atomic_xchg(count, -1) != 1)) > + return fail_fn(count); > return 0; > } > > -- > 1.7.4.1 >
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