From mboxrd@z Thu Jan 1 00:00:00 1970 From: will.deacon@arm.com (Will Deacon) Date: Thu, 9 Aug 2012 19:17:05 +0100 Subject: RFC: mutex: hung tasks on SMP platforms with asm-generic/mutex-xchg.h In-Reply-To: References: <20120807115647.GA12828@mudshark.cambridge.arm.com> <20120809144953.GC18486@mudshark.cambridge.arm.com> <20120809175019.GE18486@mudshark.cambridge.arm.com> Message-ID: <20120809181705.GG18486@mudshark.cambridge.arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Thu, Aug 09, 2012 at 07:09:02PM +0100, Nicolas Pitre wrote: > On Thu, 9 Aug 2012, Will Deacon wrote: > > On Thu, Aug 09, 2012 at 05:57:33PM +0100, Nicolas Pitre wrote: > > > On Thu, 9 Aug 2012, Nicolas Pitre wrote: > > > diff --git a/include/asm-generic/mutex-xchg.h b/include/asm-generic/mutex-xchg.h > > > index 580a6d35c7..44a66c99c8 100644 > > > --- a/include/asm-generic/mutex-xchg.h > > > +++ b/include/asm-generic/mutex-xchg.h > > > @@ -25,8 +25,11 @@ > > > static inline void > > > __mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *)) > > > { > > > - if (unlikely(atomic_xchg(count, 0) != 1)) > > > - fail_fn(count); > > > + if (unlikely(atomic_xchg(count, 0) != 1)) { > > > + /* Mark lock contention explicitly */ > > > + if (likely(atomic_xchg(count, -1) != 1)) > > > + fail_fn(count); > > > + } > > > } > > > > > > /** > > > > Doesn't this mean that we're no longer just swapping 0 for a 0 if the lock > > was taken, therefore needlessly sending the current owner down the slowpath > > on unlock? > > If the lock was taken, this means the count was either 0 or -1. If it > was 1 then we just put a 0 there and we own it. But if the cound was 0 > then we should store -1 instead, which is what the inner xchg does. If > the count was already -1 then we store -1 back. That more closely mimic > what the atomic dec does which is what we want. Ok, I just wasn't sure that marking the lock contended was required when it was previously locked, given that we'll drop into spinning on the owner anyway. I'll add a commit message to the above and re-post if that's ok? Will