Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758840Ab2HISJF (ORCPT ); Thu, 9 Aug 2012 14:09:05 -0400 Received: from relais.videotron.ca ([24.201.245.36]:61919 "EHLO relais.videotron.ca" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751477Ab2HISJD (ORCPT ); Thu, 9 Aug 2012 14:09:03 -0400 MIME-version: 1.0 Content-transfer-encoding: 7BIT Content-type: TEXT/PLAIN; CHARSET=US-ASCII Date: Thu, 09 Aug 2012 14:09:02 -0400 (EDT) From: Nicolas Pitre To: Will Deacon Cc: "linux-kernel@vger.kernel.org" , Peter Zijlstra , Ingo Molnar , Thomas Gleixner , Chris Mason , Arnd Bergmann , "linux-arm-kernel@lists.infradead.org" Subject: Re: RFC: mutex: hung tasks on SMP platforms with asm-generic/mutex-xchg.h In-reply-to: <20120809175019.GE18486@mudshark.cambridge.arm.com> Message-id: References: <20120807115647.GA12828@mudshark.cambridge.arm.com> <20120809144953.GC18486@mudshark.cambridge.arm.com> <20120809175019.GE18486@mudshark.cambridge.arm.com> User-Agent: Alpine 2.02 (LFD 1266 2009-07-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2163 Lines: 52 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: > > > Yes, that looks fine. I'd remove that if (prev < 0) entirely though. > > > We'll just swap a 0 for a 0 if the count wasn't < 0, or a 0 for a 1 if > > > the mutex just got unlocked which is also fine. This is especially > > > beneficial when a native xchg processor instruction is used. > > > > In fact, this suggestion isn't entirely correct either. The inner xchg > > in this case should be -1 and not 'count'. If 'count' is 0 and the > > mutex becomes contended in the small window between the two xchg's then > > the contention mark would be lost again. > > > > In other words, I think this should look like this: > > > > 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. Nicolas -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/