Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751553AbaFDU5J (ORCPT ); Wed, 4 Jun 2014 16:57:09 -0400 Received: from g5t1625.atlanta.hp.com ([15.192.137.8]:14898 "EHLO g5t1625.atlanta.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751010AbaFDU5G (ORCPT ); Wed, 4 Jun 2014 16:57:06 -0400 Message-ID: <1401915420.13877.20.camel@buesod1.americas.hpqcorp.net> Subject: Re: [RFC PATCH 1/3] locking/mutex: Try to acquire mutex only if it is unlocked From: Davidlohr Bueso To: Peter Zijlstra Cc: Jason Low , mingo@kernel.org, tglx@linutronix.de, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, paulmck@linux.vnet.ibm.com, tim.c.chen@linux.intel.com, peter@hurleysoftware.com, riel@redhat.com, hpa@zytor.com, walken@google.com, Waiman.Long@hp.com, aswin@hp.com, scott.norton@hp.com, chegu_vinod@hp.com Date: Wed, 04 Jun 2014 13:57:00 -0700 In-Reply-To: <20140604194322.GN13930@laptop.programming.kicks-ass.net> References: <1401908911-8947-1-git-send-email-jason.low2@hp.com> <1401908911-8947-2-git-send-email-jason.low2@hp.com> <20140604194322.GN13930@laptop.programming.kicks-ass.net> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.6.4 (3.6.4-3.fc18) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2014-06-04 at 21:43 +0200, Peter Zijlstra wrote: > On Wed, Jun 04, 2014 at 12:08:29PM -0700, Jason Low wrote: > > Upon entering the slowpath in __mutex_lock_common(), we try once more > > to acquire the mutex. We only try to acquire it if MUTEX_SHOW_NO_WAITER > > (lock->count >= 0) is true in order to avoid using the atomic xchg() > > operation whenever it is not necessary. However, we really only need > > to try to acquire if the mutex is free (lock->count == 1). > > > > This patch changes it so that we only try-acquire the mutex upon > > entering the slowpath if it is unlocked, rather than if there are > > no waiters. This helps further reduce unncessary atomic xchg() > > operations. Furthermore, this patch introduces and uses a new > > MUTEX_IS_UNLOCKED() macro to improve readbability. > > > > Signed-off-by: Jason Low > > --- > > kernel/locking/mutex.c | 10 ++++++---- > > 1 files changed, 6 insertions(+), 4 deletions(-) > > > > diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c > > index bc73d33..0925968 100644 > > --- a/kernel/locking/mutex.c > > +++ b/kernel/locking/mutex.c > > @@ -48,9 +48,10 @@ > > > > /* > > * A negative mutex count indicates that waiters are sleeping waiting for the > > - * mutex. > > + * mutex, and a count of one indicates the mutex is unlocked. > > */ > > #define MUTEX_SHOW_NO_WAITER(mutex) (atomic_read(&(mutex)->count) >= 0) > > +#define MUTEX_IS_UNLOCKED(mutex) (atomic_read(&(mutex)->count) == 1) > > So I recently saw that MUTEX_SHOW_NO_WAITER thing and cried a little; > and now you're adding more of that same nonsense. > > Please make them inline functions, also can we rename the SHOW_NO_WAITER > thing, because its not at all clear to me wtf it does; should it be > called: mutex_no_waiters() or somesuch? Agreed. In addition, how about the following helpers instead: - mutex_is_unlocked() : count > 0 - mutex_has_waiters() : count < 0, or list_empty(->wait_list) If combined with the negation, I think that pretty much covers the whole life-cycle of a mutex. The mutex.c file is full of places where we can replace crude atomic_reads with these. Thanks, Davidlohr -- 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/