From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752079AbeDFKyF (ORCPT ); Fri, 6 Apr 2018 06:54:05 -0400 Received: from foss.arm.com ([217.140.101.70]:35008 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751454AbeDFKyE (ORCPT ); Fri, 6 Apr 2018 06:54:04 -0400 Date: Fri, 6 Apr 2018 11:54:17 +0100 From: Will Deacon To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, mingo@kernel.org, boqun.feng@gmail.com, paulmck@linux.vnet.ibm.com, catalin.marinas@arm.com, Waiman Long Subject: Re: [PATCH 03/10] locking/qspinlock: Kill cmpxchg loop when claiming lock from head of queue Message-ID: <20180406105417.GA27619@arm.com> References: <1522947547-24081-1-git-send-email-will.deacon@arm.com> <1522947547-24081-4-git-send-email-will.deacon@arm.com> <20180405171912.GE4082@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180405171912.GE4082@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 05, 2018 at 07:19:12PM +0200, Peter Zijlstra wrote: > On Thu, Apr 05, 2018 at 05:59:00PM +0100, Will Deacon wrote: > > + > > + /* In the PV case we might already have _Q_LOCKED_VAL set */ > > + if ((val & _Q_TAIL_MASK) == tail) { > > /* > > * The smp_cond_load_acquire() call above has provided the > > + * necessary acquire semantics required for locking. > > */ > > old = atomic_cmpxchg_relaxed(&lock->val, val, _Q_LOCKED_VAL); > > if (old == val) > > + goto release; /* No contention */ > > } > > --- a/kernel/locking/qspinlock.c > +++ b/kernel/locking/qspinlock.c > @@ -464,8 +464,7 @@ void queued_spin_lock_slowpath(struct qs > * The smp_cond_load_acquire() call above has provided the > * necessary acquire semantics required for locking. > */ > - old = atomic_cmpxchg_relaxed(&lock->val, val, _Q_LOCKED_VAL); > - if (old == val) > + if (atomic_try_cmpxchg_release(&lock->val, &val, _Q_LOCKED_VAL)) > goto release; /* No contention */ > } > > Does that also work for you? It would generate slightly better code for > x86 (not that it would matter much on this path). Assuming you meant to use atomic_try_cmpxchg_relaxed, then that works for me too. Will