From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751941AbeDERTZ (ORCPT ); Thu, 5 Apr 2018 13:19:25 -0400 Received: from merlin.infradead.org ([205.233.59.134]:59090 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751359AbeDERTX (ORCPT ); Thu, 5 Apr 2018 13:19:23 -0400 Date: Thu, 5 Apr 2018 19:19:12 +0200 From: Peter Zijlstra To: Will Deacon 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: <20180405171912.GE4082@hirez.programming.kicks-ass.net> References: <1522947547-24081-1-git-send-email-will.deacon@arm.com> <1522947547-24081-4-git-send-email-will.deacon@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1522947547-24081-4-git-send-email-will.deacon@arm.com> User-Agent: Mutt/1.9.3 (2018-01-21) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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). From mboxrd@z Thu Jan 1 00:00:00 1970 From: peterz@infradead.org (Peter Zijlstra) Date: Thu, 5 Apr 2018 19:19:12 +0200 Subject: [PATCH 03/10] locking/qspinlock: Kill cmpxchg loop when claiming lock from head of queue In-Reply-To: <1522947547-24081-4-git-send-email-will.deacon@arm.com> References: <1522947547-24081-1-git-send-email-will.deacon@arm.com> <1522947547-24081-4-git-send-email-will.deacon@arm.com> Message-ID: <20180405171912.GE4082@hirez.programming.kicks-ass.net> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 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).