From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753811AbbINHhh (ORCPT ); Mon, 14 Sep 2015 03:37:37 -0400 Received: from smtp2.provo.novell.com ([137.65.250.81]:34585 "EHLO smtp2.provo.novell.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751329AbbINHhg (ORCPT ); Mon, 14 Sep 2015 03:37:36 -0400 From: Davidlohr Bueso To: Peter Zijlstra , Ingo Molnar , Thomas Gleixner Cc: "Paul E. McKenney" , Davidlohr Bueso , linux-kernel@vger.kernel.org, Davidlohr Bueso Subject: [PATCH -tip 2/3] sched/wake_q: Relax to acquire semantics Date: Mon, 14 Sep 2015 00:37:23 -0700 Message-Id: <1442216244-4409-2-git-send-email-dave@stgolabs.net> X-Mailer: git-send-email 2.1.4 In-Reply-To: <1442216244-4409-1-git-send-email-dave@stgolabs.net> References: <1442216244-4409-1-git-send-email-dave@stgolabs.net> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The barrier parings for wake-queues are very straightforward, and thus we can ease the barrier requirements, for archs that support it, for wake_q_add by relying on acquire semantics. As such, (i) we keep the pairing structure/logic and (ii) users, such as mqueues, can continue to rely on a full barrier after the successful [Rmw]. [Another alternative could be to just not try at all and fully downgrade to cmpxchg_relaxed() and rely on users to enable their own synchronization. But controlling this ourselves makes me sleep better at night.] Signed-off-by: Davidlohr Bueso --- kernel/sched/core.c | 12 ++++++------ 1 file changed, 6 insertions(+), 6 deletions(-) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 6ab415a..7567603 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -523,14 +523,14 @@ void wake_q_add(struct wake_q_head *head, struct task_struct *task) struct wake_q_node *node = &task->wake_q; /* - * Atomically grab the task, if ->wake_q is !nil already it means - * its already queued (either by us or someone else) and will get the - * wakeup due to that. + * Atomically grab the task. If ->wake_q is non-nil (failed cmpxchg) + * then the task is already queued (by us or someone else) and will + * get the wakeup due to that. * - * This cmpxchg() implies a full barrier, which pairs with the write - * barrier implied by the wakeup in wake_up_list(). + * Use acquire semantics to add the next pointer, which pairs with the + * write barrier implied by the wakeup in wake_up_list(). */ - if (cmpxchg(&node->next, NULL, WAKE_Q_TAIL)) + if (cmpxchg_acquire(&node->next, NULL, WAKE_Q_TAIL)) return; get_task_struct(task); -- 2.1.4