Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752013AbdIALQg (ORCPT ); Fri, 1 Sep 2017 07:16:36 -0400 Received: from bombadil.infradead.org ([65.50.211.133]:51844 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751811AbdIALQe (ORCPT ); Fri, 1 Sep 2017 07:16:34 -0400 Date: Fri, 1 Sep 2017 13:16:28 +0200 From: Peter Zijlstra To: Nicholas Piggin Cc: Ingo Molnar , Paul McKenney , Linus Torvalds , linux-kernel@vger.kernel.org, Boqun Feng Subject: Re: [PATCH] swait: add missing barrier to swake_up Message-ID: <20170901111628.23svdvyobzpdcfkk@hirez.programming.kicks-ass.net> References: <20170901061450.1450-1-npiggin@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170901061450.1450-1-npiggin@gmail.com> User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4121 Lines: 110 On Fri, Sep 01, 2017 at 04:14:50PM +1000, Nicholas Piggin wrote: > swake_up and swake_up_all test the swaitqueue outside the lock, > but they are missing the barrier that would ensure visibility > of a previous store that sets the wakeup condition with the > load that tests the swaitqueue. This could lead to a lost wakeup > if there is memory reordering. Fix this as prescribed by the > waitqueue_active comments. The below commit is in tip.. --- commit 35a2897c2a306cca344ca5c0b43416707018f434 Author: Boqun Feng Date: Thu Jun 15 12:18:28 2017 +0800 sched/wait: Remove the lockless swait_active() check in swake_up*() Steven Rostedt reported a potential race in RCU core because of swake_up(): CPU0 CPU1 ---- ---- __call_rcu_core() { spin_lock(rnp_root) need_wake = __rcu_start_gp() { rcu_start_gp_advanced() { gp_flags = FLAG_INIT } } rcu_gp_kthread() { swait_event_interruptible(wq, gp_flags & FLAG_INIT) { spin_lock(q->lock) *fetch wq->task_list here! * list_add(wq->task_list, q->task_list) spin_unlock(q->lock); *fetch old value of gp_flags here * spin_unlock(rnp_root) rcu_gp_kthread_wake() { swake_up(wq) { swait_active(wq) { list_empty(wq->task_list) } * return false * if (condition) * false * schedule(); In this case, a wakeup is missed, which could cause the rcu_gp_kthread waits for a long time. The reason of this is that we do a lockless swait_active() check in swake_up(). To fix this, we can either 1) add a smp_mb() in swake_up() before swait_active() to provide the proper order or 2) simply remove the swait_active() in swake_up(). The solution 2 not only fixes this problem but also keeps the swait and wait API as close as possible, as wake_up() doesn't provide a full barrier and doesn't do a lockless check of the wait queue either. Moreover, there are users already using swait_active() to do their quick checks for the wait queues, so it make less sense that swake_up() and swake_up_all() do this on their own. This patch then removes the lockless swait_active() check in swake_up() and swake_up_all(). Reported-by: Steven Rostedt Signed-off-by: Boqun Feng Signed-off-by: Peter Zijlstra (Intel) Cc: Krister Johansen Cc: Linus Torvalds Cc: Paul E. McKenney Cc: Paul Gortmaker Cc: Peter Zijlstra Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/20170615041828.zk3a3sfyudm5p6nl@tardis Signed-off-by: Ingo Molnar diff --git a/kernel/sched/swait.c b/kernel/sched/swait.c index 3d5610dcce11..2227e183e202 100644 --- a/kernel/sched/swait.c +++ b/kernel/sched/swait.c @@ -33,9 +33,6 @@ void swake_up(struct swait_queue_head *q) { unsigned long flags; - if (!swait_active(q)) - return; - raw_spin_lock_irqsave(&q->lock, flags); swake_up_locked(q); raw_spin_unlock_irqrestore(&q->lock, flags); @@ -51,9 +48,6 @@ void swake_up_all(struct swait_queue_head *q) struct swait_queue *curr; LIST_HEAD(tmp); - if (!swait_active(q)) - return; - raw_spin_lock_irq(&q->lock); list_splice_init(&q->task_list, &tmp); while (!list_empty(&tmp)) {