Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753214AbdHJMPQ (ORCPT ); Thu, 10 Aug 2017 08:15:16 -0400 Received: from terminus.zytor.com ([65.50.211.136]:43413 "EHLO terminus.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752435AbdHJMPM (ORCPT ); Thu, 10 Aug 2017 08:15:12 -0400 Date: Thu, 10 Aug 2017 05:10:34 -0700 From: tip-bot for Boqun Feng Message-ID: Cc: linux-kernel@vger.kernel.org, paul.gortmaker@windriver.com, rostedt@goodmis.org, mingo@kernel.org, peterz@infradead.org, tglx@linutronix.de, kjlx@templeofstupid.com, boqun.feng@gmail.com, hpa@zytor.com, paulmck@linux.vnet.ibm.com, torvalds@linux-foundation.org Reply-To: paul.gortmaker@windriver.com, rostedt@goodmis.org, mingo@kernel.org, tglx@linutronix.de, peterz@infradead.org, linux-kernel@vger.kernel.org, hpa@zytor.com, torvalds@linux-foundation.org, paulmck@linux.vnet.ibm.com, kjlx@templeofstupid.com, boqun.feng@gmail.com In-Reply-To: <20170615041828.zk3a3sfyudm5p6nl@tardis> References: <20170615041828.zk3a3sfyudm5p6nl@tardis> To: linux-tip-commits@vger.kernel.org Subject: [tip:locking/core] sched/wait: Remove the lockless swait_active() check in swake_up*() Git-Commit-ID: 35a2897c2a306cca344ca5c0b43416707018f434 X-Mailer: tip-git-log-daemon Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset=UTF-8 Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3632 Lines: 104 Commit-ID: 35a2897c2a306cca344ca5c0b43416707018f434 Gitweb: http://git.kernel.org/tip/35a2897c2a306cca344ca5c0b43416707018f434 Author: Boqun Feng AuthorDate: Thu, 15 Jun 2017 12:18:28 +0800 Committer: Ingo Molnar CommitDate: Thu, 10 Aug 2017 12:28:53 +0200 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 --- kernel/sched/swait.c | 6 ------ 1 file changed, 6 deletions(-) diff --git a/kernel/sched/swait.c b/kernel/sched/swait.c index 3d5610d..2227e18 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)) {