Messages in this thread Patch in this message | | | Date | Thu, 10 Aug 2017 05:10:34 -0700 | From | tip-bot for Boqun Feng <> | Subject | [tip:locking/core] sched/wait: Remove the lockless swait_active() check in swake_up*() |
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Commit-ID: 35a2897c2a306cca344ca5c0b43416707018f434 Gitweb: http://git.kernel.org/tip/35a2897c2a306cca344ca5c0b43416707018f434 Author: Boqun Feng <boqun.feng@gmail.com> AuthorDate: Thu, 15 Jun 2017 12:18:28 +0800 Committer: Ingo Molnar <mingo@kernel.org> 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 <rostedt@goodmis.org> Signed-off-by: Boqun Feng <boqun.feng@gmail.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Krister Johansen <kjlx@templeofstupid.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Paul Gortmaker <paul.gortmaker@windriver.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/20170615041828.zk3a3sfyudm5p6nl@tardis Signed-off-by: Ingo Molnar <mingo@kernel.org> --- 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)) {
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