4.9-stable review patch. If anyone has any objections, please let me know.
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From: Boqun Feng boqun.feng@gmail.com
commit 35a2897c2a306cca344ca5c0b43416707018f434 upstream.
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 Cc: David Chen david.chen@nutanix.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- kernel/sched/swait.c | 6 ------ 1 file changed, 6 deletions(-)
--- 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_hea 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)) {