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SubjectRe: [PATCH] swait: add missing barrier to swake_up
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

---- ----
__call_rcu_core() {

need_wake = __rcu_start_gp() {
rcu_start_gp_advanced() {
gp_flags = FLAG_INIT

rcu_gp_kthread() {
gp_flags & FLAG_INIT) {

*fetch wq->task_list here! *

list_add(wq->task_list, q->task_list)

*fetch old value of gp_flags here *


rcu_gp_kthread_wake() {
swake_up(wq) {
swait_active(wq) {

} * return false *

if (condition) * false *

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 <>
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);
raw_spin_unlock_irqrestore(&q->lock, flags);
@@ -51,9 +48,6 @@ void swake_up_all(struct swait_queue_head *q)
struct swait_queue *curr;

- if (!swait_active(q))
- return;
list_splice_init(&q->task_list, &tmp);
while (!list_empty(&tmp)) {
 \ /
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