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Subject[PATCH 4.9 17/32] sched/wait: Remove the lockless swait_active() check in swake_up*()
Date
4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

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)) {

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