Re: RCU lockup issues when CONFIG_SOFTLOCKUP_DETECTOR=n - any one else seeing this?

On Fri, Jul 28, 2017 at 07:55:30AM -0700, Paul E. McKenney wrote:

On Fri, Jul 28, 2017 at 08:54:16PM +0800, Boqun Feng wrote:

quoted

Hi Jonathan,

FWIW, there is wakeup-missing issue in swake_up() and swake_up_all():

	https://marc.info/?l=linux-kernel&m=149750022019663

and RCU begins to use swait/wake last year, so I thought this could be
relevant.

Could you try the following patch and see if it works? Thanks.

Regards,
Boqun

------------------>8
Subject: [PATCH] swait: 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) {

So the idea is that we get the old value of ->gp_flags here, correct?

quoted

   spin_lock(q->lock)

                                *fetch wq->task_list here! *

And the above fetch is really part of the swait_active() called out
below, right?

quoted

   list_add(wq->task_list, q->task_list)
   spin_unlock(q->lock);

   *fetch old value of gp_flags here *

And here we fetch the old value of ->gp_flags again, this time under
the lock, right?

quoted

                                 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 <redacted>

Even though Jonathan's testing indicates that it didn't fix this
particular problem:

Acked-by: Paul E. McKenney <redacted>

And while we are at it:

Tested-by: Paul E. McKenney <redacted>

quoted

---
 kernel/sched/swait.c | 6 ------
 1 file changed, 6 deletions(-)

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);
 	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)) {

-- 
2.13.0