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

From: [email-masked] (Paul E. McKenney)
Date: Fri, 28 Jul 2017 07:55:30 -0700

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

>    spin_lock(q->lock)
> 
>                                 *fetch wq->task_list here! *

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

>    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?

>                                  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 <[email-masked]>
> Signed-off-by: Boqun Feng <[email-masked]>

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

Acked-by: Paul E. McKenney <[email-masked]>

> ---
>  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
>