On Wed, Aug 02, 2017 at 09:07:13AM -0700, Paul E. McKenney wrote:
> On Sun, Jul 30, 2017 at 09:47:35PM +0800, Boqun Feng wrote:
> > 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 <[email protected]>
> > Signed-off-by: Boqun Feng <[email protected]>
> > Acked-by: Paul E. McKenney <[email protected]>
> > Tested-by: Paul E. McKenney <[email protected]>
>
> Hearing no objections but not hearing anything else, either, I have
> queued this for v4.14. If someone else would rather queue it, please
> let me know.
I have it too. Lets see who can get it into -tip first :-)
