Re: [PATCH tip/sched/core] swait: Remove the lockless swait_active() check in swake_up*()

[Paul E. McKenney]

On Wed, Aug 02, 2017 at 07:12:23PM +0200, Peter Zijlstra wrote:
> 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 <rostedt@xxxxxxxxxxx>
> > > Signed-off-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> > > Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > > Tested-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > 
> > 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 :-)

Heh!  We could use it as a test of Ingo's handling of multiple identical
patches.  ;-)

						Thanx, Paul