Re: [PATCH] time/tick-broadcast: Fix tick_broadcast_offline() lockdep complaint

[Paul E. McKenney]

On Tue, Jun 25, 2019 at 01:12:23AM +0200, Frederic Weisbecker wrote:
> On Fri, Jun 21, 2019 at 04:46:02PM -0700, Paul E. McKenney wrote:
> > @@ -3097,13 +3126,21 @@ static void sched_tick_remote(struct work_struct *work)
> >  	/*
> >  	 * Run the remote tick once per second (1Hz). This arbitrary
> >  	 * frequency is large enough to avoid overload but short enough
> > -	 * to keep scheduler internal stats reasonably up to date.
> > +	 * to keep scheduler internal stats reasonably up to date.  But
> > +	 * first update state to reflect hotplug activity if required.
> >  	 */
> > +	os = atomic_read(&twork->state);
> > +	if (os) {
> > +		WARN_ON_ONCE(os != TICK_SCHED_REMOTE_OFFLINING);
> > +		if (atomic_inc_not_zero(&twork->state))
> > +			return;
> 
> Using inc makes me a bit nervous here. If we do so, we should somewhow
> make sure that we never exceed a value higher than TICK_SCHED_REMOTE_OFFLINE
> by accident.
> 
> atomic_xchg() is probably a bit costlier but also safer as it allows
> us to check both the old and the new value. That path shouldn't be critically fast
> after all.

It would need to be cmpxchg() to avoid messing with the state if
the state were somehow TICK_SCHED_REMOTE_RUNNING, right?

> > +	}
> >  	queue_delayed_work(system_unbound_wq, dwork, HZ);
> >  }
> >  
> >  static void sched_tick_start(int cpu)
> >  {
> > +	int os;
> >  	struct tick_work *twork;
> >  
> >  	if (housekeeping_cpu(cpu, HK_FLAG_TICK))
> > @@ -3112,15 +3149,20 @@ static void sched_tick_start(int cpu)
> >  	WARN_ON_ONCE(!tick_work_cpu);
> >  
> >  	twork = per_cpu_ptr(tick_work_cpu, cpu);
> > -	twork->cpu = cpu;
> > -	INIT_DELAYED_WORK(&twork->work, sched_tick_remote);
> > -	queue_delayed_work(system_unbound_wq, &twork->work, HZ);
> > +	os = atomic_xchg(&twork->state, TICK_SCHED_REMOTE_RUNNING);
> > +	WARN_ON_ONCE(os == TICK_SCHED_REMOTE_RUNNING);
> 
> See if we use atomic_inc(), we would need to also WARN(os > TICK_SCHED_REMOTE_OFFLINE).

How about if I put that WARN() between the atomic_inc_not_zero() and
the return, presumably also adding braces?

							Thanx, Paul

> > +	if (os == TICK_SCHED_REMOTE_OFFLINE) {
> > +		twork->cpu = cpu;
> > +		INIT_DELAYED_WORK(&twork->work, sched_tick_remote);
> > +		queue_delayed_work(system_unbound_wq, &twork->work, HZ);
> > +	}
> >  }
> 
> Thanks.
>