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

> + }
> 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).

> + 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.

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