Re: [PATCH] tick/broadcast: Do not set oneshot_mask except was_periodic was true

[Victor Hassan]

On 4/4/2023 8:21 PM, Frederic Weisbecker wrote:
> On Tue, Apr 04, 2023 at 07:37:06PM +0800, Victor Hassan wrote:
> > > Leading to such race:
> > >
> > > * CPU 1 stop its tick, next event is in one hour
> > > * CPU 0 registers new broadcast and sets CPU 1 in tick_broadcast_oneshot_mask
> > > * CPU 1 runs into cpuidle_enter_state(), and tick_broadcast_enter() is ignored because
> > >     the CPU is already in tick_broadcast_oneshot_mask
> >
> > Yes.
> >
> > > * CPU 1 goes to sleep
> > > * CPU 0 runs the broadcast callback, sees that the next timer for CPU 1
> > >     is in one hour, program the broadcast to that deadline
> > > * CPU 1 gets an interrupt that enqueues a new timer expiring in the next jiffy
> > > * CPU 1 don't call tick_broadcast_exit and thus don't remove itself from
> > >     tick_broadcast_oneshot_mask
> >
> > I'm not sure about this... Actually, I believe CPU 1 *will* call
> > tick_broadcast_exit in this condition because I cannot find a limitation on
> > this execution path.
>
> You're right, what I wrote doesn't make sense. Let me try again:
>
> * CPU 1 stop its tick, next event is in one hour. It calls
>    tick_broadcast_enter() and goes to sleep.
>    
> * CPU 1 gets an interrupt that enqueues a new timer expiring in the next jiffy
>    (note it's not yet actually programmed in the tick device)
>    
> * CPU 1 call tick_broadcast_exit().
>
> * CPU 0 registers new broadcast device and sets CPU 1 in tick_broadcast_oneshot_mask
>
> * CPU 0 runs the broadcast callback, sees that the next timer for CPU 1
>    is in one hour (because the recently enqueued timer for CPU 1 hasn't been programmed
>    yet), so it programs the broadcast to that 1 hour deadline.
>
> * CPU 1 runs tick_nohz_idle_stop_tick() which eventually writes and program
>    dev->next_event to next jiffy
>    
> * CPU 1 runs into cpuidle_enter_state(), and tick_broadcast_enter() is ignored because
>    the CPU is already in tick_broadcast_oneshot_mask, so the dev->next_event
>    change isn't propagated to broadcast.
>
> * CPU 1 goes to sleep for 1 hour.

Hi Frederic,
  Yes, I think that make sense :)


> Does it make more sense? There might be more simple scenario of course.
>
> Thanks.