linux-kernel - Re: [PATCH] tick/broadcast: Do not set oneshot_mask except was

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Date:   Tue, 4 Apr 2023 14:21:39 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Victor Hassan <victor@...winnertech.com>
Cc:     fweisbec@...il.com, tglx@...utronix.de, mingo@...nel.org,
        jindong.yue@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tick/broadcast: Do not set oneshot_mask except
 was_periodic was true

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.

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

Thanks.