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

[Victor Hassan]

On 4/7/2023 2:51 PM, Victor Hassan wrote:
> 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 :)

Hi Frederic,
    If we have reached a consensus, may I add "Reviewed-by: Frederic" 
in the next patch?
> > Does it make more sense? There might be more simple scenario of course.
> >
> > Thanks.