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Message-ID: <87857a82-99af-a941-9374-3bbe373ff74a@allwinnertech.com>
Date: Fri, 7 Apr 2023 14:51:12 +0800
From: Victor Hassan <victor@...winnertech.com>
To: Frederic Weisbecker <frederic@...nel.org>
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 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.
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