Subject: Re: [PATCH] powerpc: irq work racing with timer interrupt can result in timer interrupt hang

From: Preeti U Murthy <[email-masked]>
Date: Sun, 11 May 2014 14:37:33 +0530

On 05/11/2014 02:33 PM, Benjamin Herrenschmidt wrote:
> On Sun, 2014-05-11 at 14:13 +0530, Preeti U Murthy wrote:
>>
>> Isn't this patch required too?
>>
>> @@ -503,12 +503,13 @@ void __timer_interrupt(void)
>>                 now = *next_tb - now;
>>                 if (now <= DECREMENTER_MAX)
>>                         set_dec((int)now);
>> -               /* We may have raced with new irq work */
>> -               if (test_irq_work_pending())
>> -                       set_dec(1);
>>                 __get_cpu_var(irq_stat).timer_irqs_others++;
>>         }
>>
>> +       /* We may have raced with new irq work */
>> +       if (test_irq_work_pending())
>> +               set_dec(1);
>> +
>>
>> The event_handler cannot be relied upon to call
>> decrementer_set_next_event() all the time. This is in the case where
>> there are no pending timers. In that case we need to have the check on
>> irq work pending at the end of __timer_interrupt() no?
> 
> I don't think we need to move the test no. If there's a pending
> irq_work, at that point, it will have done set_dec when being queued up.
> So we only care about cases where we might change the decrementer.
> 
> If the event handler doesn't call decrementer_set_next_event() then
> nothing will modify the decrementer and it will still trigger soon.

Hmm ok. Then Anton's patch covers all cases :)

Thanks!

Reviewed-by: Preeti U Murthy <[email-masked]>

Regards
Preeti U Murthy
> Cheers,
> Ben.