netdev - Re: Question about kthread_mod_delayed

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Date:   Wed, 12 Feb 2020 21:17:53 +0200
From:   Grygorii Strashko <grygorii.strashko@...com>
To:     Petr Mladek <pmladek@...e.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        <linux-rt-users@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        netdev <netdev@...r.kernel.org>,
        Richard Cochran <richardcochran@...il.com>
Subject: Re: Question about kthread_mod_delayed_work() allowed context



On 12/02/2020 17:41, Petr Mladek wrote:
> On Tue 2020-02-11 12:23:59, Grygorii Strashko wrote:
>> Hi All,
>>
>> I'd like to ask question about allowed calling context for kthread_mod_delayed_work().
>>
>> The comment to kthread_mod_delayed_work() says:
>>
>>   * This function is safe to call from any context including IRQ handler.
>>   * See __kthread_cancel_work() and kthread_delayed_work_timer_fn()
>>   * for details.
>>   */
>>
>> But it has del_timer_sync() inside which seems can't be called from hard_irq context:
>> kthread_mod_delayed_work()
>>    |-__kthread_cancel_work()
>>       |- del_timer_sync()
>> 	|- WARN_ON(in_irq() && !(timer->flags & TIMER_IRQSAFE));
> 
> It is safe because kthread_delayed_work_timer_fn() is IRQ safe.
> Note that it uses raw_spin_lock_irqsave(). It is the reason why
> the timer could have set TIMER_IRQSAFE flag, see
> KTHREAD_DELAYED_WORK_INIT().
> 
> In more details. The timer is either canceled before the callback
> is called. Or it waits for the callback but the callback is safe
> because it can't sleep.

I think, my issue (warning) could be related to the fact that kthread_init_delayed_work()
is used, which seems doesn't set TIMER_IRQSAFE flag.

> 
> 
>> My use case is related to PTP processing using PTP auxiliary worker:
>> (commit d9535cb7b760 ("ptp: introduce ptp auxiliary worker")):
>>   - periodic work A is started and res-schedules itself for every dtX
>>   - on IRQ - the work A need to be scheduled immediately
> 
> This is exactly where kthread_mod_delayed_work() need to be used
> in the IRQ context with 0 delay.
> 
> 
>> Any advice on how to proceed?
>> Can kthread_queue_work() be used even if there is delayed work is
>> scheduled already (in general, don't care if work A will be executed one
>> more time after timer expiration)?
> 
> Yes, it can be used this way. It should behave the same way as
> the workqueue API.
> 
> I am happy that there are more users for this API. I wanted to
> convert more kthreads but it was just falling down in my TODO.
> 
> I hope that I answered all questions. Feel free to ask more
> when in doubts.
> 
> Best Regards,
> Petr
> 

-- 
Best regards,
grygorii