Re: Question about kthread_mod_delayed_work() allowed context

[Grygorii Strashko]

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