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:

quoted

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.

quoted

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.

quoted

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