Re: Question about kthread_mod_delayed_work() allowed context

[Petr Mladek <pmladek@suse.com>]

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.


> 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