Re: Question about kthread_mod_delayed_work() allowed context

On Wed 2020-02-12 21:17:53, Grygorii Strashko wrote:

On 12/02/2020 17:41, Petr Mladek wrote:

quoted

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.

Great catch!

It is a bug. Would you like to send the fix for
kthread_init_delayed_work() or would you prefer me doing so?

Best Regards,
Petr