On Wed 2015-11-18 23:32:28, Thomas Gleixner wrote:

On Wed, 18 Nov 2015, Petr Mladek wrote:

quoted

timer_pending() checks whether the list of callbacks is empty.
Each callback is removed from the list before it is called,
see call_timer_fn() in __run_timers().

Sometimes we need to make sure that the callback has finished.
For example, if we want to free some resources that are accessed
by the callback.

For this purpose, this patch adds timer_active(). It checks both
the list of callbacks and the running_timer. It takes the base_lock
to see a consistent state.

I plan to use it to implement delayed works in kthread worker.
But I guess that it will have wider use. In fact, I wonder if
timer_pending() is misused in some situations.

Well. That's nice and good. But how will that new function solve
anything? After you drop the lock the state is not longer valid.

If we prevent anyone from setting up the timer and timer_pending()
returns false, we are sure that the timer will stay as is.

For example, I use it in the function try_to_cancel_kthread_work().
Any manipulation with the timer is protected by worker->lock.
If the timer is not pending but still active, I have to drop
the lock and busy wait for the timer callback. See
http://thread.gmane.org/gmane.linux.kernel.mm/141493/focus=141501


Also I wonder if the following usage in
drivers/infiniband/hw/nes/nes_cm.c is safe:

static int mini_cm_dealloc_core(struct nes_cm_core *cm_core)
{
	nes_debug(NES_DBG_CM, "De-Alloc CM Core (%p)\n", cm_core);

	if (!cm_core)
		return -EINVAL;

	barrier();

	if (timer_pending(&cm_core->tcp_timer))
		del_timer(&cm_core->tcp_timer);

	destroy_workqueue(cm_core->event_wq);
	destroy_workqueue(cm_core->disconn_wq);

We destroy the workqueue but the timer callback might still
be in progress and queue new work.


There are many more locations where I see the pattern:

      if (timer_pending())
		del_timer();
      clean_up_stuff();

IMHO, we should use:

      if (timer_active())
		del_timer_sync();
      /* really safe to free stuff */
      clean_up_stuff();

or just

   del_timer_sync();
   clean_up_stuff();


I wonder if timer_pending() is used in more racy scenarios. Or maybe,
I just miss something that makes it all safe.

Thanks,
Petr