On Wed 2015-11-18 23:32:28, Thomas Gleixner wrote: > On Wed, 18 Nov 2015, Petr Mladek wrote: > > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
