[PATCH v3 wq/for-4.0-fixes] workqueue: fix hang involving racing cancel[_delayed]_work_sync()'s for PREEMPT_NONE

cancel[_delayed]_work_sync() are implemented using __cancel_work_timer() which grabs the PENDING bit using try_to_grab_pending() and then flushes the work item with PENDING set to prevent the on-going execution of the work item from requeueing itself. try_to_grab_pending() can always grab PENDING

[PATCH v3 wq/for-4.0-fixes] workqueue: fix hang involving racing cancel[_delayed]_work_sync()'s for PREEMPT_NONE

cancel[_delayed]_work_sync() are implemented using __cancel_work_timer() which grabs the PENDING bit using try_to_grab_pending() and then flushes the work item with PENDING set to prevent the on-going execution of the work item from requeueing itself. try_to_grab_pending() can always grab PENDING