cancel_work_sync — block until a work_struct's callback has terminated
the work which is to be flushed
Returns true if work was pending.
cancel_work_sync will cancel the work if it is queued. If the work's callback appears to be running, cancel_work_sync will block until it has completed.
It is possible to use this function if the work re-queues itself. It can cancel the work even if it migrates to another workqueue, however in that case it only guarantees that work->func has completed on the last queued workqueue.
cancel_work_sync(delayed_work->work) should be used only if ->timer is not pending, otherwise it goes into a busy-wait loop until the timer expires.
The caller must ensure that workqueue_struct on which this work was last queued can't be destroyed before this function returns.