workqueue: reorganize try_to_grab_pending() and __cancel_timer_work()

* Use bool @is_dwork instead of @timer and let try_to_grab_pending()
  use to_delayed_work() to determine the delayed_work address.

* Move timer handling from __cancel_work_timer() to
  try_to_grab_pending().

* Make try_to_grab_pending() use -EAGAIN instead of -1 for
  busy-looping and drop the ret local variable.

* Add proper function comment to try_to_grab_pending().

This makes the code a bit easier to understand and will ease further
changes.  This patch doesn't make any functional change.

v2: Use @is_dwork instead of @timer.
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -1004,15 +1004,33 @@ static void cwq_dec_nr_in_flight(struct cpu_workqueue_struct *cwq, int color,
 		complete(&cwq->wq->first_flusher->done);
 }
 
-/*
- * Upon a successful return (>= 0), the caller "owns" WORK_STRUCT_PENDING bit,
- * so this work can't be re-armed in any way.
+/**
+ * try_to_grab_pending - steal work item from worklist
+ * @work: work item to steal
+ * @is_dwork: @work is a delayed_work
+ *
+ * Try to grab PENDING bit of @work.  This function can handle @work in any
+ * stable state - idle, on timer or on worklist.  Return values are
+ *
+ *  1		if @work was pending and we successfully stole PENDING
+ *  0		if @work was idle and we claimed PENDING
+ *  -EAGAIN	if PENDING couldn't be grabbed at the moment, safe to busy-retry
+ *
+ * On >= 0 return, the caller owns @work's PENDING bit.
  */
-static int try_to_grab_pending(struct work_struct *work)
+static int try_to_grab_pending(struct work_struct *work, bool is_dwork)
 {
 	struct global_cwq *gcwq;
-	int ret = -1;
 
+	/* try to steal the timer if it exists */
+	if (is_dwork) {
+		struct delayed_work *dwork = to_delayed_work(work);
+
+		if (likely(del_timer(&dwork->timer)))
+			return 1;
+	}
+
+	/* try to claim PENDING the normal way */
 	if (!test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work)))
 		return 0;
 
@@ -1022,7 +1040,7 @@ static int try_to_grab_pending(struct work_struct *work)
 	 */
 	gcwq = get_work_gcwq(work);
 	if (!gcwq)
-		return ret;
+		return -EAGAIN;
 
 	spin_lock_irq(&gcwq->lock);
 	if (!list_empty(&work->entry)) {
@@ -1038,12 +1056,14 @@ static int try_to_grab_pending(struct work_struct *work)
 			cwq_dec_nr_in_flight(get_work_cwq(work),
 				get_work_color(work),
 				*work_data_bits(work) & WORK_STRUCT_DELAYED);
-			ret = 1;
+
+			spin_unlock_irq(&gcwq->lock);
+			return 1;
 		}
 	}
 	spin_unlock_irq(&gcwq->lock);
 
-	return ret;
+	return -EAGAIN;
 }
 
 /**
@@ -2817,15 +2837,12 @@ bool flush_work_sync(struct work_struct *work)
 }
 EXPORT_SYMBOL_GPL(flush_work_sync);
 
-static bool __cancel_work_timer(struct work_struct *work,
-				struct timer_list* timer)
+static bool __cancel_work_timer(struct work_struct *work, bool is_dwork)
 {
 	int ret;
 
 	do {
-		ret = (timer && likely(del_timer(timer)));
-		if (!ret)
-			ret = try_to_grab_pending(work);
+		ret = try_to_grab_pending(work, is_dwork);
 		wait_on_work(work);
 	} while (unlikely(ret < 0));
 
@@ -2853,7 +2870,7 @@ static bool __cancel_work_timer(struct work_struct *work,
  */
 bool cancel_work_sync(struct work_struct *work)
 {
-	return __cancel_work_timer(work, NULL);
+	return __cancel_work_timer(work, false);
 }
 EXPORT_SYMBOL_GPL(cancel_work_sync);
 
@@ -2914,7 +2931,7 @@ EXPORT_SYMBOL(flush_delayed_work_sync);
  */
 bool cancel_delayed_work_sync(struct delayed_work *dwork)
 {
-	return __cancel_work_timer(&dwork->work, &dwork->timer);
+	return __cancel_work_timer(&dwork->work, true);
 }
 EXPORT_SYMBOL(cancel_delayed_work_sync);