make cancel_xxx_work_sync() return a boolean

Change cancel_work_sync() and cancel_delayed_work_sync() to return a boolean
indicating whether the work was actually cancelled.  A zero return value means
that the work was not pending/queued.

Without that kind of change it is not possible to avoid flush_workqueue()
sometimes, see the next patch as an example.

Also, this patch unifies both functions and kills the (unlikely) busy-wait
loop.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Jarek Poplawski <jarkao2@o2.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/workqueue.h

@@ -148,7 +148,7 @@ extern int keventd_up(void);
 extern void init_workqueues(void);
 int execute_in_process_context(work_func_t fn, struct execute_work *);
 
-extern void cancel_work_sync(struct work_struct *work);
+extern int cancel_work_sync(struct work_struct *work);
 
 /*
  * Kill off a pending schedule_delayed_work().  Note that the work callback
@@ -166,7 +166,7 @@ static inline int cancel_delayed_work(struct delayed_work *work)
 	return ret;
 }
 
-extern void cancel_delayed_work_sync(struct delayed_work *work);
+extern int cancel_delayed_work_sync(struct delayed_work *work);
 
 /* Obsolete. use cancel_delayed_work_sync() */
 static inline

kernel/workqueue.c

@@ -382,16 +382,16 @@ void fastcall flush_workqueue(struct workqueue_struct *wq)
 EXPORT_SYMBOL_GPL(flush_workqueue);
 
 /*
- * Upon a successful return, the caller "owns" WORK_STRUCT_PENDING bit,
+ * Upon a successful return (>= 0), the caller "owns" WORK_STRUCT_PENDING bit,
  * so this work can't be re-armed in any way.
  */
 static int try_to_grab_pending(struct work_struct *work)
 {
 	struct cpu_workqueue_struct *cwq;
-	int ret = 0;
+	int ret = -1;
 
 	if (!test_and_set_bit(WORK_STRUCT_PENDING, work_data_bits(work)))
-		return 1;
+		return 0;
 
 	/*
 	 * The queueing is in progress, or it is already queued. Try to
@@ -457,10 +457,28 @@ static void wait_on_work(struct work_struct *work)
 		wait_on_cpu_work(per_cpu_ptr(wq->cpu_wq, cpu), work);
 }
 
+static int __cancel_work_timer(struct work_struct *work,
+				struct timer_list* timer)
+{
+	int ret;
+
+	do {
+		ret = (timer && likely(del_timer(timer)));
+		if (!ret)
+			ret = try_to_grab_pending(work);
+		wait_on_work(work);
+	} while (unlikely(ret < 0));
+
+	work_clear_pending(work);
+	return ret;
+}
+
 /**
  * cancel_work_sync - block until a work_struct's callback has terminated
  * @work: 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.
@@ -476,12 +494,9 @@ static void wait_on_work(struct work_struct *work)
  * The caller must ensure that workqueue_struct on which this work was last
  * queued can't be destroyed before this function returns.
  */
-void cancel_work_sync(struct work_struct *work)
+int cancel_work_sync(struct work_struct *work)
 {
-	while (!try_to_grab_pending(work))
-		cpu_relax();
-	wait_on_work(work);
-	work_clear_pending(work);
+	return __cancel_work_timer(work, NULL);
 }
 EXPORT_SYMBOL_GPL(cancel_work_sync);
 
@@ -489,16 +504,14 @@ EXPORT_SYMBOL_GPL(cancel_work_sync);
  * cancel_delayed_work_sync - reliably kill off a delayed work.
  * @dwork: the delayed work struct
  *
+ * Returns true if @dwork was pending.
+ *
  * It is possible to use this function if @dwork rearms itself via queue_work()
  * or queue_delayed_work(). See also the comment for cancel_work_sync().
  */
-void cancel_delayed_work_sync(struct delayed_work *dwork)
+int cancel_delayed_work_sync(struct delayed_work *dwork)
 {
-	while (!del_timer(&dwork->timer) &&
-	       !try_to_grab_pending(&dwork->work))
-		cpu_relax();
-	wait_on_work(&dwork->work);
-	work_clear_pending(&dwork->work);
+	return __cancel_work_timer(&dwork->work, &dwork->timer);
 }
 EXPORT_SYMBOL(cancel_delayed_work_sync);