workqueue: protect wq->nr_drainers and ->flags with wq->mutex

We're expanding wq->mutex to cover all fields specific to each
workqueue with the end goal of replacing pwq_lock which will make
locking simpler and easier to understand.

wq->nr_drainers and ->flags are specific to each workqueue.  Protect
->nr_drainers and ->flags with wq->mutex instead of pool_mutex.

tj: Rebased on top of the current dev branch.  Updated description.
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -226,7 +226,7 @@ struct wq_device;
  * the appropriate worker_pool through its pool_workqueues.
  */
 struct workqueue_struct {
-	unsigned int		flags;		/* PL: WQ_* flags */
+	unsigned int		flags;		/* WQ: WQ_* flags */
 	struct pool_workqueue __percpu *cpu_pwqs; /* I: per-cpu pwq's */
 	struct list_head	pwqs;		/* WR: all pwqs of this wq */
 	struct list_head	list;		/* PL: list of all workqueues */
@@ -242,7 +242,7 @@ struct workqueue_struct {
 	struct list_head	maydays;	/* MD: pwqs requesting rescue */
 	struct worker		*rescuer;	/* I: rescue worker */
 
-	int			nr_drainers;	/* PL: drain in progress */
+	int			nr_drainers;	/* WQ: drain in progress */
 	int			saved_max_active; /* PW: saved pwq max_active */
 
 #ifdef CONFIG_SYSFS
@@ -2684,10 +2684,10 @@ void drain_workqueue(struct workqueue_struct *wq)
 	 * hotter than drain_workqueue() and already looks at @wq->flags.
 	 * Use __WQ_DRAINING so that queue doesn't have to check nr_drainers.
 	 */
-	mutex_lock(&wq_pool_mutex);
+	mutex_lock(&wq->mutex);
 	if (!wq->nr_drainers++)
 		wq->flags |= __WQ_DRAINING;
-	mutex_unlock(&wq_pool_mutex);
+	mutex_unlock(&wq->mutex);
 reflush:
 	flush_workqueue(wq);
 
@@ -2714,10 +2714,10 @@ void drain_workqueue(struct workqueue_struct *wq)
 
 	local_irq_enable();
 
-	mutex_lock(&wq_pool_mutex);
+	mutex_lock(&wq->mutex);
 	if (!--wq->nr_drainers)
 		wq->flags &= ~__WQ_DRAINING;
-	mutex_unlock(&wq_pool_mutex);
+	mutex_unlock(&wq->mutex);
 }
 EXPORT_SYMBOL_GPL(drain_workqueue);