writeback, cgroup: switch to rcu_work API in inode_switch_wbs()

Inode's wb switching requires two steps divided by an RCU grace period.
It's currently implemented as an RCU callback inode_switch_wbs_rcu_fn(),
which schedules inode_switch_wbs_work_fn() as a work.

Switching to the rcu_work API allows to do the same in a cleaner and
slightly shorter form.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Roman Gushchin <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Acked-by: Tejun Heo <[email protected]>
Acked-by: Dennis Zhou <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Jens Axboe <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

fs/fs-writeback.c

@@ -335,8 +335,7 @@ struct inode_switch_wbs_context {
 	struct inode		*inode;
 	struct bdi_writeback	*new_wb;
 
-	struct rcu_head		rcu_head;
-	struct work_struct	work;
+	struct rcu_work		work;
 };
 
 static void bdi_down_write_wb_switch_rwsem(struct backing_dev_info *bdi)
@@ -352,7 +351,7 @@ static void bdi_up_write_wb_switch_rwsem(struct backing_dev_info *bdi)
 static void inode_switch_wbs_work_fn(struct work_struct *work)
 {
 	struct inode_switch_wbs_context *isw =
-		container_of(work, struct inode_switch_wbs_context, work);
+		container_of(to_rcu_work(work), struct inode_switch_wbs_context, work);
 	struct inode *inode = isw->inode;
 	struct backing_dev_info *bdi = inode_to_bdi(inode);
 	struct address_space *mapping = inode->i_mapping;
@@ -469,16 +468,6 @@ static void inode_switch_wbs_work_fn(struct work_struct *work)
 	atomic_dec(&isw_nr_in_flight);
 }
 
-static void inode_switch_wbs_rcu_fn(struct rcu_head *rcu_head)
-{
-	struct inode_switch_wbs_context *isw = container_of(rcu_head,
-				struct inode_switch_wbs_context, rcu_head);
-
-	/* needs to grab bh-unsafe locks, bounce to work item */
-	INIT_WORK(&isw->work, inode_switch_wbs_work_fn);
-	queue_work(isw_wq, &isw->work);
-}
-
 /**
  * inode_switch_wbs - change the wb association of an inode
  * @inode: target inode
@@ -536,7 +525,8 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
 	 * lock so that stat transfer can synchronize against them.
 	 * Let's continue after I_WB_SWITCH is guaranteed to be visible.
 	 */
-	call_rcu(&isw->rcu_head, inode_switch_wbs_rcu_fn);
+	INIT_RCU_WORK(&isw->work, inode_switch_wbs_work_fn);
+	queue_rcu_work(isw_wq, &isw->work);
 	return;
 
 out_free: