bdi: Make wb->bdi a proper reference

Make wb->bdi a proper refcounted reference to bdi for all bdi_writeback
structures except for the one embedded inside struct backing_dev_info.
That will allow us to simplify bdi unregistration.
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>

mm/backing-dev.c

@@ -294,6 +294,8 @@ static int wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi,
 
 	memset(wb, 0, sizeof(*wb));
 
+	if (wb != &bdi->wb)
+		bdi_get(bdi);
 	wb->bdi = bdi;
 	wb->last_old_flush = jiffies;
 	INIT_LIST_HEAD(&wb->b_dirty);
@@ -314,8 +316,10 @@ static int wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi,
 	wb->dirty_sleep = jiffies;
 
 	wb->congested = wb_congested_get_create(bdi, blkcg_id, gfp);
-	if (!wb->congested)
-		return -ENOMEM;
+	if (!wb->congested) {
+		err = -ENOMEM;
+		goto out_put_bdi;
+	}
 
 	err = fprop_local_init_percpu(&wb->completions, gfp);
 	if (err)
@@ -335,6 +339,9 @@ static int wb_init(struct bdi_writeback *wb, struct backing_dev_info *bdi,
 	fprop_local_destroy_percpu(&wb->completions);
 out_put_cong:
 	wb_congested_put(wb->congested);
+out_put_bdi:
+	if (wb != &bdi->wb)
+		bdi_put(bdi);
 	return err;
 }
 
@@ -372,6 +379,8 @@ static void wb_exit(struct bdi_writeback *wb)
 
 	fprop_local_destroy_percpu(&wb->completions);
 	wb_congested_put(wb->congested);
+	if (wb != &wb->bdi->wb)
+		bdi_put(wb->bdi);
 }
 
 #ifdef CONFIG_CGROUP_WRITEBACK