bcache: Add comments for blkdev_put() in registration code path

Add comments to explain why in register_bcache() blkdev_put() won't
be called in two location. Add comments to explain why blkdev_put()
must be called in register_cache() when cache_alloc() failed.
Signed-off-by: Coly Li <colyli@suse.de>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/super.c

@@ -2189,6 +2189,12 @@ static int register_cache(struct cache_sb *sb, struct page *sb_page,
 
 	ret = cache_alloc(ca);
 	if (ret != 0) {
+		/*
+		 * If we failed here, it means ca->kobj is not initialized yet,
+		 * kobject_put() won't be called and there is no chance to
+		 * call blkdev_put() to bdev in bch_cache_release(). So we
+		 * explicitly call blkdev_put() here.
+		 */
 		blkdev_put(bdev, FMODE_READ|FMODE_WRITE|FMODE_EXCL);
 		if (ret == -ENOMEM)
 			err = "cache_alloc(): -ENOMEM";
@@ -2329,6 +2335,7 @@ static ssize_t register_bcache(struct kobject *k, struct kobj_attribute *attr,
 		mutex_lock(&bch_register_lock);
 		ret = register_bdev(sb, sb_page, bdev, dc);
 		mutex_unlock(&bch_register_lock);
+		/* blkdev_put() will be called in cached_dev_free() */
 		if (ret < 0)
 			goto err;
 	} else {
@@ -2337,6 +2344,7 @@ static ssize_t register_bcache(struct kobject *k, struct kobj_attribute *attr,
 		if (!ca)
 			goto err_close;
 
+		/* blkdev_put() will be called in bch_cache_release() */
 		if (register_cache(sb, sb_page, bdev, ca) != 0)
 			goto err;
 	}