bcache: Fix bcache device claiming

Commit 2736e8eeb0 ("block: use the holder as indication for exclusive
opens") introduced a change that blkdev_put() has to get exclusive
holder of the bdev as an argument. However it overlooked that
register_bdev() and register_cache() overwrite the bdev->bd_holder field
in the block device to point to the real owning object which was not
available at the time we called blkdev_get_by_path(). Messing with bdev
internals like this is a layering violation and it also causes
blkdev_put() to issue warning about mismatching holders.

Fix bcache to reopen the block device with appropriate holder once it is
available which also restores the behavior that multiple bcache caches
cannot claim the same device which was broken by commit 29499ab060
("bcache: don't pass a stack address to blkdev_get_by_path").

Fixes: 2736e8eeb0 ("block: use the holder as indication for exclusive opens")
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Kent Overstreet <kent.overstreet@linux.dev>
Acked-by: Coly Li <colyli@suse.de>
Link: https://lore.kernel.org/r/20230622164658.12861-2-jack@suse.cz
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/super.c

@@ -1369,7 +1369,7 @@ static void cached_dev_free(struct closure *cl)
 		put_page(virt_to_page(dc->sb_disk));
 
 	if (!IS_ERR_OR_NULL(dc->bdev))
-		blkdev_put(dc->bdev, bcache_kobj);
+		blkdev_put(dc->bdev, dc);
 
 	wake_up(&unregister_wait);
 
@@ -1453,7 +1453,6 @@ static int register_bdev(struct cache_sb *sb, struct cache_sb_disk *sb_disk,
 
 	memcpy(&dc->sb, sb, sizeof(struct cache_sb));
 	dc->bdev = bdev;
-	dc->bdev->bd_holder = dc;
 	dc->sb_disk = sb_disk;
 
 	if (cached_dev_init(dc, sb->block_size << 9))
@@ -2218,7 +2217,7 @@ void bch_cache_release(struct kobject *kobj)
 		put_page(virt_to_page(ca->sb_disk));
 
 	if (!IS_ERR_OR_NULL(ca->bdev))
-		blkdev_put(ca->bdev, bcache_kobj);
+		blkdev_put(ca->bdev, ca);
 
 	kfree(ca);
 	module_put(THIS_MODULE);
@@ -2345,7 +2344,6 @@ static int register_cache(struct cache_sb *sb, struct cache_sb_disk *sb_disk,
 
 	memcpy(&ca->sb, sb, sizeof(struct cache_sb));
 	ca->bdev = bdev;
-	ca->bdev->bd_holder = ca;
 	ca->sb_disk = sb_disk;
 
 	if (bdev_max_discard_sectors((bdev)))
@@ -2359,7 +2357,7 @@ static int register_cache(struct cache_sb *sb, struct cache_sb_disk *sb_disk,
 		 * call blkdev_put() to bdev in bch_cache_release(). So we
 		 * explicitly call blkdev_put() here.
 		 */
-		blkdev_put(bdev, bcache_kobj);
+		blkdev_put(bdev, ca);
 		if (ret == -ENOMEM)
 			err = "cache_alloc(): -ENOMEM";
 		else if (ret == -EPERM)
@@ -2516,10 +2514,11 @@ static ssize_t register_bcache(struct kobject *k, struct kobj_attribute *attr,
 	char *path = NULL;
 	struct cache_sb *sb;
 	struct cache_sb_disk *sb_disk;
-	struct block_device *bdev;
+	struct block_device *bdev, *bdev2;
-	void *holder;
+	void *holder = NULL;
 	ssize_t ret;
 	bool async_registration = false;
+	bool quiet = false;
 
 #ifdef CONFIG_BCACHE_ASYNC_REGISTRATION
 	async_registration = true;
@@ -2548,24 +2547,9 @@ static ssize_t register_bcache(struct kobject *k, struct kobj_attribute *attr,
 
 	ret = -EINVAL;
 	err = "failed to open device";
-	bdev = blkdev_get_by_path(strim(path), BLK_OPEN_READ | BLK_OPEN_WRITE,
+	bdev = blkdev_get_by_path(strim(path), BLK_OPEN_READ, NULL, NULL);
-				  bcache_kobj, NULL);
+	if (IS_ERR(bdev))
-	if (IS_ERR(bdev)) {
-		if (bdev == ERR_PTR(-EBUSY)) {
-			dev_t dev;
-
-			mutex_lock(&bch_register_lock);
-			if (lookup_bdev(strim(path), &dev) == 0 &&
-			    bch_is_open(dev))
-				err = "device already registered";
-			else
-				err = "device busy";
-			mutex_unlock(&bch_register_lock);
-			if (attr == &ksysfs_register_quiet)
-				goto done;
-		}
 		goto out_free_sb;
-	}
 
 	err = "failed to set blocksize";
 	if (set_blocksize(bdev, 4096))
@@ -2582,6 +2566,32 @@ static ssize_t register_bcache(struct kobject *k, struct kobj_attribute *attr,
 		goto out_put_sb_page;
 	}
 
+	/* Now reopen in exclusive mode with proper holder */
+	bdev2 = blkdev_get_by_dev(bdev->bd_dev, BLK_OPEN_READ | BLK_OPEN_WRITE,
+				  holder, NULL);
+	blkdev_put(bdev, NULL);
+	bdev = bdev2;
+	if (IS_ERR(bdev)) {
+		ret = PTR_ERR(bdev);
+		bdev = NULL;
+		if (ret == -EBUSY) {
+			dev_t dev;
+
+			mutex_lock(&bch_register_lock);
+			if (lookup_bdev(strim(path), &dev) == 0 &&
+			    bch_is_open(dev))
+				err = "device already registered";
+			else
+				err = "device busy";
+			mutex_unlock(&bch_register_lock);
+			if (attr == &ksysfs_register_quiet) {
+				quiet = true;
+				ret = size;
+			}
+		}
+		goto out_free_holder;
+	}
+
 	err = "failed to register device";
 
 	if (async_registration) {
@@ -2619,7 +2629,6 @@ static ssize_t register_bcache(struct kobject *k, struct kobj_attribute *attr,
 			goto out_free_sb;
 	}
 
-done:
 	kfree(sb);
 	kfree(path);
 	module_put(THIS_MODULE);
@@ -2631,7 +2640,8 @@ out_free_holder:
 out_put_sb_page:
 	put_page(virt_to_page(sb_disk));
 out_blkdev_put:
-	blkdev_put(bdev, register_bcache);
+	if (bdev)
+		blkdev_put(bdev, holder);
 out_free_sb:
 	kfree(sb);
 out_free_path:
@@ -2640,6 +2650,7 @@ out_free_path:
 out_module_put:
 	module_put(THIS_MODULE);
 out:
+	if (!quiet)
 		pr_info("error %s: %s\n", path?path:"", err);
 	return ret;
 }