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Commit 538f72cd authored by Wang Shilong's avatar Wang Shilong Committed by Chris Mason
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Btrfs: fix protection between walking backrefs and root deletion 



There is a race condition between resolving indirect ref and root deletion,
and we should gurantee that root can not be destroyed to avoid accessing
broken tree here.

Here we fix it by holding @subvol_srcu, and we will release it as soon
as we have held root node lock.

Signed-off-by: default avatarWang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: default avatarJosef Bacik <jbacik@fb.com>
Signed-off-by: default avatarChris Mason <clm@fb.com>
parent 3c9665df

fs/btrfs/backref.c

+12 −1
Original line number Diff line number Diff line
@@ -301,23 +301,34 @@ static int __resolve_indirect_ref(struct btrfs_fs_info *fs_info, 
	int ret = 0;

	int root_level;

	int level = ref->level;

	int index;



	root_key.objectid = ref->root_id;

	root_key.type = BTRFS_ROOT_ITEM_KEY;

	root_key.offset = (u64)-1;



	index = srcu_read_lock(&fs_info->subvol_srcu);



	root = btrfs_read_fs_root_no_name(fs_info, &root_key);

	if (IS_ERR(root)) {

		srcu_read_unlock(&fs_info->subvol_srcu, index);

		ret = PTR_ERR(root);

		goto out;

	}



	root_level = btrfs_old_root_level(root, time_seq);



	if (root_level + 1 == level)

	if (root_level + 1 == level) {

		srcu_read_unlock(&fs_info->subvol_srcu, index);

		goto out;

	}



	path->lowest_level = level;

	ret = btrfs_search_old_slot(root, &ref->key_for_search, path, time_seq);



	/* root node has been locked, we can release @subvol_srcu safely here */

	srcu_read_unlock(&fs_info->subvol_srcu, index);



	pr_debug("search slot in root %llu (level %d, ref count %d) returned "

		 "%d for key (%llu %u %llu)\n",

		 ref->root_id, level, ref->count, ret,