From 538f72cdf03cad1c21c551ea542c8ce7d9fa2d81 Mon Sep 17 00:00:00 2001 From: Wang Shilong Date: Thu, 23 Jan 2014 13:47:48 +0800 Subject: [PATCH] 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: Wang Shilong Signed-off-by: Josef Bacik Signed-off-by: Chris Mason --- fs/btrfs/backref.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c index 15384968a84a..10ae5700ab1e 100644 --- a/fs/btrfs/backref.c +++ b/fs/btrfs/backref.c @@ -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,