Btrfs: use btrfs_read_lock_root_node in get_old_root

get_old_root could race with root node updates because we weren't locking
the node early enough. Use btrfs_read_lock_root_node to grab the root locked
in the very beginning and release the lock as soon as possible (just like
btrfs_search_slot does).

Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>

1 files changed, 16 insertions, 4 deletions

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 836e4e03edca..2cde7b0a0106 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -1143,6 +1143,13 @@ tree_mod_log_rewind(struct btrfs_fs_info *fs_info, struct extent_buffer *eb,
         return eb_rewin;
 }
 
+/*
+ * get_old_root() rewinds the state of @root's root node to the given @time_seq
+ * value. If there are no changes, the current root->root_node is returned. If
+ * anything changed in between, there's a fresh buffer allocated on which the
+ * rewind operations are done. In any case, the returned buffer is read locked.
+ * Returns NULL on error (with no locks held).
+ */
 static inline struct extent_buffer *
 get_old_root(struct btrfs_root *root, u64 time_seq)
 {
@@ -1151,6 +1158,7 @@ get_old_root(struct btrfs_root *root, u64 time_seq)
         struct tree_mod_root *old_root;
         u64 old_generation;
 
+        eb = btrfs_read_lock_root_node(root);
         tm = __tree_mod_log_oldest_root(root->fs_info, root, time_seq);
         if (!tm)
                 return root->node;
@@ -1173,15 +1181,21 @@ get_old_root(struct btrfs_root *root, u64 time_seq)
                 /* there's a root replace operation for the current root */
                 eb = alloc_dummy_extent_buffer(tm->index << PAGE_CACHE_SHIFT,
                                                root->nodesize);
+        }
+        btrfs_tree_read_unlock(root->node);
+        free_extent_buffer(root->node);
+        if (!eb)
+                return NULL;
+        btrfs_tree_read_lock(eb);
+        if (old_root->logical != root->node->start) {
                 btrfs_set_header_bytenr(eb, eb->start);
                 btrfs_set_header_backref_rev(eb, BTRFS_MIXED_BACKREF_REV);
                 btrfs_set_header_owner(eb, root->root_key.objectid);
         }
-        if (!eb)
-                return NULL;
         btrfs_set_header_level(eb, old_root->level);
         btrfs_set_header_generation(eb, old_generation);
         __tree_mod_log_rewind(eb, time_seq, tm);
+        extent_buffer_get(eb);
 
         return eb;
 }
@@ -2612,9 +2626,7 @@ int btrfs_search_old_slot(struct btrfs_root *root, struct btrfs_key *key,
 
 again:
         b = get_old_root(root, time_seq);
-        extent_buffer_get(b);
         level = btrfs_header_level(b);
-        btrfs_tree_read_lock(b);
         p->locks[level] = BTRFS_READ_LOCK;
 
         while (b) {