Btrfs: resolve tree mod log locking issue in btrfs_next_leaf

With the tree mod log, we may end up with two roots (the current root and a
rewinded version of it) both pointing to two leaves, l1 and l2, of which l2
had already been cow-ed in the current transaction. If we don't rewind any
tree blocks, we cannot have two roots both pointing to an already cowed tree
block.

Now there is btrfs_next_leaf, which has a leaf locked and wants a lock on
the next (right) leaf. And there is push_leaf_left, which has a (cowed!)
leaf locked and wants a lock on the previous (left) leaf.

In order to solve this dead lock situation, we use try_lock in
btrfs_next_leaf (only in case it's called with a tree mod log time_seq
paramter) and if we fail to get a lock on the next leaf, we give up our lock
on the current leaf and retry from the very beginning.

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

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index b98f860..8206b39 100644 (file)
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -5119,6 +5119,18 @@ again:
 
                if (!path->skip_locking) {
                        ret = btrfs_try_tree_read_lock(next);
+                       if (!ret && time_seq) {
+                               /*
+                                * If we don't get the lock, we may be racing
+                                * with push_leaf_left, holding that lock while
+                                * itself waiting for the leaf we've currently
+                                * locked. To solve this situation, we give up
+                                * on our lock and cycle.
+                                */
+                               btrfs_release_path(path);
+                               cond_resched();
+                               goto again;
+                       }
                        if (!ret) {
                                btrfs_set_path_blocking(path);
                                btrfs_tree_read_lock(next);