From d42244a0d36ad0939c5f173ebf15841a0678899c Mon Sep 17 00:00:00 2001 From: Jan Schmidt Date: Fri, 22 Jun 2012 14:51:15 +0200 Subject: [PATCH] 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 --- fs/btrfs/ctree.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c index b98f860..8206b39 100644 --- 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); -- 2.7.4