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authorJan Schmidt <list.btrfs@jan-o-sch.net>2012-06-22 14:51:15 +0200
committerJan Schmidt <list.btrfs@jan-o-sch.net>2012-06-27 16:34:40 +0200
commitd42244a0d36ad0939c5f173ebf15841a0678899c (patch)
tree3bd49c2f50a0786230f882756c8ee683c73ed9e0 /fs
parent19956c7e94a7a58d6df8c4db5ae62f9109a7c663 (diff)
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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>
Diffstat (limited to 'fs')
-rw-r--r--fs/btrfs/ctree.c12
1 files changed, 12 insertions, 0 deletions
diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index b98f8604f4f..8206b390058 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);