Commit d42244a0 authored Jun 22, 2012 by Jan Schmidt

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>

parent 19956c7e

fs/btrfs/ctree.c

+12 −0

Original line number	Diff line number	Diff line
		@@ -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);

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