On 06/22/2012 07:00 AM, Jan Schmidt wrote:
While debugging my tree mod log, after several hours of successful iteration I
finally reached a dead lock. I got stacks with btrfs_next_leaf and
push_leaf_left and looked into those.
If I'm not mistaken, there is at least one deadlock situation between those two
(I'm currently thinking about a second one). Basically, the problem is that
btrfs_next_leaf has a leaf locked and wants a lock for the next (right) leaf,
while push_leaf_left has a lock on another leaf and wants a lock for the
previous (left) leaf.
Assume that we've got two roots (subvolumes), both referencing the same two
leafs in two really small trees:
r1 r2
| \ / |
| X |
| / \ |
l1 l2
Commented pseudo code that is meant to summarize the relevant code from ctree.c:
Thread A in push_leaf_left, path is currently r2->l2:
btrfs_assert_tree_locked(path->nodes[1]); /* r2 */
/* also holds a lock at path->nodes[0] -> l2 */
left = read_node_slot(root, path->nodes[1], slot - 1); /* l1 */
btrfs_tree_lock(left);
-> blocking to get lock on l1
Thread B in btrfs_next_leaf, path is currently r1->l1:
path->keep_locks = 1;
btrfs_search_slot(...); /* locks r1, l1 */
level = 1;
while ...
slot = path->slots[level] + 1;
next = read_block_for_search(... slot ...);
btrfs_tree_read_lock(next); /* l2 */
-> blocking to get lock on l2
l2 shouldn't be locked anymore, if we're in push_leaf_left it's because
we cow'ed l2 and are holding a lock on it, so really it has a lock on
l2' and the btrfs_next_leaf is trying to get a lock on l2 which it
should be free to do. Thanks,
Josef
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