From d42244a0d36ad0939c5f173ebf15841a0678899c Mon Sep 17 00:00:00 2001
From: Jan Schmidt <list.btrfs@jan-o-sch.net>
Date: Fri, 22 Jun 2012 14:51:15 +0200
Subject: 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>
---
 fs/btrfs/ctree.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

(limited to 'fs')

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index b98f8604f4f6..8206b3900587 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);
-- 
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