Btrfs: fix race in run_clustered_refs

With commit

commit d1270cd91f308c9d22b2804720c36ccd32dbc35e
Author: Arne Jansen <sensille@gmx.net>
Date:   Tue Sep 13 15:16:43 2011 +0200

     Btrfs: put back delayed refs that are too new

I added a window where the delayed_ref's head->ref_mod code can diverge
from the sum of the remaining refs, because we release the head->mutex
in the middle. This leads to btrfs_lookup_extent_info returning wrong
numbers. This patch fixes this by adjusting the head's ref_mod with each
delayed ref we run.

Signed-off-by: Arne Jansen <sensille@gmx.net>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 1bb408f737fb..f16411d3c252 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -2318,6 +2318,23 @@ static noinline int run_clustered_refs(struct btrfs_trans_handle *trans,
                 ref->in_tree = 0;
                 rb_erase(&ref->rb_node, &delayed_refs->root);
                 delayed_refs->num_entries--;
+                if (locked_ref) {
+                        /*
+                         * when we play the delayed ref, also correct the
+                         * ref_mod on head
+                         */
+                        switch (ref->action) {
+                        case BTRFS_ADD_DELAYED_REF:
+                        case BTRFS_ADD_DELAYED_EXTENT:
+                                locked_ref->node.ref_mod -= ref->ref_mod;
+                                break;
+                        case BTRFS_DROP_DELAYED_REF:
+                                locked_ref->node.ref_mod += ref->ref_mod;
+                                break;
+                        default:
+                                WARN_ON(1);
+                        }
+                }
                 spin_unlock(&delayed_refs->lock);
 
                 ret = run_one_delayed_ref(trans, root, ref, extent_op,