[PATCH] Btrfs: fix freeing delayed ref head while still holding its mutex V2

[PATCH] Btrfs: fix freeing delayed ref head while still holding its mutex V2

[Josef Bacik]

I hit this error when reproducing a bug that would end in a transaction
abort.  We take the delayed ref head's mutex to keep anybody from processing
it while we're destroying it, but we fail to drop the mutex before we carry
on and free the damned thing.  Fix this by doing the remove logic for the
head ourselves and unlock the mutex, that way we can avoid use after free's
or hung tasks waiting on that mutex to come back so they know the delayed
ref completed.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
---
V1->V2: don't duplicate the freeing stuff, just unlock if we have a head.

 fs/btrfs/disk-io.c |    8 +++++---
 1 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 12ef591..42f83aa 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3615,11 +3615,11 @@ int btrfs_destroy_delayed_refs(struct btrfs_transaction *trans,
 	}
 
 	while ((node = rb_first(&delayed_refs->root)) != NULL) {
-		ref = rb_entry(node, struct btrfs_delayed_ref_node, rb_node);
+		struct btrfs_delayed_ref_head *head = NULL;
 
+		ref = rb_entry(node, struct btrfs_delayed_ref_node, rb_node);
 		atomic_set(&ref->refs, 1);
 		if (btrfs_delayed_ref_is_head(ref)) {
-			struct btrfs_delayed_ref_head *head;
 
 			head = btrfs_delayed_node_to_head(ref);
 			if (!mutex_trylock(&head->mutex)) {
@@ -3641,10 +3641,12 @@ int btrfs_destroy_delayed_refs(struct btrfs_transaction *trans,
 				delayed_refs->num_heads_ready--;
 			list_del_init(&head->cluster);
 		}
+
 		ref->in_tree = 0;
 		rb_erase(&ref->rb_node, &delayed_refs->root);
 		delayed_refs->num_entries--;
-
+		if (head)
+			mutex_unlock(&head->mutex);
 		spin_unlock(&delayed_refs->lock);
 		btrfs_put_delayed_ref(ref);
 
-- 
1.7.7.6

Re: [PATCH] Btrfs: fix freeing delayed ref head while still holding its mutex V2

[Zach Brown]

> V1->V2: don't duplicate the freeing stuff, just unlock if we have a head.

Nice, that's what I was picturing if we needed the freeing stuff to be
covered by the mutex.  Thanks for cleaning it up.

- z