Btrfs: fix deadlock in wait_for_more_refs

Commit a168650c introduced a waiting mechanism to prevent busy waiting in
btrfs_run_delayed_refs. This can deadlock with btrfs_run_ordered_operations,
where a tree_mod_seq is held while waiting for the io to complete, while
the end_io calls btrfs_run_delayed_refs.
This whole mechanism is unnecessary. If not enough runnable refs are
available to satisfy count, just return as count is more like a guideline
than a strict requirement.
In case we have to run all refs, commit transaction makes sure that no
other threads are working in the transaction anymore, so we just assert
here that no refs are blocked.

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

1 files changed, 0 insertions, 8 deletions

diff --git a/fs/btrfs/delayed-ref.c b/fs/btrfs/delayed-ref.c
index da7419ed01b..7561431af50 100644
--- a/fs/btrfs/delayed-ref.c
+++ b/fs/btrfs/delayed-ref.c
@@ -662,9 +662,6 @@ int btrfs_add_delayed_tree_ref(struct btrfs_fs_info *fs_info,
         add_delayed_tree_ref(fs_info, trans, &ref->node, bytenr,
                                    num_bytes, parent, ref_root, level, action,
                                    for_cow);
-        if (!need_ref_seq(for_cow, ref_root) &&
-            waitqueue_active(&fs_info->tree_mod_seq_wait))
-                wake_up(&fs_info->tree_mod_seq_wait);
         spin_unlock(&delayed_refs->lock);
         if (need_ref_seq(for_cow, ref_root))
                 btrfs_qgroup_record_ref(trans, &ref->node, extent_op);
@@ -713,9 +710,6 @@ int btrfs_add_delayed_data_ref(struct btrfs_fs_info *fs_info,
         add_delayed_data_ref(fs_info, trans, &ref->node, bytenr,
                                    num_bytes, parent, ref_root, owner, offset,
                                    action, for_cow);
-        if (!need_ref_seq(for_cow, ref_root) &&
-            waitqueue_active(&fs_info->tree_mod_seq_wait))
-                wake_up(&fs_info->tree_mod_seq_wait);
         spin_unlock(&delayed_refs->lock);
         if (need_ref_seq(for_cow, ref_root))
                 btrfs_qgroup_record_ref(trans, &ref->node, extent_op);
@@ -744,8 +738,6 @@ int btrfs_add_delayed_extent_op(struct btrfs_fs_info *fs_info,
                                    num_bytes, BTRFS_UPDATE_DELAYED_HEAD,
                                    extent_op->is_data);
 
-        if (waitqueue_active(&fs_info->tree_mod_seq_wait))
-                wake_up(&fs_info->tree_mod_seq_wait);
         spin_unlock(&delayed_refs->lock);
         return 0;
 }