Btrfs: add waitqueue instead of doing busy waiting for more delayed refs

Now that we may be holding back delayed refs for a limited period, we
might end up having no runnable delayed refs. Without this commit, we'd
do busy waiting in that thread until another (runnable) ref arives.
Instead, we're detecting this situation and use a waitqueue, such that
we only try to run more refs after
	a) another runnable ref was added  or
	b) delayed refs are no longer held back

Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>

1 files changed, 8 insertions, 0 deletions

diff --git a/fs/btrfs/delayed-ref.c b/fs/btrfs/delayed-ref.c
index ee181989d444..66e4f29505a3 100644
--- a/fs/btrfs/delayed-ref.c
+++ b/fs/btrfs/delayed-ref.c
@@ -664,6 +664,9 @@ int btrfs_add_delayed_tree_ref(struct btrfs_fs_info *fs_info,
                                    num_bytes, parent, ref_root, level, action,
                                    for_cow);
         BUG_ON(ret);
+        if (!need_ref_seq(for_cow, ref_root) &&
+            waitqueue_active(&delayed_refs->seq_wait))
+                wake_up(&delayed_refs->seq_wait);
         spin_unlock(&delayed_refs->lock);
         return 0;
 }
@@ -712,6 +715,9 @@ int btrfs_add_delayed_data_ref(struct btrfs_fs_info *fs_info,
                                    num_bytes, parent, ref_root, owner, offset,
                                    action, for_cow);
         BUG_ON(ret);
+        if (!need_ref_seq(for_cow, ref_root) &&
+            waitqueue_active(&delayed_refs->seq_wait))
+                wake_up(&delayed_refs->seq_wait);
         spin_unlock(&delayed_refs->lock);
         return 0;
 }
@@ -739,6 +745,8 @@ int btrfs_add_delayed_extent_op(struct btrfs_fs_info *fs_info,
                                    extent_op->is_data);
         BUG_ON(ret);
 
+        if (waitqueue_active(&delayed_refs->seq_wait))
+                wake_up(&delayed_refs->seq_wait);
         spin_unlock(&delayed_refs->lock);
         return 0;
 }