[PATCH 5/5] btrfs: stop running all delayed refs during snapshot

[Josef Bacik <josef@toxicpanda.com>]

This was added a very long time ago to work around issues with delayed
refs and with qgroups.  Both of these issues have since been properly
fixed, so all this does is cause a lot of lock contention with anybody
else who is running delayed refs.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/transaction.c | 23 ++++++++++++-----------
 1 file changed, 12 insertions(+), 11 deletions(-)

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index c3b3b524b8c3..8d34d7e0adb6 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -1184,10 +1184,6 @@ static noinline int commit_cowonly_roots(struct btrfs_trans_handle *trans)
 	btrfs_tree_unlock(eb);
 	free_extent_buffer(eb);
 
-	if (ret)
-		return ret;
-
-	ret = btrfs_run_delayed_refs(trans, (unsigned long)-1);
 	if (ret)
 		return ret;
 
@@ -1205,10 +1201,6 @@ static noinline int commit_cowonly_roots(struct btrfs_trans_handle *trans)
 	if (ret)
 		return ret;
 
-	/* run_qgroups might have added some more refs */
-	ret = btrfs_run_delayed_refs(trans, (unsigned long)-1);
-	if (ret)
-		return ret;
 again:
 	while (!list_empty(&fs_info->dirty_cowonly_roots)) {
 		struct btrfs_root *root;
@@ -1223,15 +1215,24 @@ static noinline int commit_cowonly_roots(struct btrfs_trans_handle *trans)
 		ret = update_cowonly_root(trans, root);
 		if (ret)
 			return ret;
-		ret = btrfs_run_delayed_refs(trans, (unsigned long)-1);
-		if (ret)
-			return ret;
 	}
 
+	/* Now flush any delayed refs generated by updating all of the roots. */
+	ret = btrfs_run_delayed_refs(trans, (unsigned long)-1);
+	if (ret)
+		return ret;
+
 	while (!list_empty(dirty_bgs) || !list_empty(io_bgs)) {
 		ret = btrfs_write_dirty_block_groups(trans);
 		if (ret)
 			return ret;
+
+		/*
+		 * We're writing the dirty block groups, which could generate
+		 * delayed refs, which could generate more dirty block groups,
+		 * so we want to keep this flushing in this loop to make sure
+		 * everything gets run.
+		 */
 		ret = btrfs_run_delayed_refs(trans, (unsigned long)-1);
 		if (ret)
 			return ret;
-- 
2.24.1