btrfs: use a bit to track the existence of tree mod log users

The tree modification log functions are called very frequently, basically
they are called every time a btree is modified (a pointer added or removed
to a node, a new root for a btree is set, etc). Because of that, to avoid
heavy lock contention on the lock that protects the list of tree mod log
users, we have checks that test the emptiness of the list with a full
memory barrier before the checks, so that when there are no tree mod log
users we avoid taking the lock.

Replace the memory barrier and list emptiness check with a test for a new
bit set at fs_info->flags. This bit is used to indicate when there are
tree mod log users, set whenever a user is added to the list and cleared
when the last user is removed from the list. This makes the intention a
bit more obvious and possibly more efficient (assuming test_bit() may be
cheaper than a full memory barrier on some architectures).

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -562,6 +562,9 @@ enum {
 
 	/* Indicate that we can't trust the free space tree for caching yet */
 	BTRFS_FS_FREE_SPACE_TREE_UNTRUSTED,
+
+	/* Indicate whether there are any tree modification log users */
+	BTRFS_FS_TREE_MOD_LOG_USERS,
 };
 
 /*

fs/btrfs/tree-mod-log.c

@@ -60,6 +60,7 @@ u64 btrfs_get_tree_mod_seq(struct btrfs_fs_info *fs_info,
 	if (!elem->seq) {
 		elem->seq = btrfs_inc_tree_mod_seq(fs_info);
 		list_add_tail(&elem->list, &fs_info->tree_mod_seq_list);
+		set_bit(BTRFS_FS_TREE_MOD_LOG_USERS, &fs_info->flags);
 	}
 	write_unlock(&fs_info->tree_mod_log_lock);
 
@@ -83,7 +84,9 @@ void btrfs_put_tree_mod_seq(struct btrfs_fs_info *fs_info,
 	list_del(&elem->list);
 	elem->seq = 0;
 
-	if (!list_empty(&fs_info->tree_mod_seq_list)) {
+	if (list_empty(&fs_info->tree_mod_seq_list)) {
+		clear_bit(BTRFS_FS_TREE_MOD_LOG_USERS, &fs_info->flags);
+	} else {
 		struct btrfs_seq_list *first;
 
 		first = list_first_entry(&fs_info->tree_mod_seq_list,
@@ -166,8 +169,7 @@ static noinline int tree_mod_log_insert(struct btrfs_fs_info *fs_info,
 static inline bool tree_mod_dont_log(struct btrfs_fs_info *fs_info,
 				    struct extent_buffer *eb)
 {
-	smp_mb();
-	if (list_empty(&(fs_info)->tree_mod_seq_list))
+	if (!test_bit(BTRFS_FS_TREE_MOD_LOG_USERS, &fs_info->flags))
 		return true;
 	if (eb && btrfs_header_level(eb) == 0)
 		return true;
@@ -185,8 +187,7 @@ static inline bool tree_mod_dont_log(struct btrfs_fs_info *fs_info,
 static inline bool tree_mod_need_log(const struct btrfs_fs_info *fs_info,
 				    struct extent_buffer *eb)
 {
-	smp_mb();
-	if (list_empty(&(fs_info)->tree_mod_seq_list))
+	if (!test_bit(BTRFS_FS_TREE_MOD_LOG_USERS, &fs_info->flags))
 		return false;
 	if (eb && btrfs_header_level(eb) == 0)
 		return false;