[PATCH 1/2] btrfs: Remove superflous lock acquisition in __del_reloc_root

Nikolay Borisov
2020-02-21T13:11:29Z

__del_reloc_root is called from:

a) Transaction commit via:
btrfs_transaction_commit
commit_fs_roots
btrfs_update_reloc_root
__del_reloc_root

b) From the relocation thread with the following call chains:

relocate_block_group
merge_reloc_roots
insert_dirty_subvol
btrfs_update_reloc_root
__del_reloc_root

c) merge_reloc_roots
free_reloc_roots
__del_reloc_roots

(The above call chain can called from btrfs_recover_relocation as well
but for the purpose of this fix this is irrelevant).

The commont data structure that needs protecting is
fs_info->reloc_ctl->reloc_list as reloc roots are anchored at this list.
Turns out it's no needed to hold the trans_lock in __del_reloc_root
since consistency is already guaranteed by call chain b) above holding
a transaction while calling insert_dirty_subvol, meaning we cannot have
a concurrent transaction commit. For call chain c) above a snapshot of
the fs_info->reloc_ctl->reloc_list is taken with reloc_mutex held and
free_reloc_roots is called on this local snapshot.

Those invariants are sufficient to prevent racing calls to
__del_reloc_root alongside other users of the list, as such it's fine
to drop the lock acquisition.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
---
fs/btrfs/relocation.c | 2 --
1 file changed, 2 deletions(-)

diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index 8076c340749f..e5cb64409f7c 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -1381,9 +1381,7 @@ static void __del_reloc_root(struct btrfs_root *root)
BUG_ON((struct btrfs_root *)node->data != root);
}

- spin_lock(&fs_info->trans_lock);
list_del_init(&root->root_list);
- spin_unlock(&fs_info->trans_lock);
kfree(node);
}

--
2.17.1