[PATCH] btrfs: relocation: fix misplaced barrier in reloc_root_is_dead

From: Baptiste Lepers
Date: Wed May 26 2021 - 02:10:25 EST

Fix a misplaced barrier in reloc_root_is_dead. The
BTRFS_ROOT_DEAD_RELOC_TREE bit should be checked before accessing

The fix forces the order documented in the original commit:
"In the cross section C-D, either caller gets the DEAD bit set, avoiding
access reloc_root no matter if it's safe or not. Or caller get the DEAD
bit cleared, then access reloc_root, which is already NULL, nothing will
be wrong."

static bool reloc_root_is_dead()
smp_rmb(); <--- misplaced
if (test_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state))
return true;
return false;

static bool have_reloc_root(struct btrfs_root *root)
if (reloc_root_is_dead(root))
return false;
<--- the barrier should happen here
if (!root->reloc_root)
return false;
return true;

Fixes: 6282675e6708e ("btrfs: relocation: fix reloc_root lifespan and access")
Signed-off-by: Baptiste Lepers <baptiste.lepers@xxxxxxxxx>
fs/btrfs/relocation.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index b70be2ac2e9e..8cddcce56bf4 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -289,15 +289,14 @@ static int update_backref_cache(struct btrfs_trans_handle *trans,

static bool reloc_root_is_dead(struct btrfs_root *root)
+ bool is_dead = test_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state);
* Pair with set_bit/clear_bit in clean_dirty_subvols and
* btrfs_update_reloc_root. We need to see the updated bit before
* trying to access reloc_root
- if (test_bit(BTRFS_ROOT_DEAD_RELOC_TREE, &root->state))
- return true;
- return false;
+ return is_dead;