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Subject[PATCH] btrfs: transaction: Fix misplaced barrier in btrfs_record_root_in_trans
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Per comment, record_root_in_trans orders the writes of the root->state
and root->last_trans:
set_bit(BTRFS_ROOT_IN_TRANS_SETUP, &root->state);
smp_wmb();
root->last_trans = trans->transid;

But the barrier that enforces the order on the read side is misplaced:
smp_rmb(); <-- misplaced
if (root->last_trans == trans->transid &&
<-- missing barrier here -->
!test_bit(BTRFS_ROOT_IN_TRANS_SETUP, &root->state))

This patches fixes the ordering and wraps the racy accesses with
READ_ONCE and WRITE_ONCE calls to avoid load/store tearing.

Fixes: 7585717f304f5 ("Btrfs: fix relocation races")
Signed-off-by: Baptiste Lepers <baptiste.lepers@gmail.com>
---
fs/btrfs/transaction.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 14b9fdc8aaa9..a609222e6704 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -437,7 +437,7 @@ static int record_root_in_trans(struct btrfs_trans_handle *trans,
(unsigned long)root->root_key.objectid,
BTRFS_ROOT_TRANS_TAG);
spin_unlock(&fs_info->fs_roots_radix_lock);
- root->last_trans = trans->transid;
+ WRITE_ONCE(root->last_trans, trans->transid);

/* this is pretty tricky. We don't want to
* take the relocation lock in btrfs_record_root_in_trans
@@ -489,7 +489,7 @@ int btrfs_record_root_in_trans(struct btrfs_trans_handle *trans,
struct btrfs_root *root)
{
struct btrfs_fs_info *fs_info = root->fs_info;
- int ret;
+ int ret, last_trans;

if (!test_bit(BTRFS_ROOT_SHAREABLE, &root->state))
return 0;
@@ -498,8 +498,9 @@ int btrfs_record_root_in_trans(struct btrfs_trans_handle *trans,
* see record_root_in_trans for comments about IN_TRANS_SETUP usage
* and barriers
*/
+ last_trans = READ_ONCE(root->last_trans);
smp_rmb();
- if (root->last_trans == trans->transid &&
+ if (last_trans == trans->transid &&
!test_bit(BTRFS_ROOT_IN_TRANS_SETUP, &root->state))
return 0;

--
2.17.1
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