Messages in this thread Patch in this message | | | From | Baptiste Lepers <> | Subject | [PATCH] btrfs: transaction: Fix misplaced barrier in btrfs_record_root_in_trans | Date | Mon, 6 Sep 2021 11:25:59 +1000 |
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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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