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It skips reloc_mutex when root->last_trans matches the current transaction and BTRFS_ROOT_IN_TRANS_SETUP is clear. The writer side publishes that state in two phases: it sets IN_TRANS_SETUP before updating root->last_trans, then clears the bit after btrfs_init_reloc_root() finishes. However, the reader-side smp_rmb() is before both loads, so it does not order the last_trans load against the later bit test. A reader can observe the new last_trans value while missing the setup bit and return before the relocation-root setup is complete. Read root->last_trans first, then issue the read barrier before testing IN_TRANS_SETUP. Also use clear_bit_unlock() for the writer's final clear and test_bit_acquire() for the successful fast path, so the lockless return observes the setup done before the bit was cleared. Fixes: 7585717f304f ("Btrfs: fix relocation races") Signed-off-by: Cen Zhang --- fs/btrfs/transaction.c | 18 ++++++++++-------- 1 file changed, 10 insertions(+), 8 deletions(-) diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c index 8dd77c4..ac9ffa8 100644 --- a/fs/btrfs/transaction.c +++ b/fs/btrfs/transaction.c @@ -454,12 +454,12 @@ static int record_root_in_trans(struct btrfs_trans_ha= ndle *trans, * * When this is zero, they can trust root->last_trans and fly * through btrfs_record_root_in_trans without having to take the - * lock. smp_wmb() makes sure that all the writes above are - * done before we pop in the zero below + * lock. smp_wmb() makes sure readers that see the last_trans + * update also see IN_TRANS_SETUP set, and clear_bit_unlock() + * publishes the relocation setup before we clear the bit. */ ret =3D btrfs_init_reloc_root(trans, root); - smp_mb__before_atomic(); - clear_bit(BTRFS_ROOT_IN_TRANS_SETUP, &root->state); + clear_bit_unlock(BTRFS_ROOT_IN_TRANS_SETUP, &root->state); } return ret; } @@ -497,10 +497,12 @@ int btrfs_record_root_in_trans(struct btrfs_trans_han= dle *trans, * see record_root_in_trans for comments about IN_TRANS_SETUP usage * and barriers */ - smp_rmb(); - if (btrfs_get_root_last_trans(root) =3D=3D trans->transid && - !test_bit(BTRFS_ROOT_IN_TRANS_SETUP, &root->state)) - return 0; + if (btrfs_get_root_last_trans(root) =3D=3D trans->transid) { + /* Order the last_trans load before testing IN_TRANS_SETUP. */ + smp_rmb(); + if (!test_bit_acquire(BTRFS_ROOT_IN_TRANS_SETUP, &root->state)) + return 0; + } =20 mutex_lock(&fs_info->reloc_mutex); ret =3D record_root_in_trans(trans, root, 0); --=20 2.43.0