From: Filipe Manana Date: Mon, 22 Sep 2025 11:09:14 +0000 (+0100) Subject: btrfs: use smp_mb__after_atomic() when forcing COW in create_pending_snapshot() X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=45c222468d33202c07c41c113301a4b9c8451b8f;p=thirdparty%2Fkernel%2Fstable.git btrfs: use smp_mb__after_atomic() when forcing COW in create_pending_snapshot() After setting the BTRFS_ROOT_FORCE_COW flag on the root we are doing a full write barrier, smp_wmb(), but we don't need to, all we need is a smp_mb__after_atomic(). The use of the smp_wmb() is from the old days when we didn't use a bit and used instead an int field in the root to signal if cow is forced. After the int field was changed to a bit in the root's state (flags field), we forgot to update the memory barrier in create_pending_snapshot() to smp_mb__after_atomic(), but we did the change in commit_fs_roots() after clearing BTRFS_ROOT_FORCE_COW. That happened in commit 27cdeb7096b8 ("Btrfs: use bitfield instead of integer data type for the some variants in btrfs_root"). On the reader side, in should_cow_block(), we also use the counterpart smp_mb__before_atomic() which generates further confusion. So change the smp_wmb() to smp_mb__after_atomic(). In fact we don't even need any barrier at all since create_pending_snapshot() is called in the critical section of a transaction commit and therefore no one can concurrently join/attach the transaction, or start a new one, until the transaction is unblocked. By the time someone starts a new transaction and enters should_cow_block(), a lot of implicit memory barriers already took place by having acquired several locks such as fs_info->trans_lock and extent buffer locks on the root node at least. Nevertlheless, for consistency use smp_mb__after_atomic() after setting the force cow bit in create_pending_snapshot(). Signed-off-by: Filipe Manana Reviewed-by: David Sterba Signed-off-by: David Sterba --- diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c index 23bcd3c0fc1c2..89ae0c7a610aa 100644 --- a/fs/btrfs/transaction.c +++ b/fs/btrfs/transaction.c @@ -1802,7 +1802,7 @@ static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans, } /* see comments in should_cow_block() */ set_bit(BTRFS_ROOT_FORCE_COW, &root->state); - smp_wmb(); + smp_mb__after_atomic(); btrfs_set_root_node(new_root_item, tmp); /* record when the snapshot was created in key.offset */