5.4-stable review patch. If anyone has any objections, please let me know.
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From: Filipe Manana fdmanana@suse.com
[ Upstream commit 45c222468d33202c07c41c113301a4b9c8451b8f ]
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 fdmanana@suse.com Reviewed-by: David Sterba dsterba@suse.com Signed-off-by: David Sterba dsterba@suse.com Signed-off-by: Sasha Levin sashal@kernel.org --- fs/btrfs/transaction.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c index 094b024bbf0cf..6618b42defed7 100644 --- a/fs/btrfs/transaction.c +++ b/fs/btrfs/transaction.c @@ -1546,7 +1546,7 @@ 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 */