On Tue, Nov 24, 2020 at 04:44:13PM +0200, Nikolay Borisov wrote:
> btrfs_transaction::state is protected by btrfs_fs_info::trans_lock and
> all accesses to this variable should be synchornized by it. However,
> there are 2 exceptions, namely checking if currently running transaction
> has entered its commit phase in start_transaction and in
> btrfs_should_end_transaction.
>
> Annotate those 2, unlocked accesses with READ_ONCE respectively. Also
> remove the full memory barrier in start_transaction as it provides no
> ordering guarantees due to being unpaired. All other accsess to
> transaction state happen under trans_lock being held.
>
> Signed-off-by: Nikolay Borisov <nbori...@suse.com>
> ---
> fs/btrfs/transaction.c | 5 ++---
> 1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
> index df5687a92798..e5a5c3604a9b 100644
> --- a/fs/btrfs/transaction.c
> +++ b/fs/btrfs/transaction.c
> @@ -695,8 +695,7 @@ start_transaction(struct btrfs_root *root, unsigned int
> num_items,
> h->can_flush_pending_bgs = true;
> INIT_LIST_HEAD(&h->new_bgs);
>
> - smp_mb();
> - if (cur_trans->state >= TRANS_STATE_COMMIT_START &&
> + if (READ_ONCE(cur_trans->state) >= TRANS_STATE_COMMIT_START &&
> may_wait_transaction(fs_info, type)) {
> current->journal_info = h;
> btrfs_commit_transaction(h);
> @@ -912,7 +911,7 @@ int btrfs_should_end_transaction(struct
> btrfs_trans_handle *trans)
> {
> struct btrfs_transaction *cur_trans = trans->transaction;
>
> - if (cur_trans->state >= TRANS_STATE_COMMIT_START ||
> + if (READ_ONCE(cur_trans->state) >= TRANS_STATE_COMMIT_START ||
> test_bit(BTRFS_DELAYED_REFS_FLUSHING,
> &cur_trans->delayed_refs.flags))
> return 1;
> --
> 2.25.1
>
I think some parts of this have already gone in and the conditional has
been lightly refactored, so this patch no longer applies to
kdave/misc-next.
Assuming this is a change for adherence to best practices rather than a
fix for a specific issue (in which case it would be nice to have the
issue described in the message), the patch looks good to me otherwise.
