Re: [PATCH] btrfs: transaction: Fix misplaced barrier in btrfs_record_root_in_trans

[David Sterba]

On Mon, Sep 06, 2021 at 11:25:59AM +1000, Baptiste Lepers wrote:
> 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@xxxxxxxxx>
> ---
>  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))

Aren't the smp_rmb barriers supposed to be used before the condition?