Re: [PATCH 8/9] xfs: add log item precommit operation

["Darrick J. Wong" <djwong@kernel.org>]

On Mon, Jun 27, 2022 at 10:43:35AM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> For inodes that are dirty, we have an attached cluster buffer that
> we want to use to track the dirty inode through the AIL.
> Unfortunately, locking the cluster buffer and adding it to the
> transaction when the inode is first logged in a transaction leads to
> buffer lock ordering inversions.
> 
> The specific problem is ordering against the AGI buffer. When
> modifying unlinked lists, the buffer lock order is AGI -> inode
> cluster buffer as the AGI buffer lock serialises all access to the
> unlinked lists. Unfortunately, functionality like xfs_droplink()
> logs the inode before calling xfs_iunlink(), as do various directory
> manipulation functions. The inode can be logged way down in the
> stack as far as the bmapi routines and hence, without a major
> rewrite of lots of APIs there's no way we can avoid the inode being
> logged by something until after the AGI has been logged.
> 
> As we are going to be using ordered buffers for inode AIL tracking,
> there isn't a need to actually lock that buffer against modification
> as all the modifications are captured by logging the inode item
> itself. Hence we don't actually need to join the cluster buffer into
> the transaction until just before it is committed. This means we do
> not perturb any of the existing buffer lock orders in transactions,
> and the inode cluster buffer is always locked last in a transaction
> that doesn't otherwise touch inode cluster buffers.
> 
> We do this by introducing a precommit log item method. A log item
> method is used because it is likely dquots will be moved to this
> same ordered buffer tracking scheme and hence will need a similar

Oh?

> callout. This commit just introduces the mechanism; the inode item
> implementation is in followup commits.
> 
> The precommit items need to be sorted into consistent order as we
> may be locking multiple items here. Hence if we have two dirty
> inodes in cluster buffers A and B, and some other transaction has
> two separate dirty inodes in the same cluster buffers, locking them
> in different orders opens us up to ABBA deadlocks. Hence we sort the
> items on the transaction based on the presence of a sort log item
> method.
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
>  fs/xfs/xfs_icache.c |  1 +
>  fs/xfs/xfs_trans.c  | 91 +++++++++++++++++++++++++++++++++++++++++++++
>  fs/xfs/xfs_trans.h  |  6 ++-
>  3 files changed, 96 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/xfs/xfs_icache.c b/fs/xfs/xfs_icache.c
> index 9fc324a29535..374b3bafaeb0 100644
> --- a/fs/xfs/xfs_icache.c
> +++ b/fs/xfs/xfs_icache.c
> @@ -914,6 +914,7 @@ xfs_reclaim_inode(
>  	ip->i_checked = 0;
>  	spin_unlock(&ip->i_flags_lock);
>  
> +	ASSERT(!ip->i_itemp || ip->i_itemp->ili_item.li_buf == NULL);
>  	xfs_iunlock(ip, XFS_ILOCK_EXCL);
>  
>  	XFS_STATS_INC(ip->i_mount, xs_ig_reclaims);
> diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c
> index 82cf0189c0db..0acb31093d9f 100644
> --- a/fs/xfs/xfs_trans.c
> +++ b/fs/xfs/xfs_trans.c
> @@ -844,6 +844,90 @@ xfs_trans_committed_bulk(
>  	spin_unlock(&ailp->ail_lock);
>  }
>  
> +/*
> + * Sort transaction items prior to running precommit operations. This will
> + * attempt to order the items such that they will always be locked in the same
> + * order. Items that have no sort function are moved to the end of the list
> + * and so are locked last (XXX: need to check the logic matches the comment).
> + *
> + * This may need refinement as different types of objects add sort functions.
> + *
> + * Function is more complex than it needs to be because we are comparing 64 bit
> + * values and the function only returns 32 bit values.
> + */
> +static int
> +xfs_trans_precommit_sort(
> +	void			*unused_arg,
> +	const struct list_head	*a,
> +	const struct list_head	*b)
> +{
> +	struct xfs_log_item	*lia = container_of(a,
> +					struct xfs_log_item, li_trans);
> +	struct xfs_log_item	*lib = container_of(b,
> +					struct xfs_log_item, li_trans);
> +	int64_t			diff;
> +
> +	/*
> +	 * If both items are non-sortable, leave them alone. If only one is
> +	 * sortable, move the non-sortable item towards the end of the list.
> +	 */
> +	if (!lia->li_ops->iop_sort && !lib->li_ops->iop_sort)
> +		return 0;
> +	if (!lia->li_ops->iop_sort)
> +		return 1;
> +	if (!lib->li_ops->iop_sort)
> +		return -1;
> +
> +	diff = lia->li_ops->iop_sort(lia) - lib->li_ops->iop_sort(lib);

I'm kinda surprised the iop_sort method doesn't take both log item
pointers, like most sorting-comparator functions?  But I'll see, maybe
you're doing something clever wrt ordering of log items of differing
types, and hence the ->iop_sort implementations are required to return
some absolute priority or something.

--D

> +	if (diff < 0)
> +		return -1;
> +	if (diff > 0)
> +		return 1;
> +	return 0;
> +}
> +
> +/*
> + * Run transaction precommit functions.
> + *
> + * If there is an error in any of the callouts, then stop immediately and
> + * trigger a shutdown to abort the transaction. There is no recovery possible
> + * from errors at this point as the transaction is dirty....
> + */
> +static int
> +xfs_trans_run_precommits(
> +	struct xfs_trans	*tp)
> +{
> +	struct xfs_mount	*mp = tp->t_mountp;
> +	struct xfs_log_item	*lip, *n;
> +	int			error = 0;
> +
> +	/*
> +	 * Sort the item list to avoid ABBA deadlocks with other transactions
> +	 * running precommit operations that lock multiple shared items such as
> +	 * inode cluster buffers.
> +	 */
> +	list_sort(NULL, &tp->t_items, xfs_trans_precommit_sort);
> +
> +	/*
> +	 * Precommit operations can remove the log item from the transaction
> +	 * if the log item exists purely to delay modifications until they
> +	 * can be ordered against other operations. Hence we have to use
> +	 * list_for_each_entry_safe() here.
> +	 */
> +	list_for_each_entry_safe(lip, n, &tp->t_items, li_trans) {
> +		if (!test_bit(XFS_LI_DIRTY, &lip->li_flags))
> +			continue;
> +		if (lip->li_ops->iop_precommit) {
> +			error = lip->li_ops->iop_precommit(tp, lip);
> +			if (error)
> +				break;
> +		}
> +	}
> +	if (error)
> +		xfs_force_shutdown(mp, SHUTDOWN_CORRUPT_INCORE);
> +	return error;
> +}
> +
>  /*
>   * Commit the given transaction to the log.
>   *
> @@ -869,6 +953,13 @@ __xfs_trans_commit(
>  
>  	trace_xfs_trans_commit(tp, _RET_IP_);
>  
> +	error = xfs_trans_run_precommits(tp);
> +	if (error) {
> +		if (tp->t_flags & XFS_TRANS_PERM_LOG_RES)
> +			xfs_defer_cancel(tp);
> +		goto out_unreserve;
> +	}
> +
>  	/*
>  	 * Finish deferred items on final commit. Only permanent transactions
>  	 * should ever have deferred ops.
> diff --git a/fs/xfs/xfs_trans.h b/fs/xfs/xfs_trans.h
> index 9561f193e7e1..64062e3b788b 100644
> --- a/fs/xfs/xfs_trans.h
> +++ b/fs/xfs/xfs_trans.h
> @@ -71,10 +71,12 @@ struct xfs_item_ops {
>  	void (*iop_format)(struct xfs_log_item *, struct xfs_log_vec *);
>  	void (*iop_pin)(struct xfs_log_item *);
>  	void (*iop_unpin)(struct xfs_log_item *, int remove);
> -	uint (*iop_push)(struct xfs_log_item *, struct list_head *);
> +	uint64_t (*iop_sort)(struct xfs_log_item *lip);
> +	int (*iop_precommit)(struct xfs_trans *tp, struct xfs_log_item *lip);
>  	void (*iop_committing)(struct xfs_log_item *lip, xfs_csn_t seq);
> -	void (*iop_release)(struct xfs_log_item *);
>  	xfs_lsn_t (*iop_committed)(struct xfs_log_item *, xfs_lsn_t);
> +	uint (*iop_push)(struct xfs_log_item *, struct list_head *);
> +	void (*iop_release)(struct xfs_log_item *);

Why did these get moved?

--D

>  	int (*iop_recover)(struct xfs_log_item *lip,
>  			   struct list_head *capture_list);
>  	bool (*iop_match)(struct xfs_log_item *item, uint64_t id);
> -- 
> 2.36.1
>