Re: [PATCH 02/24] xfs: add an inode item lock

Amir Goldstein
2020-05-22T06:46:07Z

On Fri, May 22, 2020 at 6:51 AM Dave Chinner <david@fromorbit.com> wrote:
>
> From: Dave Chinner <dchinner@redhat.com>
>
> The inode log item is kind of special in that it can be aggregating
> new changes in memory at the same time time existing changes are
> being written back to disk. This means there are fields in the log
> item that are accessed concurrently from contexts that don't share
> any locking at all.
>
> e.g. updating ili_last_fields occurs at flush time under the
> ILOCK_EXCL and flush lock at flush time, under the flush lock at IO
> completion time, and is read under the ILOCK_EXCL when the inode is
> logged. Hence there is no actual serialisation between reading the
> field during logging of the inode in transactions vs clearing the
> field in IO completion.
>
> We currently get away with this by the fact that we are only
> clearing fields in IO completion, and nothing bad happens if we
> accidentally log more of the inode than we actually modify. Worst
> case is we consume a tiny bit more memory and log bandwidth.
>
> However, if we want to do more complex state manipulations on the
> log item that requires updates at all three of these potential
> locations, we need to have some mechanism of serialising those
> operations. To do this, introduce a spinlock into the log item to
> serialise internal state.
>
> This could be done via the xfs_inode i_flags_lock, but this then
> leads to potential lock inversion issues where inode flag updates
> need to occur inside locks that best nest inside the inode log item
> locks (e.g. marking inodes stale during inode cluster freeing).
> Using a separate spinlock avoids these sorts of problems and
> simplifies future code.
>
> This does not touch the use of ili_fields in the item formatting
> code - that is entirely protected by the ILOCK_EXCL at this point in
> time, so it remains untouched.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> fs/xfs/libxfs/xfs_trans_inode.c | 44 ++++++++++++++++++---------------
> fs/xfs/xfs_file.c | 9 ++++---
> fs/xfs/xfs_inode.c | 20 +++++++++------
> fs/xfs/xfs_inode_item.c | 7 ++++++
> fs/xfs/xfs_inode_item.h | 3 ++-
> 5 files changed, 51 insertions(+), 32 deletions(-)
>
> diff --git a/fs/xfs/libxfs/xfs_trans_inode.c b/fs/xfs/libxfs/xfs_trans_inode.c
> index b5dfb66548422..510b996008221 100644
> --- a/fs/xfs/libxfs/xfs_trans_inode.c
> +++ b/fs/xfs/libxfs/xfs_trans_inode.c
> @@ -81,15 +81,19 @@ xfs_trans_ichgtime(
> */
> void
> xfs_trans_log_inode(
> - xfs_trans_t *tp,
> - xfs_inode_t *ip,
> - uint flags)
> + struct xfs_trans *tp,
> + struct xfs_inode *ip,
> + uint flags)
> {
> - struct inode *inode = VFS_I(ip);
> + struct xfs_inode_log_item *iip = ip->i_itemp;
> + struct inode *inode = VFS_I(ip);
> + uint iversion_flags = 0;
>
> ASSERT(ip->i_itemp != NULL);
> ASSERT(xfs_isilocked(ip, XFS_ILOCK_EXCL));
>
> + tp->t_flags |= XFS_TRANS_DIRTY;
> +
> /*
> * Don't bother with i_lock for the I_DIRTY_TIME check here, as races
> * don't matter - we either will need an extra transaction in 24 hours
> @@ -102,15 +106,6 @@ xfs_trans_log_inode(
> spin_unlock(&inode->i_lock);
> }
>
> - /*
> - * Record the specific change for fdatasync optimisation. This
> - * allows fdatasync to skip log forces for inodes that are only
> - * timestamp dirty. We do this before the change count so that
> - * the core being logged in this case does not impact on fdatasync
> - * behaviour.
> - */
> - ip->i_itemp->ili_fsync_fields |= flags;
> -
> /*
> * First time we log the inode in a transaction, bump the inode change
> * counter if it is configured for this to occur. While we have the
> @@ -120,13 +115,21 @@ xfs_trans_log_inode(
> * set however, then go ahead and bump the i_version counter
> * unconditionally.
> */
> - if (!test_and_set_bit(XFS_LI_DIRTY, &ip->i_itemp->ili_item.li_flags) &&
> - IS_I_VERSION(VFS_I(ip))) {
> - if (inode_maybe_inc_iversion(VFS_I(ip), flags & XFS_ILOG_CORE))
> - flags |= XFS_ILOG_CORE;
> + if (!test_and_set_bit(XFS_LI_DIRTY, &iip->ili_item.li_flags) &&
> + IS_I_VERSION(inode)) {
> + if (inode_maybe_inc_iversion(inode, flags & XFS_ILOG_CORE))
> + iversion_flags = XFS_ILOG_CORE;
> }
>
> - tp->t_flags |= XFS_TRANS_DIRTY;
> + /*
> + * Record the specific change for fdatasync optimisation. This
> + * allows fdatasync to skip log forces for inodes that are only
> + * timestamp dirty. We do this before the change count so that
> + * the core being logged in this case does not impact on fdatasync
> + * behaviour.
> + */
> + spin_lock(&iip->ili_lock);
> + iip->ili_fsync_fields |= flags;
>
> /*
> * Always OR in the bits from the ili_last_fields field.
> @@ -135,8 +138,9 @@ xfs_trans_log_inode(
> * See the big comment in xfs_iflush() for an explanation of
> * this coordination mechanism.
> */
> - flags |= ip->i_itemp->ili_last_fields;
> - ip->i_itemp->ili_fields |= flags;
> + flags |= iip->ili_last_fields | iversion_flags;
> + iip->ili_fields |= flags;
> + spin_unlock(&iip->ili_lock);
> }
>
> int
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 403c90309a8ff..0abf770b77498 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -94,6 +94,7 @@ xfs_file_fsync(
> {
> struct inode *inode = file->f_mapping->host;
> struct xfs_inode *ip = XFS_I(inode);
> + struct xfs_inode_log_item *iip = ip->i_itemp;
> struct xfs_mount *mp = ip->i_mount;
> int error = 0;
> int log_flushed = 0;
> @@ -137,13 +138,15 @@ xfs_file_fsync(
> xfs_ilock(ip, XFS_ILOCK_SHARED);
> if (xfs_ipincount(ip)) {
> if (!datasync ||
> - (ip->i_itemp->ili_fsync_fields & ~XFS_ILOG_TIMESTAMP))
> - lsn = ip->i_itemp->ili_last_lsn;
> + (iip->ili_fsync_fields & ~XFS_ILOG_TIMESTAMP))
> + lsn = iip->ili_last_lsn;
> }
>
> if (lsn) {
> error = xfs_log_force_lsn(mp, lsn, XFS_LOG_SYNC, &log_flushed);
> - ip->i_itemp->ili_fsync_fields = 0;
> + spin_lock(&iip->ili_lock);
> + iip->ili_fsync_fields = 0;
> + spin_unlock(&iip->ili_lock);
> }
> xfs_iunlock(ip, XFS_ILOCK_SHARED);
>
> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> index ca9f2688b745d..57781c0dbbec5 100644
> --- a/fs/xfs/xfs_inode.c
> +++ b/fs/xfs/xfs_inode.c
> @@ -2683,9 +2683,11 @@ xfs_ifree_cluster(
> continue;
>
> iip = ip->i_itemp;
> + spin_lock(&iip->ili_lock);
> iip->ili_last_fields = iip->ili_fields;
> iip->ili_fields = 0;
> iip->ili_fsync_fields = 0;
> + spin_unlock(&iip->ili_lock);
> xfs_trans_ail_copy_lsn(mp->m_ail, &iip->ili_flush_lsn,
> &iip->ili_item.li_lsn);
>
> @@ -2721,6 +2723,7 @@ xfs_ifree(
> {
> int error;
> struct xfs_icluster xic = { 0 };
> + struct xfs_inode_log_item *iip = ip->i_itemp;
>
> ASSERT(xfs_isilocked(ip, XFS_ILOCK_EXCL));
> ASSERT(VFS_I(ip)->i_nlink == 0);
> @@ -2758,7 +2761,9 @@ xfs_ifree(
> ip->i_df.if_format = XFS_DINODE_FMT_EXTENTS;
>
> /* Don't attempt to replay owner changes for a deleted inode */
> - ip->i_itemp->ili_fields &= ~(XFS_ILOG_AOWNER|XFS_ILOG_DOWNER);
> + spin_lock(&iip->ili_lock);
> + iip->ili_fields &= ~(XFS_ILOG_AOWNER|XFS_ILOG_DOWNER);
> + spin_unlock(&iip->ili_lock);
>
> /*
> * Bump the generation count so no one will be confused
> @@ -3814,20 +3819,19 @@ xfs_iflush_int(
> * know that the information those bits represent is permanently on
> * disk. As long as the flush completes before the inode is logged
> * again, then both ili_fields and ili_last_fields will be cleared.
> - *
> - * We can play with the ili_fields bits here, because the inode lock
> - * must be held exclusively in order to set bits there and the flush
> - * lock protects the ili_last_fields bits. Store the current LSN of the
> - * inode so that we can tell whether the item has moved in the AIL from
> - * xfs_iflush_done(). In order to read the lsn we need the AIL lock,
> - * because it is a 64 bit value that cannot be read atomically.
> */
> error = 0;
> flush_out:
> + spin_lock(&iip->ili_lock);
> iip->ili_last_fields = iip->ili_fields;
> iip->ili_fields = 0;
> iip->ili_fsync_fields = 0;
> + spin_unlock(&iip->ili_lock);
>
> + /*
> + * Store the current LSN of the inode so that we can tell whether the
> + * item has moved in the AIL from xfs_iflush_done().
> + */
> xfs_trans_ail_copy_lsn(mp->m_ail, &iip->ili_flush_lsn,
> &iip->ili_item.li_lsn);
>
> diff --git a/fs/xfs/xfs_inode_item.c b/fs/xfs/xfs_inode_item.c
> index b17384aa8df40..6ef9cbcfc94a7 100644
> --- a/fs/xfs/xfs_inode_item.c
> +++ b/fs/xfs/xfs_inode_item.c
> @@ -637,6 +637,7 @@ xfs_inode_item_init(
> iip = ip->i_itemp = kmem_zone_zalloc(xfs_ili_zone, 0);
>
> iip->ili_inode = ip;
> + spin_lock_init(&iip->ili_lock);
> xfs_log_item_init(mp, &iip->ili_item, XFS_LI_INODE,
> &xfs_inode_item_ops);
> }
> @@ -738,7 +739,11 @@ xfs_iflush_done(
> list_for_each_entry_safe(blip, n, &tmp, li_bio_list) {
> list_del_init(&blip->li_bio_list);
> iip = INODE_ITEM(blip);
> +
> + spin_lock(&iip->ili_lock);
> iip->ili_last_fields = 0;
> + spin_unlock(&iip->ili_lock);
> +
> xfs_ifunlock(iip->ili_inode);
> }
> list_del(&tmp);
> @@ -762,9 +767,11 @@ xfs_iflush_abort(
> * Clear the inode logging fields so no more flushes are
> * attempted.
> */
> + spin_lock(&iip->ili_lock);
> iip->ili_last_fields = 0;
> iip->ili_fields = 0;
> iip->ili_fsync_fields = 0;
> + spin_unlock(&iip->ili_lock);
> }
> /*
> * Release the inode's flush lock since we're done with it.
> diff --git a/fs/xfs/xfs_inode_item.h b/fs/xfs/xfs_inode_item.h
> index 4de5070e07655..1234e8cd3726d 100644
> --- a/fs/xfs/xfs_inode_item.h
> +++ b/fs/xfs/xfs_inode_item.h
> @@ -18,7 +18,8 @@ struct xfs_inode_log_item {
> struct xfs_inode *ili_inode; /* inode ptr */
> xfs_lsn_t ili_flush_lsn; /* lsn at last flush */
> xfs_lsn_t ili_last_lsn; /* lsn at last transaction */
> - unsigned short ili_lock_flags; /* lock flags */
> + spinlock_t ili_lock; /* internal state lock */

"internal state" is a fluid term.
It would be more useful to document "Protects ..." as in i_lock/f_lock.

For verifying unchanged logic from code re-organization and locking
balance:

Reviewed-by: Amir Goldstein <amir73il@gmail.com>

Thanks,
Amir.