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

Dave Chinner
2020-05-22T03:50:39Z

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 */
+ unsigned short ili_lock_flags; /* inode lock flags */
unsigned int ili_last_fields; /* fields when flushed */
unsigned int ili_fields; /* fields to be logged */
unsigned int ili_fsync_fields; /* logged since last fsync */
--
2.26.2.761.g0e0b3e54be