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>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>