xfs: ensure log tail is always up to date

xfs: ensure log tail is always up to date

Whenever we write an iclog, we call xlog_assign_tail_lsn() to update
the current tail before we write it into the iclog header. This
means we have to take the AIL lock on every iclog write just to
check if the tail of the log has moved.

This doesn't avoid races with log tail updates - the log tail could
move immediately after we assign the tail to the iclog header and
hence by the time the iclog reaches stable storage the tail LSN has
moved forward in memory. Hence the log tail LSN in the iclog header
is really just a point in time snapshot of the current state of the
AIL.

With this in mind, if we simply update the in memory log->l_tail_lsn
every time it changes in the AIL, there is no need to update the in
memory value when we are writing it into an iclog - it will already
be up-to-date in memory and checking the AIL again will not change
this. Hence xlog_state_release_iclog() does not need to check the
AIL to update the tail lsn and can just sample it directly without
needing to take the AIL lock.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>