the intent is that the sync command ensures that all current IO has gone
to disc without allowing file system activity
meanwhile--that's what the rlock of mainlock does--
but syncproc is allowed to run
concurrently with file system activity
to encourage the disc to be reasonably up to date
but without ensuring it.
there are interlocks on the buffers themselves (for instance)
to prevent confusion between several processes active
at once, whether for file system activity or syncproc.