ReadyNAS 6.9.0 kernel(?) crash

My ReadyNAS, recently upgraded to 6.9.0, froze up sometime early Sunday.  I didn't notice until various rsync jobs started failing.  On investigation, it responded to "ping" but little else --- I could not ssh into it, nor connect to the admin webpage, nor of course rsync.  Some of the rsync jobs didn't return failure but just hung for many hours, suggesting that enough of the TCP stack was alive to prevent connection timeouts.  When I went to look at the box, I found its display lit up and showing "__dentry_kill+5c", which looks suspiciously like part of a kernel stack traceback.

I force powered it off by holding down the power button for 5 sec, and then rebooted it and it seems to have come back fine.  But there's something not quite right about 6.9.0.