9.3-RELEASE still instapanics on multi-mps(4) servers

[Garrett Wollman]

Remember about six months ago when I tested 9-stable on one of my big
NFS servers, and had it panic in the middle of the USB probe, but
ultimately bisected the problem to an update to the mps(4) driver?  I
had to stop investigating and get the server working (which I did, by
installing 9.2 instead of something newer and presumably faster).  I'm
at the point now where I'd like to upgrade my file servers to 9.3, and
I can't, because of this issue, so it's time to start tracking down
the bug again.

I have two test servers now.  9.3 works just fine on one of them, and
panics on the other.  The one it works on is slightly older, and has
an mpt(4) controller for the boot drives, as opposed to the system
where the panic happens, which is mps(4)-only.  Both systems have two
SAS2116 controllers for external drives; the ones on the working
system have drives attached, and the ones on the non-working system
are not connected to anything (and in fact disabled in the BIOS for
now).

Any experts want to suggest where to start (besides, obviously,
attaching a serial console, which I haven't done yet)?  I saw one
change in the svn logs for 9.3 prior to the release which looked like
it might be a relevant fix, but it clearly hasn't improved anything
for my servers.

-GAWollman