[6.1-PRERELEASE/amd64] Kernel panic during heavy UFS traffic

[Jason Harmening]

Last night I ran into a series of kernel panics that seemed to be related to
heavy UFS traffic.  I ran into two consecutive panics when trying to mount a
UFS-formatted DVD-RAM as a regular user (though not when I mounted it as
root).  The system seemed to actually succeed in mounting the disk, as it
was marked dirty after the ensuing panic.  Upon rebooting after the second
panic, I saw another two consecutive panics which happened whenever I tried
to do something fairly disk-intensive (e.g. starting the X server + KDE)
while the bgfsck was still running from the last panic.  Ultimately I
rebooted in single-user mode, ran fsck manually, and have experienced no
further panics.  I suspect these panics may be related to UFS deadlocks, as
in all cases the application that was attempting disk access hung for
several seconds before the panic, followed by a few seconds of total system
hang, followed by the automatic reboot.

I'm running 6.1-PRELEASE/amd64 from 12 March on an Athlon 64 x2 (SMP) with
SCHED_ULE+PREEMPTION--dangerous combination I know, but it's been rock solid
for months until now.  If anyone is interested, I'll try to reproduce this
panic with a dump/backtrace.  It may be one of the UFS deadlock issues
that's already under investigation for 6.1-RELEASE.

Thanks,
Jason Harmening