6.4-PRELEASE sporadically panicking with fatal trap 12

[barbara]

> barbara wrote:
> > Hello,
> > I'm running 6.4-PRELEASE, last built on 2008-10-05 with /usr/src updated on the same day.
> > I had a panic that looks to me very similiar to the one described here (hence the subject): 
> > http://lists.freebsd.org/pipermail/freebsd-stable/2008-September/045405.html
> >
> > What caught my curiosity is the message:
> > 	
> > 	Unread portion of the kernel message buffer:
> >
> > 	acd0: WARNING - TEST_UNIT_READY read data overrun 18>0
> >
> > 	kernel trap 12 with interrupts disabled
> >
> > I don't have atapicam built in the kernel and it wasn't loaded, and I'm pretty sure no media was inserted in my dvdrw unit since the last boot.
> > The other report has a similar message too (acd1: WARNING - READ_TOC read data overrun 18>12)
> >
> >
> > Here's the backtrace:
> >   
> Interesting.  I ran 6.3 for a bit before I changed over to 7.0.  Neither
> 6.3 or 7.0 exhibited this problem.
> 
> I'm at 7.1 prerelease #4 now, and I'm using Fluxbox instead of Gnome. 
> The system has been up six days with no problems.  I'll probably try
> using Gnome again after 7.1 release is out.  There's also a patch to ATA
> that I might try.  Or possibly I'll just wait for 7.1. :-)
> 

Obviously I was confused when I wrote about atapicam, in fact the message is about acd0.
Anyway I'm sure that no media was inserted during the whole uptime.
I'm running both 6 and 7 stable and I've never seen this before too.

Few minutes ago, while cron was running, the system froze for a couple of minutes and the these lines was added to /var/log/messages:

	acd0: WARNING - PREVENT_ALLOW taskqueue timeout - completing request directly
	acd0: WARNING - PREVENT_ALLOW freeing taskqueue zombie request

and again, no media was inserted.
The only change I did in the last days was enabling powerd, I have no idea if this could be related.