Can't panic from debugger

[Robert Watson]

On Sun, 23 Dec 2007, Erwin Lansing wrote:

> The amd64 nodes in the pointyhat cluster are starting to behave quite 
> interestingly.  They stop to respond to ssh, but are still answering ping. 
> More worrying is that I cannot get a useful dump out of it, as a panic from 
> the debugger just hangs there, and all I am left with is to pull the plug. 
> This even happens on a normal working system after entering the debugger, of 
> which there is a typescript below.
>
> FreeBSD hammer1.isc.gumbysoft.com 8.0-CURRENT FreeBSD 8.0-CURRENT #11: Sat 
> Dec 8 17:18:09 UTC 2007 
> root at danner.isc.gumbysoft.com:/usr/obj/usr/src/sys/HAMMER amd64
>
> http://people.freebsd.org/~erwin/hammer1

I discovered yesterday that I was seeing the same problem on a dual-cpu, 
dual-core box in the netperf cluster:

cheetah-rwatson# sysctl debug.kdb.enter=1
debug.kdb.enter:K DB0: enter: sysctl debug.kdb.enter
[thread pid 1136 tid 100091 ]
Stopped at      kdb_enter+0x3d: movq    $0,0x603f38(%rip)
db> show pcpu
cpuid        = 0
curthread    = 0xffffff00032e56a0: pid 1136 "sysctl"
curpcb       = 0xffffffffabae7d40
fpcurthread  = none
idlethread   = 0xffffff00010d56a0: pid 11 "idle: cpu0"
spin locks held:
db> panic
panic: from debugger
cpuid = 0
<wedge>

I *can* get a coredump if I directly "call doadump" and then "reset", but I 
can't get one if I just do "panic".

Robert N M Watson
Computer Laboratory
University of Cambridge