[9fans] ps hang

[erik quanstrom <quanstro@coraid.com>]

i know this has been reported before.  and i've
guessed at exactly this problem, but it's happened
again here.  and this time i have a bit of evidence.

i haven't quite tracked everything down, or
what the original problem is (that is, who
owns the lock and more importantly, why)
but the hang is because of the qlock of
p->debug in procopen.

minooka# acid -k 55053 /n/dump/2009/0819/386/9pccpu
/n/dump/2009/0819/386/9pccpu:386 plan 9 boot image
/sys/lib/acid/port
/sys/lib/acid/386
acid: stk()
sched()+0x12d /sys/src/9/port/proc.c:155
qlock(q=0xf03d9244)+0x141 /sys/src/9/port/qlock.c:48
procopen(omode=0x0,c=0xf6e52ab8)+0x193 /sys/src/9/port/devproc.c:369
namec(aname=0xf709f318,amode=0x3,omode=0x0,perm=0x0)+0x6d9 /sys/src/9/port/chan.c:1491
sysopen(arg=0xf041c1a0)+0xa9 /sys/src/9/port/sysfile.c:279
syscall(ureg=0xf6810e6c)+0x1b5 /sys/src/9/pc/trap.c:712
_syscallintr()+0x18 /sys/src/9/pc/plan9l.s:45
0xf6810e6c ?file?:0

- erik