[9fans] Lock loop in malloc()

[Lucio De Re <lucio@proxima.alt.za>]

While attempting to compile Bison (yeah, still gnawing at that
bone!) I have managed to jam cpp more or less solid.  That's compiling
scan-code-c.c which reduces to compiling scan-code.c.

However, it does not seem to be Bison that's at fault: it seems that an
invocation of alloc() tries to set a lock and never succeeds or gives up.

This is a summary, with some help from acid, subject to some very limited
knowledge on my part:

	term% acid 3208
	/proc/3208/text:386 plan 9 executable
	/sys/lib/acid/port
	/sys/lib/acid/386
	acid: lstk()
	sleep()+0x7 /sys/src/libc/9syscall/sleep.s:5
	lock(lk=0x1f2f8)+0xb7 /sys/src/libc/port/lock.c:25
	plock(p=0x18310)+0x16 /sys/src/libc/port/malloc.c:81
	poolalloc(p=0x18310,n=0x20)+0xf /sys/src/libc/port/pool.c:1223
	malloc(size=0x18)+0x1c /sys/src/libc/port/malloc.c:207
	domalloc(size=0x18)+0xf /sys/src/cmd/cpp/cpp.c:271
	lookup(tp=0xab4a8,install=0x1)+0x74 /sys/src/cmd/cpp/nlist.c:213
	dodefine(trp=0xdfffeeac)+0x40 /sys/src/cmd/cpp/macro.c:23
	control(trp=0xdfffeeac)+0x4b2 /sys/src/cmd/cpp/cpp.c:133
	process(trp=0xdfffeeac)+0xec /sys/src/cmd/cpp/cpp.c:70
	main(argc=0xb,argv=0xdfffef0c)+0x8a /sys/src/cmd/cpp/cpp.c:35
	_main+0x31 /sys/src/libc/386/main9.s:16
	acid: lstk()
	sleep()+0x7 /sys/src/libc/9syscall/sleep.s:5
	lock(lk=0x1f2f8)+0xb7 /sys/src/libc/port/lock.c:25
		i=0x3e8
	plock(p=0x18310)+0x16 /sys/src/libc/port/malloc.c:81
		pv=0x1f2f8
	poolalloc(p=0x18310,n=0x20)+0xf /sys/src/libc/port/pool.c:1223
		v=0x1928
	malloc(size=0x18)+0x1c /sys/src/libc/port/malloc.c:207
		v=0x18310
	domalloc(size=0x18)+0xf /sys/src/cmd/cpp/cpp.c:271
		p=0x0
	lookup(tp=0xab4a8,install=0x1)+0x74 /sys/src/cmd/cpp/nlist.c:213
		h=0x6f
		np=0x1
	dodefine(trp=0xdfffeeac)+0x40 /sys/src/cmd/cpp/macro.c:23
		dots=0x0
		tp=0xab4a8
		np=0x27100
		args=0x3e
		narg=0x204b6
		err=0x6
		atp=0x27140
		def=0x3876
		tap=0x204b6
	control(trp=0xdfffeeac)+0x4b2 /sys/src/cmd/cpp/cpp.c:133
		tp=0xab498
		np=0x27100
	process(trp=0xdfffeeac)+0xec /sys/src/cmd/cpp/cpp.c:70
		anymacros=0x80000020
	main(argc=0xb,argv=0xdfffef0c)+0x8a /sys/src/cmd/cpp/cpp.c:35
		ebuf=0x3a707063
		tr=0xab498
	_main+0x31 /sys/src/libc/386/main9.s:16
	acid:

I've no idea how to track this problem down, let alone fix it.  But this
problem is reproducible, albeit not using a small code base.  It is
mildly possible that my Plan 9 installation is not altogether pristine
and is causing this situation, but I can't think how.

++L