a different approach to panic.  for details on the new
rules see the already-appled patch to panic(9nix).

the idea here is to *never* hang a machine.  it's too
annoying.

Reference: /n/atom/patch/applied2013/panicreliable
Date: Sun Dec 29 16:15:51 CET 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/nix/port/edf.c	Sun Dec 29 16:15:32 2013
+++ /sys/src/nix/port/edf.c	Sun Dec 29 16:15:33 2013
@@ -131,7 +131,6 @@
 deadlineintr(Ureg*, Timer *t)
 {
 	/* Proc reached deadline */
-	extern int panicking;
 	Proc *p;
 
 	if(panicking || active.exiting)
@@ -206,7 +205,6 @@
 releaseintr(Ureg*, Timer *t)
 {
 	Proc *p;
-	extern int panicking;
 	Sched *sch;
 	Schedq *rq;
 
--- /sys/src/nix/port/rdb.c	Sun Dec 29 16:15:34 2013
+++ /sys/src/nix/port/rdb.c	Sun Dec 29 16:15:35 2013
@@ -59,7 +59,7 @@
 	uchar *a;
 	char *p, *req;
 
-	delconsdevs();		/* turn off serial console and kprint */
+	synccons();
 //	scrprint("Plan 9 debugger\n");
 	iprint("Edebugger reset\n");
 	for(;;){
--- /sys/src/nix/port/portfns.h	Sun Dec 29 16:15:36 2013
+++ /sys/src/nix/port/portfns.h	Sun Dec 29 16:15:36 2013
@@ -51,7 +51,6 @@
 uintptr		dbgpc(Proc*);
 int		decref(Ref*);
 void		delay(int);
-void		delconsdevs(void);
 Proc*		dequeueproc(Sched*, Schedq*, Proc*);
 Chan*		devattach(int, char*);
 Block*		devbread(Chan*, long, vlong);
@@ -342,6 +341,7 @@
 void		startwaitstats(int);
 int		swapcount(ulong);
 void		swapinit(void);
+void		synccons(void);
 void		syscallfmt(int, va_list list);
 void		sysretfmt(int, va_list, Ar0*, uvlong, uvlong);
 void*		sysexecregs(uintptr, uint, uint);
--- /sys/src/nix/port/portdat.h	Sun Dec 29 16:15:38 2013
+++ /sys/src/nix/port/portdat.h	Sun Dec 29 16:15:39 2013
@@ -995,8 +995,9 @@
  * console flags &c
  */
 enum {
-	Ciprint		= 2,		/* call this fn from iprint */
-	Cntorn		= 4,		/* change \n to \r\n */
+	Csync		= 1<<0,	/* use in synchronous mode (e.g. panic) */
+	Ciprint		= 1<<1,	/* call this fn from iprint */
+	Cntorn		= 1<<2,	/* change \n to \r\n */
 };
 
 /*
@@ -1050,13 +1051,13 @@
 extern	Ref	noteidalloc;
 extern	int	nphysseg;
 extern	int	nsyscall;
+extern	int	panicking;
 extern	Pgalloc	pga;
 extern	Physseg	physseg[];
 extern	Procalloc	procalloc;
 extern	uint	qiomaxatomic;
 extern	Sched	run[];
 extern	char*	statename[];
-extern	char*	sysname;
 extern	Lockstats lockstats;
 extern	QLockstats qlockstats;
 extern	Waitstats waitstats;
--- /sys/src/nix/port/devuart.c	Sun Dec 29 16:15:40 2013
+++ /sys/src/nix/port/devuart.c	Sun Dec 29 16:15:41 2013
@@ -433,7 +433,7 @@
 		}
 		p->putc = kbdcr2nl;
 		addkbdq(p->iq, -1);
-		addconsdev(p->oq, uartputs, 2, 0);
+		addconsdev(p->oq, uartputs, 2, Csync|Ciprint|Cntorn);
 //		consuart = p;
 		p->opens++;
 		p->console = 1;
--- /sys/src/nix/port/devcons.c	Sun Dec 29 16:15:43 2013
+++ /sys/src/nix/port/devcons.c	Sun Dec 29 16:15:44 2013
@@ -30,13 +30,14 @@
 
 static	Lock	consdevslock;
 static	int	nconsdevs = 3;
-static	Consdev	consdevs[Nconsdevs] =			/* keep this order */
+static	Consdev	consdevs[Nconsdevs] =
 {
-	{nil, nil,	kmesgputs,	0},			/* kmesg */
-	{nil, nil,	kprintputs,	Ciprint},		/* kprint */
-	{nil, nil,	uartputs,	Ciprint|Cntorn},	/* serial */
+	{nil, nil,	kmesgputs,	0, },			/* kmesg */
+	{nil, nil,	kprintputs,	Csync|Ciprint, },		/* kprint */
+	{nil, nil,	uartputs,		Csync|Ciprint|Cntorn, },	/* serial (botch! must be 2d) */
 };
 
+static	int	syncprint;
 static	int	nkbdqs;
 static	int	nkbdprocs;
 static	Queue*	kbdqs[Nconsdevs];
@@ -75,8 +76,8 @@
 	.ie	= kbd.istage + sizeof(kbd.istage),
 };
 
-char	*sysname;
-vlong	fasthz;
+static	char	*sysname;
+static	vlong	fasthz;
 
 static void	seedrand(void);
 static int	readtime(ulong, char*, int);
@@ -112,12 +113,11 @@
 	Consdev *c;
 
 	ilock(&consdevslock);
-	if(i < 0)
-		i = nconsdevs;
-	else
-		flags |= consdevs[i].flags;
-	if(nconsdevs == Nconsdevs)
+	if(i < 0){
+		if(nconsdevs == Nconsdevs)
 		panic("Nconsdevs too small");
+		i = nconsdevs;
+	}
 	c = &consdevs[i];
 	c->flags = flags;
 	c->q = q;
@@ -128,13 +128,6 @@
 	return i;
 }
 
-void
-delconsdevs(void)
-{
-	nconsdevs = 2;	/* throw away serial consoles and kprint */
-	consdevs[1].q = nil;
-}
-
 static void
 conskbdqproc(void *a)
 {
@@ -275,14 +268,25 @@
 	Chan *cc;
 	Queue *q;
 
+	if(syncprint){
+		usewrite = 0;
+		if((c->flags & Csync) == 0)
+			return;
+	}
+
+	if(!islo())
+		usewrite = 0;
+	if(usewrite == 0 && (c->flags & Ciprint) == 0)
+		return;
+
 	if((cc = c->c) != nil && usewrite)
 		cc->dev->write(cc, s, n, 0);
-	else if((q = c->q) != nil && !qisclosed(q))
+	else if(!syncprint && (q = c->q) != nil && !qisclosed(q)){
 		if(usewrite)
 			qwrite(q, s, n);
 		else
 			qiwrite(q, s, n);
-	else if(c->fn != nil)
+	}else
 		c->fn(s, n);
 }
 
@@ -299,9 +303,6 @@
 	char *s, *t;
 	int i, len, m;
 
-	if(!islo())
-		usewrite = 0;
-
 	for(i = 0; i < nconsdevs; i++){
 		c = &consdevs[i];
 		len = n;
@@ -372,7 +373,7 @@
 iprint(char *fmt, ...)
 {
 	Mpl pl;
-	int i, n, locked;
+	int n, locked;
 	va_list arg;
 	char buf[PRINTSIZE];
 
@@ -381,13 +382,7 @@
 	n = vseprint(buf, buf+sizeof(buf), fmt, arg) - buf;
 	va_end(arg);
 	locked = iprintcanlock(&iprintlock);
-	for(i = 0; i < nconsdevs; i++)
-		if((consdevs[i].flags&Ciprint) != 0){
-			if(consdevs[i].q != nil)
-				qiwrite(consdevs[i].q, buf, n);
-			else
-				consdevs[i].fn(buf, n);
-		}
+	putstrn0(buf, n, 0);
 	if(locked)
 		unlock(&iprintlock);
 	splx(pl);
@@ -395,35 +390,38 @@
 	return n;
 }
 
+void
+synccons(void)
+{
+	syncprint = 1;
+/*	prflush();	*/
+}
+
 #pragma profile 0
 void
 panic(char *fmt, ...)
 {
-	int n;
-	Mpl pl;
+	char *p, buf[PRINTSIZE];
 	va_list arg;
-	char buf[PRINTSIZE];
-
-	consdevs[1].q = nil;	/* don't try to write to /dev/kprint */
+	static pmno = -1;
 
-	if(panicking)
-		for(;;);
-	panicking = 1;
+	if(ainc(&panicking)>1){
+		if(m->machno == pmno)
+			archreset();
+		ndnr();
+	}
+	pmno = m->machno;
+	synccons();
 
-	pl = splhi();
-	seprint(buf, buf+sizeof buf, "panic: cpu%d: ", m->machno);
+	splhi();	/* never to release */
+	p = seprint(buf, buf+sizeof buf, "panic: cpu%d: ", m->machno);
 	va_start(arg, fmt);
-	n = vseprint(buf+strlen(buf), buf+sizeof(buf), fmt, arg) - buf;
+	vseprint(p, buf+sizeof(buf), fmt, arg);
 	va_end(arg);
 	iprint("%s\n", buf);
-	if(consdebug)
-		(*consdebug)();
-	splx(pl);
-	prflush();
-	buf[n] = '\n';
-	putstrn(buf, n+1);
+	if(consdebug != nil)
+		consdebug();
 	dumpstack();
-	delay(1000);	/* give time to consoles */
 
 	exit(1);
 }