cleanup exit() which still allowed splhi'ing.  that bit has been
removed and we put the console into synchronous mode.  this
allows us to reboot without worring about interrupts calling sched
again.

if anyone sees nix double panic after this change, please post a note to this list.

Reference: /n/atom/patch/applied2013/exitcleanup
Date: Sun Dec 29 22:30:47 CET 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/nix/k10/main.c	Sun Dec 29 22:27:19 2013
+++ /sys/src/nix/k10/main.c	Sun Dec 29 22:27:19 2013
@@ -8,7 +8,6 @@
 #include "io.h"
 #include "apic.h"
 
-uintptr	kseg0 = KZERO;
 Sys*	sys;
 char	dbgflg[256];
 
@@ -40,9 +39,6 @@
 	m->splpc = 0;
 	m->online = 1;
 
-	/*
-	 * CAUTION: no time sync done, etc.
-	 */
 	DBG("Wait for the thunderbirds!\n");
 	while(!active.thunderbirdsarego)
 		monmwait(&active.thunderbirdsarego, 0);
@@ -62,16 +58,12 @@
 	ainc(&active.nonline);
 
 	schedinit();
-
 	panic("squidboy returns");
 }
 
 /*
- * Rendezvous with other cores.
- * wait until they are initialized.
- * Sync TSC with them.
- * We assume other processors that could boot had time to
- * set online to 1 by now.
+ * Wait for other cores to be initialized and sync tsc counters.
+ * Assume other cores have had time to set active.online=1.
  */
 static void
 nixsquids(void)
@@ -142,9 +134,7 @@
 	consputs = cgaconsputs;
 
 	vsvminit(MACHSTKSZ);
-
 	sys->nmach = 1;
-
 	fmtinit();
 	print("\nnix\n");
 
@@ -164,20 +154,19 @@
 	mallocinit();
 	archpciinit();
 
-  	if(getconf("*maxmach") != nil)
-  		maxmach = atoi(getconf("*maxmach"));
-	mpsinit(maxmach);		/* acpi */
-
-	umeminit();
-	trapinit();
-	printinit();
-
 	/*
 	 * This is necessary with GRUB and QEMU. Without it an interrupt can occur
 	 * at a weird vector, because the vector base is likely different, causing
 	 * havoc. Do it before any APIC initialisation.
 	 */
 	i8259init(32);
+	if(getconf("*maxmach") != nil)
+		maxmach = atoi(getconf("*maxmach"));
+	mpsinit(maxmach);		/* acpi */
+
+	umeminit();
+	trapinit();
+	printinit();
 
 	procinit0();
 	lapiconline();
@@ -301,9 +290,7 @@
 	 * Kernel Stack
 	 *
 	 * N.B. make sure there's enough space for syscall to check
-	 *	for valid args and
-	 *	space for gotolabel's return PC
-	 * AMD64 stack must be quad-aligned.
+	 * for valid args and space for gotolabel's return PC
 	 */
 	p->sched.pc = PTR2UINT(init0);
 	p->sched.sp = PTR2UINT(p->kstack+KSTACK-sizeof(up->arg)-sizeof(uintptr));
@@ -342,43 +329,6 @@
 	ready(p);
 }
 
-static void
-shutdown(int ispanic)
-{
-	int ms, once;
-
-	ilock(&active);
-	if(ispanic)
-		active.ispanic = ispanic;
-	else if(m->machno == 0 && m->online == 0)
-		active.ispanic = 0;
-	once = m->online;
-	m->online = 0;
-	adec(&active.nonline);
-	active.exiting = 1;
-	iunlock(&active);
-
-	if(once)
-		iprint("cpu%d: exiting\n", m->machno);
-
-	spllo();
-	for(ms = 5*1000; ms > 0; ms -= TK2MS(2)){
-		delay(TK2MS(2));
-		if(active.nonline == 0 && consactive() == 0)
-			break;
-	}
-
-	if(active.ispanic && m->machno == 0){
-		if(cpuserver)
-			delay(30000);
-		else
-			for(;;)
-				halt();
-	}
-	else
-		delay(1000);
-}
-
 void
 apshut(void *v)
 {
@@ -386,8 +336,8 @@
 
 	i = (int)(uintptr)v;
 	procwired(up, i);
-	sched();
 	splhi();
+	m->online = 0;
 	adec(&active.nonline);
 	ndnr();
 }
@@ -402,7 +352,6 @@
 
 //	writeconf();
 	procwired(up, 0);
-	sched();
 
 	for(i = 1; i < active.nonline; i++)
 		kproc("apshut", apshut, (void*)i);
@@ -435,6 +384,23 @@
 void
 exit(int ispanic)
 {
-	shutdown(ispanic);
+	synccons();
+
+	/* accounting */
+	if(!m->online)
+		ndnr();
+	m->online = 0;
+	iprint("cpu%d: exiting\n", m->machno);
+	adec(&active.nonline);
+	ainc(&active.exiting);
+
+	/* wait (terminals wait forever) */
+	if(ispanic && !cpuserver)
+		ndnr();
+	for(int ms = 0; ms < 5000; ms += 1){
+		if(active.nonline == 0)
+			break;
+		delay(1);
+	}
 	archreset();
 }
--- /sys/src/nix/k10/dat.h	Sun Dec 29 22:27:19 2013
+++ /sys/src/nix/k10/dat.h	Sun Dec 29 22:27:19 2013
@@ -281,7 +281,6 @@
 
 struct
 {
-	Lock;
 	int	nonline;			/* # of active CPUs */
 	int	nbooting;			/* # of CPUs waiting for the bsp to go */
 	int	exiting;			/* shutdown */
@@ -298,11 +297,6 @@
 
 extern register Mach* m;			/* R15 */
 extern register Proc* up;			/* R14 */
-
-extern uintptr kseg0;
-
-#pragma	varargck	type	"R"	u64int
-#pragma	varargck	type	"W"	uintptr
 
 /*
  * Horrid.