runtime: minor cleanup

implement runtime.casp on amd64.
keep simultaneous panic messages separate.

R=r
CC=golang-dev
https://golang.org/cl/4188053

src/pkg/runtime/amd64/asm.s

@@ -317,6 +317,26 @@ TEXT runtime·cas(SB), 7, $0
 	MOVL	$1, AX
 	RET
 
+// bool casp(void **val, void *old, void *new)
+// Atomically:
+//	if(*val == old){
+//		*val = new;
+//		return 1;
+//	} else
+//		return 0;
+TEXT runtime·casp(SB), 7, $0
+	MOVQ	8(SP), BX
+	MOVQ	16(SP), AX
+	MOVQ	24(SP), CX
+	LOCK
+	CMPXCHGQ	CX, 0(BX)
+	JZ 3(PC)
+	MOVL	$0, AX
+	RET
+	MOVL	$1, AX
+	RET
+
+
 // void jmpdefer(fn, sp);
 // called from deferreturn.
 // 1. pop the caller

src/pkg/runtime/mfinal.c

@@ -5,7 +5,9 @@
 #include "runtime.h"
 #include "malloc.h"
 
-// TODO(rsc): Why not just use mheap.Lock?
+// Lock to protect finalizer data structures.
+// Cannot reuse mheap.Lock because the finalizer
+// maintenance requires allocation.
 static Lock finlock;
 
 // Finalizer hash table.  Direct hash, linear scan, at most 3/4 full.

src/pkg/runtime/proc.c

@@ -63,7 +63,6 @@ struct Sched {
 	int32 mcount;	// number of ms that have been created
 	int32 mcpu;	// number of ms executing on cpu
 	int32 mcpumax;	// max number of ms allowed on cpu
-	int32 gomaxprocs;
 	int32 msyscall;	// number of ms in system calls
 
 	int32 predawn;	// running initialization, don't run new gs.
@@ -73,6 +72,7 @@ struct Sched {
 };
 
 Sched runtime·sched;
+int32 gomaxprocs;
 
 // Scheduling helpers.  Sched must be locked.
 static void gput(G*);	// put/get on ghead/gtail
@@ -116,13 +116,13 @@ runtime·schedinit(void)
 	// For debugging:
 	// Allocate internal symbol table representation now,
 	// so that we don't need to call malloc when we crash.
-	// findfunc(0);
+	// runtime·findfunc(0);
 
-	runtime·sched.gomaxprocs = 1;
+	runtime·gomaxprocs = 1;
 	p = runtime·getenv("GOMAXPROCS");
 	if(p != nil && (n = runtime·atoi(p)) != 0)
-		runtime·sched.gomaxprocs = n;
-	runtime·sched.mcpumax = runtime·sched.gomaxprocs;
+		runtime·gomaxprocs = n;
+	runtime·sched.mcpumax = runtime·gomaxprocs;
 	runtime·sched.mcount = 1;
 	runtime·sched.predawn = 1;
 
@@ -376,7 +376,7 @@ runtime·starttheworld(void)
 {
 	runtime·lock(&runtime·sched);
 	runtime·gcwaiting = 0;
-	runtime·sched.mcpumax = runtime·sched.gomaxprocs;
+	runtime·sched.mcpumax = runtime·gomaxprocs;
 	matchmg();
 	runtime·unlock(&runtime·sched);
 }
@@ -1019,6 +1019,7 @@ runtime·panic(Eface e)
 	}
 
 	// ran out of deferred calls - old-school panic now
+	runtime·startpanic();
 	printpanics(g->panic);
 	runtime·dopanic(0);
 }
@@ -1151,10 +1152,10 @@ runtime·gomaxprocsfunc(int32 n)
 	int32 ret;
 
 	runtime·lock(&runtime·sched);
-	ret = runtime·sched.gomaxprocs;
+	ret = runtime·gomaxprocs;
 	if (n <= 0)
 		n = ret;
-	runtime·sched.gomaxprocs = n;
+	runtime·gomaxprocs = n;
 	runtime·sched.mcpumax = n;
 	// handle fewer procs?
 	if(runtime·sched.mcpu > runtime·sched.mcpumax) {

src/pkg/runtime/runtime.c

@@ -8,7 +8,7 @@ enum {
 	maxround = sizeof(uintptr),
 };
 
-int32	runtime·panicking	= 0;
+uint32	runtime·panicking;
 
 int32
 runtime·gotraceback(void)
@@ -21,14 +21,24 @@ runtime·gotraceback(void)
 	return runtime·atoi(p);
 }
 
+static Lock paniclk;
+
 void
-runtime·dopanic(int32 unused)
+runtime·startpanic(void)
 {
-	if(runtime·panicking) {
-		runtime·printf("double panic\n");
+	if(m->dying) {
+		runtime·printf("panic during panic\n");
 		runtime·exit(3);
 	}
-	runtime·panicking++;
+	m->dying = 1;
+	runtime·xadd(&runtime·panicking, 1);
+	runtime·lock(&paniclk);
+}
+
+void
+runtime·dopanic(int32 unused)
+{
+	static bool didothers;
 
 	if(g->sig != 0)
 		runtime·printf("\n[signal %x code=%p addr=%p pc=%p]\n",
@@ -37,9 +47,23 @@ runtime·dopanic(int32 unused)
 	runtime·printf("\n");
 	if(runtime·gotraceback()){
 		runtime·traceback(runtime·getcallerpc(&unused), runtime·getcallersp(&unused), 0, g);
-		runtime·tracebackothers(g);
+		if(!didothers) {
+			didothers = true;
+			runtime·tracebackothers(g);
+		}
 	}
-	
+	runtime·unlock(&paniclk);
+	if(runtime·xadd(&runtime·panicking, -1) != 0) {
+		// Some other m is panicking too.
+		// Let it print what it needs to print.
+		// Wait forever without chewing up cpu.
+		// It will exit when it's done.
+		static Lock deadlock;
+		runtime·lock(&deadlock);
+		runtime·lock(&deadlock);
+	}
+
+	runtime·panicking = 1;  // so we don't dump another stack trace for breakpoint trap
 	runtime·breakpoint();  // so we can grab it in a debugger
 	runtime·exit(2);
 }
@@ -73,6 +97,7 @@ runtime·throwinit(void)
 void
 runtime·throw(int8 *s)
 {
+	runtime·startpanic();
 	runtime·printf("throw: %s\n", s);
 	runtime·dopanic(0);
 	*(int32*)0 = 0;	// not reached

src/pkg/runtime/runtime.h

@@ -224,6 +224,7 @@ struct	M
 	int32	locks;
 	int32	nomemprof;
 	int32	waitnextg;
+	int32	dying;
 	Note	havenextg;
 	G*	nextg;
 	M*	alllink;	// on allm
@@ -358,7 +359,7 @@ G*	runtime·allg;
 M*	runtime·allm;
 int32	runtime·goidgen;
 extern	int32	runtime·gomaxprocs;
-extern	int32	runtime·panicking;
+extern	uint32	runtime·panicking;
 extern	int32	runtime·gcwaiting;		// gc is waiting to run
 int8*	runtime·goos;
 extern	bool	runtime·iscgo;
@@ -455,6 +456,7 @@ void	runtime·gettime(int64*, int32*);
 int32	runtime·callers(int32, uintptr*, int32);
 int64	runtime·nanotime(void);
 void	runtime·dopanic(int32);
+void	runtime·startpanic(void);
 
 #pragma	varargck	argpos	runtime·printf	1
 #pragma	varargck	type	"d"	int32