runtime: avoid write barriers with traceback info

Unlike the gc runtime, libgo stores traceback information in location
    structs, which contain strings.  Therefore, copying location structs
    around appears to require write barriers, although in fact write
    barriers are never important because the strings are never allocated
    in Go memory.  They come from libbacktrace.
    
    Some of the generated write barriers come at times when write barriers
    are not permitted.  For example, exitsyscall, marked
    nowritebarrierrec, calls exitsyscallfast which calls traceGoSysExit
    which calls traceEvent which calls traceStackID which calls
    trace.stackTab.put which copies location values into memory allocated
    by tab.newStack.  This write barrier can be invoked when there is no
    p, causing a crash.
    
    This change fixes the problem by ensuring that location values are
    copied around in the tracing code with no write barriers.
    
    This was found by fixing the compiler to fully implement
    //go:nowritebarrierrec; CL to follow.
    
    Reviewed-on: https://go-review.googlesource.com/134226

From-SVN: r264282

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-70bd9801911f8ed27df410d36a928166c37a68fd
+baf07c40960dc4f8df9da97281870d80d4245b18
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/proc.go

@@ -1140,7 +1140,7 @@ func startTheWorldWithSema(emitTraceEvent bool) int64 {
 func kickoff() {
 	gp := getg()
 
-	if gp.traceback != nil {
+	if gp.traceback != 0 {
 		gtraceback(gp)
 	}
 
@@ -3097,7 +3097,7 @@ func newproc(fn uintptr, arg unsafe.Pointer) *g {
 	} else {
 		resetNewG(newg, &sp, &spsize)
 	}
-	newg.traceback = nil
+	newg.traceback = 0
 
 	if readgstatus(newg) != _Gdead {
 		throw("newproc1: new g is not Gdead")

libgo/go/runtime/runtime2.go

@@ -431,7 +431,7 @@ type g struct {
 
 	isSystemGoroutine bool // whether goroutine is a "system" goroutine
 
-	traceback *tracebackg // stack traceback buffer
+	traceback uintptr // stack traceback buffer
 
 	context      g_ucontext_t // saved context for setcontext
 	stackcontext [10]uintptr  // split-stack context

libgo/go/runtime/trace.go

@@ -135,6 +135,7 @@ var trace struct {
 }
 
 // traceBufHeader is per-P tracing buffer.
+//go:notinheap
 type traceBufHeader struct {
 	link      traceBufPtr              // in trace.empty/full
 	lastTicks uint64                   // when we wrote the last event
@@ -747,7 +748,8 @@ func (tab *traceStackTable) put(pcs []location) uint32 {
 	stk.n = len(pcs)
 	stkpc := stk.stack()
 	for i, pc := range pcs {
-		stkpc[i] = pc
+		// Use memmove to avoid write barrier.
+		memmove(unsafe.Pointer(&stkpc[i]), unsafe.Pointer(&pc), unsafe.Sizeof(pc))
 	}
 	part := int(hash % uintptr(len(tab.tab)))
 	stk.link = tab.tab[part]

libgo/go/runtime/traceback_gccgo.go

@@ -186,7 +186,7 @@ func tracebackothers(me *g) {
 	if gp != nil && gp != me {
 		print("\n")
 		goroutineheader(gp)
-		gp.traceback = (*tracebackg)(noescape(unsafe.Pointer(&tb)))
+		gp.traceback = (uintptr)(noescape(unsafe.Pointer(&tb)))
 		getTraceback(me, gp)
 		printtrace(tb.locbuf[:tb.c], nil)
 		printcreatedby(gp)
@@ -220,7 +220,7 @@ func tracebackothers(me *g) {
 			print("\tgoroutine in C code; stack unavailable\n")
 			printcreatedby(gp)
 		} else {
-			gp.traceback = (*tracebackg)(noescape(unsafe.Pointer(&tb)))
+			gp.traceback = (uintptr)(noescape(unsafe.Pointer(&tb)))
 			getTraceback(me, gp)
 			printtrace(tb.locbuf[:tb.c], nil)
 			printcreatedby(gp)

libgo/runtime/proc.c

@@ -338,7 +338,7 @@ runtime_mcall(FuncVal *fv)
 		gp = runtime_g();
 		mp = gp->m;
 
-		if(gp->traceback != nil)
+		if(gp->traceback != 0)
 			gtraceback(gp);
 	}
 	if (gp == nil || !gp->fromgogo) {
@@ -443,7 +443,7 @@ void getTraceback(G* me, G* gp)
 #endif
 	getcontext(ucontext_arg(&me->context[0]));
 
-	if (gp->traceback != nil) {
+	if (gp->traceback != 0) {
 		runtime_gogo(gp);
 	}
 }
@@ -457,8 +457,8 @@ gtraceback(G* gp)
 	Traceback* traceback;
 	M* holdm;
 
-	traceback = gp->traceback;
-	gp->traceback = nil;
+	traceback = (Traceback*)gp->traceback;
+	gp->traceback = 0;
 	holdm = gp->m;
 	if(holdm != nil && holdm != g->m)
 		runtime_throw("gtraceback: m is not nil");
@@ -510,7 +510,7 @@ runtime_mstart(void *arg)
 	// multiple times via the setcontext call in mcall.
 	getcontext(ucontext_arg(&gp->context[0]));
 
-	if(gp->traceback != nil) {
+	if(gp->traceback != 0) {
 		// Got here from getTraceback.
 		// I'm not sure this ever actually happens--getTraceback
 		// may always go to the getcontext call in mcall.