runtime: in getTraceback, set gp->m before gogo

Currently, when collecting a traceback for another goroutine,
    getTraceback calls gogo(gp) switching to gp, which will resume in
    mcall, which will call gtraceback, which will set up gp->m. There
    is a gap between setting the current running g to gp and setting
    gp->m. If a profiling signal arrives in between, sigtramp will
    see a non-nil gp with a nil m, and will seg fault. Fix this by
    setting up gp->m first.
    
    Fixes golang/go#29448.
    
    Reviewed-on: https://go-review.googlesource.com/c/156038

From-SVN: r267658

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-2ce291eaee427799bfcde256929dab89e0ab61eb
+c257303eaef143663216e483857d5b259e05753f
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/pprof/pprof_test.go

@@ -946,3 +946,38 @@ func TestAtomicLoadStore64(t *testing.T) {
 	atomic.StoreUint64(&flag, 1)
 	<-done
 }
+
+func TestTracebackAll(t *testing.T) {
+	// With gccgo, if a profiling signal arrives at the wrong time
+	// during traceback, it may crash or hang. See issue #29448.
+	f, err := ioutil.TempFile("", "proftraceback")
+	if err != nil {
+		t.Fatalf("TempFile: %v", err)
+	}
+	defer os.Remove(f.Name())
+	defer f.Close()
+
+	if err := StartCPUProfile(f); err != nil {
+		t.Fatal(err)
+	}
+	defer StopCPUProfile()
+
+	ch := make(chan int)
+	defer close(ch)
+
+	count := 10
+	for i := 0; i < count; i++ {
+		go func() {
+			<-ch // block
+		}()
+	}
+
+	N := 10000
+	if testing.Short() {
+		N = 500
+	}
+	buf := make([]byte, 10*1024)
+	for i := 0; i < N; i++ {
+		runtime.Stack(buf, true)
+	}
+}

libgo/runtime/proc.c

@@ -442,6 +442,11 @@ void getTraceback(G*, G*) __asm__(GOSYM_PREFIX "runtime.getTraceback");
 // goroutine stored in the traceback field, which is me.
 void getTraceback(G* me, G* gp)
 {
+	M* holdm;
+
+	holdm = gp->m;
+	gp->m = me->m;
+
 #ifdef USING_SPLIT_STACK
 	__splitstack_getcontext((void*)(&me->stackcontext[0]));
 #endif
@@ -450,6 +455,8 @@ void getTraceback(G* me, G* gp)
 	if (gp->traceback != 0) {
 		runtime_gogo(gp);
 	}
+
+	gp->m = holdm;
 }
 
 // Do a stack trace of gp, and then restore the context to
@@ -459,17 +466,11 @@ void
 gtraceback(G* gp)
 {
 	Traceback* traceback;
-	M* holdm;
 
 	traceback = (Traceback*)gp->traceback;
 	gp->traceback = 0;
-	holdm = gp->m;
-	if(holdm != nil && holdm != g->m)
-		runtime_throw("gtraceback: m is not nil");
-	gp->m = traceback->gp->m;
 	traceback->c = runtime_callers(1, traceback->locbuf,
 		sizeof traceback->locbuf / sizeof traceback->locbuf[0], false);
-	gp->m = holdm;
 	runtime_gogo(traceback->gp);
 }