runtime: run getgcmask on the M stack.

LGTM=rsc
R=rsc, khr
CC=golang-codereviews
https://golang.org/cl/132660043

src/pkg/runtime/export_test.go

@@ -111,8 +111,19 @@ func ParForIters(desc *ParFor, tid uint32) (uint32, uint32) {
 	return begin, end
 }
 
+// in mgc0.c
 //go:noescape
-func GCMask(x interface{}) []byte
+func getgcmask(data unsafe.Pointer, typ *_type, array **byte, len *uint)
+
+func GCMask(x interface{}) (ret []byte) {
+	e := (*eface)(unsafe.Pointer(&x))
+	s := (*slice)(unsafe.Pointer(&ret))
+	onM(func() {
+		getgcmask(e.data, e._type, &s.array, &s.len)
+		s.cap = s.len
+	})
+	return
+}
 
 func testSchedLocalQueue()
 func testSchedLocalQueueSteal()

src/pkg/runtime/mgc0.c

@@ -1805,7 +1805,7 @@ getgcmaskcb(Stkframe *frame, void *ctxt)
 	Stkframe *frame0;
 
 	frame0 = ctxt;
-	if(frame0->sp >= frame->varp - frame->sp && frame0->sp < frame->varp) {
+	if(frame->sp <= frame0->sp && frame0->sp < frame->varp) {
 		*frame0 = *frame;
 		return false;
 	}
@@ -1865,7 +1865,7 @@ runtime·getgcmask(byte *p, Type *t, byte **mask, uintptr *len)
 	frame.fn = nil;
 	frame.sp = (uintptr)p;
 	cb = getgcmaskcb;
-	runtime·gentraceback((uintptr)runtime·getcallerpc(&p), (uintptr)runtime·getcallersp(&p), 0, g, 0, nil, 1000, &cb, &frame, false);
+	runtime·gentraceback(g->m->curg->sched.pc, g->m->curg->sched.sp, 0, g->m->curg, 0, nil, 1000, &cb, &frame, false);
 	if(frame.fn != nil) {
 		Func *f;
 		StackMap *stackmap;

src/pkg/runtime/stubs.goc

@@ -54,13 +54,6 @@ func runtime·gomcache() (ret *MCache) {
 	ret = g->m->mcache;
 }
 
-// For testing.
-// TODO: find a better place for this.
-func GCMask(x Eface) (mask Slice) {
-	runtime·getgcmask(x.data, x.type, &mask.array, &mask.len);
-	mask.cap = mask.len;
-}
-
 #pragma textflag NOSPLIT
 func reflect·typelinks() (ret Slice) {
         extern Type *runtime·typelink[], *runtime·etypelink[];