runtime: fix racefuncenter argument corruption.

Revision 6a88e1893941 corrupts the argument to
racefuncenter by pushing the data block pointer
to the stack.

Fixes #4885.

R=dvyukov, rsc
CC=golang-dev
https://golang.org/cl/7381053

src/pkg/runtime/race.c

@@ -94,10 +94,7 @@ runtime·racefuncenter1(uintptr pc)
 {
 	// If the caller PC is lessstack, use slower runtime·callers
 	// to walk across the stack split to find the real caller.
-	// Same thing if the PC is on the heap, which should be a
-	// closure trampoline.
-	if(pc == (uintptr)runtime·lessstack ||
-		(pc >= (uintptr)runtime·mheap->arena_start && pc < (uintptr)runtime·mheap->arena_used))
+	if(pc == (uintptr)runtime·lessstack)
 		runtime·callers(2, &pc, 1);
 
 	m->racecall = true;
@@ -162,8 +159,7 @@ memoryaccess(void *addr, uintptr callpc, uintptr pc, bool write)
 		m->racecall = true;
 		racectx = g->racectx;
 		if(callpc) {
-			if(callpc == (uintptr)runtime·lessstack ||
-				(callpc >= (uintptr)runtime·mheap->arena_start && callpc < (uintptr)runtime·mheap->arena_used))
+			if(callpc == (uintptr)runtime·lessstack)
 				runtime·callers(3, &callpc, 1);
 			runtime∕race·FuncEnter(racectx, (void*)callpc);
 		}
@@ -198,8 +194,7 @@ rangeaccess(void *addr, uintptr size, uintptr step, uintptr callpc, uintptr pc,
 		m->racecall = true;
 		racectx = g->racectx;
 		if(callpc) {
-			if(callpc == (uintptr)runtime·lessstack ||
-				(callpc >= (uintptr)runtime·mheap->arena_start && callpc < (uintptr)runtime·mheap->arena_used))
+			if(callpc == (uintptr)runtime·lessstack)
 				runtime·callers(3, &callpc, 1);
 			runtime∕race·FuncEnter(racectx, (void*)callpc);
 		}

src/pkg/runtime/race_amd64.s

@@ -4,8 +4,11 @@
 
 // +build race
 
-TEXT	runtime·racefuncenter(SB),7,$0
-	PUSHQ	DX // save function entry context (for closures)
+// func runtime·racefuncenter(pc uintptr)
+TEXT	runtime·racefuncenter(SB), 7, $16
+	MOVQ	DX, saved-8(SP) // save function entry context (for closures)
+	MOVQ	pc+0(FP), DX
+	MOVQ	DX, arg-16(SP)
 	CALL	runtime·racefuncenter1(SB)
-	POPQ	DX
+	MOVQ	saved-8(SP), DX
 	RET