runtime: fix race detector when map keys/values are passed by pointer.

Now that the map implementation is reading the keys and values from
arbitrary memory (instead of from stack slots), it needs to tell the
race detector when it does so.

Fixes #6875.

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/36360043

src/pkg/runtime/hashmap.c

@@ -991,9 +991,10 @@ reflect·makemap(MapType *t, Hmap *ret)
 void
 runtime·mapaccess1(MapType *t, Hmap *h, byte *ak, byte *av)
 {
-	if(raceenabled && h != nil)
+	if(raceenabled && h != nil) {
 		runtime·racereadpc(h, runtime·getcallerpc(&t), runtime·mapaccess1);
-
+		runtime·racereadpc(ak, runtime·getcallerpc(&t), runtime·mapaccess1);
+	}
 	if(h == nil || h->count == 0) {
 		av = t->elem->zero;
 	} else {
@@ -1021,8 +1022,10 @@ runtime·mapaccess1(MapType *t, Hmap *h, byte *ak, byte *av)
 void
 runtime·mapaccess2(MapType *t, Hmap *h, byte *ak, byte *av, bool pres)
 {
-	if(raceenabled && h != nil)
+	if(raceenabled && h != nil) {
 		runtime·racereadpc(h, runtime·getcallerpc(&t), runtime·mapaccess2);
+		runtime·racereadpc(ak, runtime·getcallerpc(&t), runtime·mapaccess2);
+	}
 
 	if(h == nil || h->count == 0) {
 		av = t->elem->zero;
@@ -1097,8 +1100,11 @@ runtime·mapassign1(MapType *t, Hmap *h, byte *ak, byte *av)
 	if(h == nil)
 		runtime·panicstring("assignment to entry in nil map");
 
-	if(raceenabled)
+	if(raceenabled) {
 		runtime·racewritepc(h, runtime·getcallerpc(&t), runtime·mapassign1);
+		runtime·racereadpc(ak, runtime·getcallerpc(&t), runtime·mapassign1);
+		runtime·racereadpc(av, runtime·getcallerpc(&t), runtime·mapassign1);
+	}
 
 	hash_insert(t, h, ak, av);
 
@@ -1121,8 +1127,10 @@ runtime·mapdelete(MapType *t, Hmap *h, byte *ak)
 	if(h == nil)
 		return;
 
-	if(raceenabled)
+	if(raceenabled) {
 		runtime·racewritepc(h, runtime·getcallerpc(&t), runtime·mapdelete);
+		runtime·racereadpc(ak, runtime·getcallerpc(&t), runtime·mapdelete);
+	}
 
 	hash_remove(t, h, ak);