runtime: don't run finalizers if we're still on the g0 stack.

R=golang-dev, rsc, dvyukov, khr
CC=golang-dev
https://golang.org/cl/11386044

src/pkg/runtime/mgc0.c

@@ -2098,7 +2098,7 @@ runtime·gc(int32 force)
 	runtime·starttheworld();
 	m->locks--;
 
-	// now that gc is done and we're back on g stack, kick off finalizer thread if needed
+	// now that gc is done, kick off finalizer thread if needed
 	if(finq != nil) {
 		runtime·lock(&finlock);
 		// kick off or wake up goroutine to run queued finalizers
@@ -2109,11 +2109,12 @@ runtime·gc(int32 force)
 			runtime·ready(fing);
 		}
 		runtime·unlock(&finlock);
-		// give the queued finalizers, if any, a chance to run
-		runtime·gosched();
 	}
 	if(g->preempt)  // restore the preemption request in case we've cleared it in newstack
 		g->stackguard0 = StackPreempt;
+	// give the queued finalizers, if any, a chance to run
+	if(g != m->g0)
+		runtime·gosched();
 }
 
 static void

src/pkg/runtime/stack.c

@@ -105,7 +105,7 @@ runtime·stackalloc(uint32 n)
 		m->stackinuse++;
 		return v;
 	}
-	return runtime·mallocgc(n, 0, FlagNoProfiling|FlagNoGC|FlagNoZero|FlagNoInvokeGC);
+	return runtime·mallocgc(n, 0, FlagNoProfiling|FlagNoGC|FlagNoZero);
 }
 
 void