runtime: mark extra M's G as dead when not in use

Currently the extra Ms created for cgo callbacks have a corresponding
G that's kept in syscall state with only a call to goexit on its
stack. This leads to confusing output from runtime.NumGoroutines and
in tracebacks:

goroutine 17 [syscall, locked to thread]:
runtime.goexit()
	.../src/runtime/asm_amd64.s:2197 +0x1

Fix this by putting this goroutine into state _Gdead when it's not in
use instead of _Gsyscall. To keep the goroutine counts correct, we
also add one to sched.ngsys while the goroutine is in _Gdead. The
effect of this is as if the goroutine simply doesn't exist when it's
not in use.

Fixes #16631.
Fixes #16714.

Change-Id: Ieae08a2febd4b3d00bef5c23fd6ca88fb2bb0087
Reviewed-on: https://go-review.googlesource.com/45030
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/crash_cgo_test.go

@@ -395,3 +395,12 @@ func TestRaceSignal(t *testing.T) {
 		t.Errorf("expected %q got %s", want, got)
 	}
 }
+
+func TestCgoNumGoroutine(t *testing.T) {
+	t.Parallel()
+	got := runTestProg(t, "testprogcgo", "NumGoroutine")
+	want := "OK\n"
+	if got != want {
+		t.Errorf("expected %q got %v", want, got)
+	}
+}

src/runtime/proc.go

@@ -1435,6 +1435,10 @@ func needm(x byte) {
 	// Initialize this thread to use the m.
 	asminit()
 	minit()
+
+	// mp.curg is now a real goroutine.
+	casgstatus(mp.curg, _Gdead, _Gsyscall)
+	atomic.Xadd(&sched.ngsys, -1)
 }
 
 var earlycgocallback = []byte("fatal error: cgo callback before cgo call\n")
@@ -1477,9 +1481,11 @@ func oneNewExtraM() {
 	gp.stktopsp = gp.sched.sp
 	gp.gcscanvalid = true
 	gp.gcscandone = true
-	// malg returns status as Gidle, change to Gsyscall before adding to allg
-	// where GC will see it.
-	casgstatus(gp, _Gidle, _Gsyscall)
+	// malg returns status as _Gidle. Change to _Gdead before
+	// adding to allg where GC can see it. We use _Gdead to hide
+	// this from tracebacks and stack scans since it isn't a
+	// "real" goroutine until needm grabs it.
+	casgstatus(gp, _Gidle, _Gdead)
 	gp.m = mp
 	mp.curg = gp
 	mp.locked = _LockInternal
@@ -1492,6 +1498,12 @@ func oneNewExtraM() {
 	// put on allg for garbage collector
 	allgadd(gp)
 
+	// gp is now on the allg list, but we don't want it to be
+	// counted by gcount. It would be more "proper" to increment
+	// sched.ngfree, but that requires locking. Incrementing ngsys
+	// has the same effect.
+	atomic.Xadd(&sched.ngsys, +1)
+
 	// Add m to the extra list.
 	mnext := lockextra(true)
 	mp.schedlink.set(mnext)
@@ -1528,6 +1540,10 @@ func dropm() {
 	// with no pointer manipulation.
 	mp := getg().m
 
+	// Return mp.curg to dead state.
+	casgstatus(mp.curg, _Gsyscall, _Gdead)
+	atomic.Xadd(&sched.ngsys, +1)
+
 	// Block signals before unminit.
 	// Unminit unregisters the signal handling stack (but needs g on some systems).
 	// Setg(nil) clears g, which is the signal handler's cue not to run Go handlers.

src/runtime/testdata/testprogcgo/numgoroutine.go

@@ -0,0 +1,97 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+/*
+#include <stddef.h>
+#include <pthread.h>
+
+extern void CallbackNumGoroutine();
+
+static void* thread2(void* arg __attribute__ ((unused))) {
+	CallbackNumGoroutine();
+	return NULL;
+}
+
+static void CheckNumGoroutine() {
+	pthread_t tid;
+	pthread_create(&tid, NULL, thread2, NULL);
+	pthread_join(tid, NULL);
+}
+*/
+import "C"
+
+import (
+	"fmt"
+	"runtime"
+	"strings"
+)
+
+var baseGoroutines int
+
+func init() {
+	register("NumGoroutine", NumGoroutine)
+}
+
+func NumGoroutine() {
+	// Test that there are just the expected number of goroutines
+	// running. Specifically, test that the spare M's goroutine
+	// doesn't show up.
+	//
+	// On non-Windows platforms there's a signal handling thread
+	// started by os/signal.init in addition to the main
+	// goroutine.
+	if runtime.GOOS != "windows" {
+		baseGoroutines = 1
+	}
+	if _, ok := checkNumGoroutine("first", 1+baseGoroutines); !ok {
+		return
+	}
+
+	// Test that the goroutine for a callback from C appears.
+	if C.CheckNumGoroutine(); !callbackok {
+		return
+	}
+
+	// Make sure we're back to the initial goroutines.
+	if _, ok := checkNumGoroutine("third", 1+baseGoroutines); !ok {
+		return
+	}
+
+	fmt.Println("OK")
+}
+
+func checkNumGoroutine(label string, want int) (string, bool) {
+	n := runtime.NumGoroutine()
+	if n != want {
+		fmt.Printf("%s NumGoroutine: want %d; got %d\n", label, want, n)
+		return "", false
+	}
+
+	sbuf := make([]byte, 32<<10)
+	sbuf = sbuf[:runtime.Stack(sbuf, true)]
+	n = strings.Count(string(sbuf), "goroutine ")
+	if n != want {
+		fmt.Printf("%s Stack: want %d; got %d:\n%s\n", label, want, n, string(sbuf))
+		return "", false
+	}
+	return string(sbuf), true
+}
+
+var callbackok bool
+
+//export CallbackNumGoroutine
+func CallbackNumGoroutine() {
+	stk, ok := checkNumGoroutine("second", 2+baseGoroutines)
+	if !ok {
+		return
+	}
+	if !strings.Contains(stk, "CallbackNumGoroutine") {
+		fmt.Printf("missing CallbackNumGoroutine from stack:\n%s\n", stk)
+		return
+	}
+
+	callbackok = true
+}