runtime: fix arm/arm64/ppc64/mips64 to dropm when necessary

Fixes #13881.

Change-Id: Idff77db381640184ddd2b65022133bb226168800
Reviewed-on: https://go-review.googlesource.com/18449
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

2 files changed, 68 insertions, 0 deletions

diff --git a/src/runtime/testdata/testprogcgo/dropm.go b/src/runtime/testdata/testprogcgo/dropm.go
new file mode 100644
index 0000000000..80ccdcc608
--- /dev/null
+++ b/src/runtime/testdata/testprogcgo/dropm.go
@@ -0,0 +1,57 @@
+// Copyright 2016 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.
+
+// Test that a sequence of callbacks from C to Go get the same m.
+// This failed to be true on arm and arm64, which was the root cause
+// of issue 13881.
+
+package main
+
+/*
+#include <stddef.h>
+#include <pthread.h>
+
+extern void GoCheckM();
+
+static void* thread(void* arg __attribute__ ((unused))) {
+	GoCheckM();
+	return NULL;
+}
+
+static void CheckM() {
+	pthread_t tid;
+	pthread_create(&tid, NULL, thread, NULL);
+	pthread_join(tid, NULL);
+	pthread_create(&tid, NULL, thread, NULL);
+	pthread_join(tid, NULL);
+}
+*/
+import "C"
+
+import (
+	"fmt"
+	"os"
+)
+
+func init() {
+	register("EnsureDropM", EnsureDropM)
+}
+
+var savedM uintptr
+
+//export GoCheckM
+func GoCheckM() {
+	m := runtime_getm_for_test()
+	if savedM == 0 {
+		savedM = m
+	} else if savedM != m {
+		fmt.Printf("m == %x want %x\n", m, savedM)
+		os.Exit(1)
+	}
+}
+
+func EnsureDropM() {
+	C.CheckM()
+	fmt.Println("OK")
+}
diff --git a/src/runtime/testdata/testprogcgo/dropm_stub.go b/src/runtime/testdata/testprogcgo/dropm_stub.go
new file mode 100644
index 0000000000..4c3f46ade4
--- /dev/null
+++ b/src/runtime/testdata/testprogcgo/dropm_stub.go
@@ -0,0 +1,11 @@
+// Copyright 2016 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
+
+import _ "unsafe" // for go:linkname
+
+// Defined in the runtime package.
+//go:linkname runtime_getm_for_test runtime.getm
+func runtime_getm_for_test() uintptr