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>

8 files changed, 110 insertions, 12 deletions

diff --git a/src/runtime/asm_arm.s b/src/runtime/asm_arm.s
index d8757fd0b9..09fbc952e0 100644
--- a/src/runtime/asm_arm.s
+++ b/src/runtime/asm_arm.s
@@ -556,7 +556,13 @@ TEXT	·cgocallback_gofunc(SB),NOSPLIT,$8-12
 	// lots of space, but the linker doesn't know. Hide the call from
 	// the linker analysis by using an indirect call.
 	CMP	$0, g
-	B.NE	havem
+	B.EQ	needm
+
+	MOVW	g_m(g), R8
+	MOVW	R8, savedm-4(SP)
+	B	havem
+
+needm:
 	MOVW	g, savedm-4(SP) // g is zero, so is m.
 	MOVW	$runtime·needm(SB), R0
 	BL	(R0)
@@ -577,8 +583,6 @@ TEXT	·cgocallback_gofunc(SB),NOSPLIT,$8-12
 	MOVW	R13, (g_sched+gobuf_sp)(R3)
 
 havem:
-	MOVW	g_m(g), R8
-	MOVW	R8, savedm-4(SP)
 	// Now there's a valid m, and we're running on its m->g0.
 	// Save current m->g0->sched.sp on stack and then set it to SP.
 	// Save current sp in m->g0->sched.sp in preparation for
diff --git a/src/runtime/asm_arm64.s b/src/runtime/asm_arm64.s
index 732abe13af..ab5d5b5e5f 100644
--- a/src/runtime/asm_arm64.s
+++ b/src/runtime/asm_arm64.s
@@ -586,7 +586,13 @@ nocgo:
 	// lots of space, but the linker doesn't know. Hide the call from
 	// the linker analysis by using an indirect call.
 	CMP	$0, g
-	BNE	havem
+	BEQ	needm
+
+	MOVD	g_m(g), R8
+	MOVD	R8, savedm-8(SP)
+	B	havem
+
+needm:
 	MOVD	g, savedm-8(SP) // g is zero, so is m.
 	MOVD	$runtime·needm(SB), R0
 	BL	(R0)
@@ -608,8 +614,6 @@ nocgo:
 	MOVD	R0, (g_sched+gobuf_sp)(R3)
 
 havem:
-	MOVD	g_m(g), R8
-	MOVD	R8, savedm-8(SP)
 	// Now there's a valid m, and we're running on its m->g0.
 	// Save current m->g0->sched.sp on stack and then set it to SP.
 	// Save current sp in m->g0->sched.sp in preparation for
diff --git a/src/runtime/asm_mips64x.s b/src/runtime/asm_mips64x.s
index 7d3d7c2ae2..08482fed23 100644
--- a/src/runtime/asm_mips64x.s
+++ b/src/runtime/asm_mips64x.s
@@ -486,7 +486,13 @@ nocgo:
 	// In this case, we're running on the thread stack, so there's
 	// lots of space, but the linker doesn't know. Hide the call from
 	// the linker analysis by using an indirect call.
-	BNE	g, havem
+	BEQ	g, needm
+
+	MOVV	g_m(g), R3
+	MOVV	R3, savedm-8(SP)
+	JMP	havem
+
+needm:
 	MOVV	g, savedm-8(SP) // g is zero, so is m.
 	MOVV	$runtime·needm(SB), R4
 	JAL	(R4)
@@ -507,8 +513,6 @@ nocgo:
 	MOVV	R29, (g_sched+gobuf_sp)(R1)
 
 havem:
-	MOVV	g_m(g), R3
-	MOVV	R3, savedm-8(SP)
 	// Now there's a valid m, and we're running on its m->g0.
 	// Save current m->g0->sched.sp on stack and then set it to SP.
 	// Save current sp in m->g0->sched.sp in preparation for
diff --git a/src/runtime/asm_ppc64x.s b/src/runtime/asm_ppc64x.s
index f3b193ae31..6d003b04e1 100644
--- a/src/runtime/asm_ppc64x.s
+++ b/src/runtime/asm_ppc64x.s
@@ -602,7 +602,13 @@ nocgo:
 	// lots of space, but the linker doesn't know. Hide the call from
 	// the linker analysis by using an indirect call.
 	CMP	g, $0
-	BNE	havem
+	BEQ	needm
+
+	MOVD	g_m(g), R8
+	MOVD	R8, savedm-8(SP)
+	BR	havem
+
+needm:
 	MOVD	g, savedm-8(SP) // g is zero, so is m.
 	MOVD	$runtime·needm(SB), R12
 	MOVD	R12, CTR
@@ -624,8 +630,6 @@ nocgo:
 	MOVD	R1, (g_sched+gobuf_sp)(R3)
 
 havem:
-	MOVD	g_m(g), R8
-	MOVD	R8, savedm-8(SP)
 	// Now there's a valid m, and we're running on its m->g0.
 	// Save current m->g0->sched.sp on stack and then set it to SP.
 	// Save current sp in m->g0->sched.sp in preparation for
diff --git a/src/runtime/crash_cgo_test.go b/src/runtime/crash_cgo_test.go
index 9422a08620..92b4f0ca71 100644
--- a/src/runtime/crash_cgo_test.go
+++ b/src/runtime/crash_cgo_test.go
@@ -134,3 +134,12 @@ func TestCgoExecSignalMask(t *testing.T) {
 		t.Errorf("expected %q, got %v", want, got)
 	}
 }
+
+func TestEnsureDropM(t *testing.T) {
+	// Test for issue 13881.
+	got := runTestProg(t, "testprogcgo", "EnsureDropM")
+	want := "OK\n"
+	if got != want {
+		t.Errorf("expected %q, got %v", want, got)
+	}
+}
diff --git a/src/runtime/proc.go b/src/runtime/proc.go
index b14aabde3d..545e134cc2 100644
--- a/src/runtime/proc.go
+++ b/src/runtime/proc.go
@@ -1443,6 +1443,11 @@ func dropm() {
 	unlockextra(mp)
 }
 
+// A helper function for EnsureDropM.
+func getm() uintptr {
+	return uintptr(unsafe.Pointer(getg().m))
+}
+
 var extram uintptr
 
 // lockextra locks the extra list and returns the list head.
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