From 1effdc091b441c4b1ada4327c1422cd360f80f98 Mon Sep 17 00:00:00 2001
From: Ian Gudger <igudger@google.com>
Date: Tue, 21 Jan 2020 16:59:24 -0800
Subject: TMutex based on sync.Mutex.

Updates #231

PiperOrigin-RevId: 290854399
---
 pkg/sync/BUILD            |  2 ++
 pkg/sync/tmutex_test.go   | 71 +++++++++++++++++++++++++++++++++++++++++++++++
 pkg/sync/tmutex_unsafe.go | 49 ++++++++++++++++++++++++++++++++
 3 files changed, 122 insertions(+)
 create mode 100644 pkg/sync/tmutex_test.go
 create mode 100644 pkg/sync/tmutex_unsafe.go

diff --git a/pkg/sync/BUILD b/pkg/sync/BUILD
index e8cd16b8f..97c4b3b1e 100644
--- a/pkg/sync/BUILD
+++ b/pkg/sync/BUILD
@@ -38,6 +38,7 @@ go_library(
         "race_unsafe.go",
         "seqcount.go",
         "syncutil.go",
+        "tmutex_unsafe.go",
     ],
     importpath = "gvisor.dev/gvisor/pkg/sync",
 )
@@ -48,6 +49,7 @@ go_test(
     srcs = [
         "downgradable_rwmutex_test.go",
         "seqcount_test.go",
+        "tmutex_test.go",
     ],
     embed = [":sync"],
 )
diff --git a/pkg/sync/tmutex_test.go b/pkg/sync/tmutex_test.go
new file mode 100644
index 000000000..c640bae23
--- /dev/null
+++ b/pkg/sync/tmutex_test.go
@@ -0,0 +1,71 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package sync
+
+import (
+	"sync"
+	"testing"
+	"unsafe"
+)
+
+// TestStructSize verifies that syncMutex's size hasn't drifted from the
+// standard library's version.
+//
+// The correctness of this package relies on these remaining in sync.
+func TestStructSize(t *testing.T) {
+	const (
+		got  = unsafe.Sizeof(syncMutex{})
+		want = unsafe.Sizeof(sync.Mutex{})
+	)
+	if got != want {
+		t.Errorf("got sizeof(syncMutex) = %d, want = sizeof(sync.Mutex) = %d", got, want)
+	}
+}
+
+// TestFieldValues verifies that the semantics of syncMutex.state from the
+// standard library's implementation.
+//
+// The correctness of this package relies on these remaining in sync.
+func TestFieldValues(t *testing.T) {
+	var m TMutex
+	m.Lock()
+	if got := *m.state(); got != mutexLocked {
+		t.Errorf("got locked sync.Mutex.state = %d, want = %d", got, mutexLocked)
+	}
+	m.Unlock()
+	if got := *m.state(); got != mutexUnlocked {
+		t.Errorf("got unlocked sync.Mutex.state = %d, want = %d", got, mutexUnlocked)
+	}
+}
+
+func TestDoubleTryLock(t *testing.T) {
+	var m TMutex
+	if !m.TryLock() {
+		t.Fatal("failed to aquire lock")
+	}
+	if m.TryLock() {
+		t.Fatal("unexpectedly succeeded in aquiring locked mutex")
+	}
+}
+
+func TestTryLockAfterLock(t *testing.T) {
+	var m TMutex
+	m.Lock()
+	if m.TryLock() {
+		t.Fatal("unexpectedly succeeded in aquiring locked mutex")
+	}
+}
+
+func TestTryLockUnlock(t *testing.T) {
+	var m TMutex
+	if !m.TryLock() {
+		t.Fatal("failed to aquire lock")
+	}
+	m.Unlock()
+	if !m.TryLock() {
+		t.Fatal("failed to aquire lock after unlock")
+	}
+}
diff --git a/pkg/sync/tmutex_unsafe.go b/pkg/sync/tmutex_unsafe.go
new file mode 100644
index 000000000..3c32f8371
--- /dev/null
+++ b/pkg/sync/tmutex_unsafe.go
@@ -0,0 +1,49 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build go1.13
+// +build !go1.15
+
+// When updating the build constraint (above), check that syncMutex matches the
+// standard library sync.Mutex definition.
+
+package sync
+
+import (
+	"sync"
+	"sync/atomic"
+	"unsafe"
+)
+
+// TMutex is a try lock.
+type TMutex struct {
+	sync.Mutex
+}
+
+type syncMutex struct {
+	state int32
+	sema  uint32
+}
+
+func (m *TMutex) state() *int32 {
+	return &(*syncMutex)(unsafe.Pointer(&m.Mutex)).state
+}
+
+const (
+	mutexUnlocked = 0
+	mutexLocked   = 1
+)
+
+// TryLock tries to aquire the mutex. It returns true if it succeeds and false
+// otherwise. TryLock does not block.
+func (m *TMutex) TryLock() bool {
+	if atomic.CompareAndSwapInt32(m.state(), mutexUnlocked, mutexLocked) {
+		if RaceEnabled {
+			RaceAcquire(unsafe.Pointer(&m.Mutex))
+		}
+		return true
+	}
+	return false
+}
-- 
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