From d02b74a5dcfed4bfc8f2f8e545bca4d2afabb296 Mon Sep 17 00:00:00 2001
From: Googler <noreply@google.com>
Date: Fri, 27 Apr 2018 10:37:02 -0700
Subject: Check in gVisor.

PiperOrigin-RevId: 194583126
Change-Id: Ica1d8821a90f74e7e745962d71801c598c652463
---
 pkg/tmutex/tmutex.go | 71 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 71 insertions(+)
 create mode 100644 pkg/tmutex/tmutex.go

(limited to 'pkg/tmutex/tmutex.go')

diff --git a/pkg/tmutex/tmutex.go b/pkg/tmutex/tmutex.go
new file mode 100644
index 000000000..61779654f
--- /dev/null
+++ b/pkg/tmutex/tmutex.go
@@ -0,0 +1,71 @@
+// Copyright 2016 The Netstack 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 tmutex provides the implementation of a mutex that implements an
+// efficient TryLock function in addition to Lock and Unlock.
+package tmutex
+
+import (
+	"sync/atomic"
+)
+
+// Mutex is a mutual exclusion primitive that implements TryLock in addition
+// to Lock and Unlock.
+type Mutex struct {
+	v  int32
+	ch chan struct{}
+}
+
+// Init initializes the mutex.
+func (m *Mutex) Init() {
+	m.v = 1
+	m.ch = make(chan struct{}, 1)
+}
+
+// Lock acquires the mutex. If it is currently held by another goroutine, Lock
+// will wait until it has a chance to acquire it.
+func (m *Mutex) Lock() {
+	// Uncontended case.
+	if atomic.AddInt32(&m.v, -1) == 0 {
+		return
+	}
+
+	for {
+		// Try to acquire the mutex again, at the same time making sure
+		// that m.v is negative, which indicates to the owner of the
+		// lock that it is contended, which will force it to try to wake
+		// someone up when it releases the mutex.
+		if v := atomic.LoadInt32(&m.v); v >= 0 && atomic.SwapInt32(&m.v, -1) == 1 {
+			return
+		}
+
+		// Wait for the mutex to be released before trying again.
+		<-m.ch
+	}
+}
+
+// TryLock attempts to acquire the mutex without blocking. If the mutex is
+// currently held by another goroutine, it fails to acquire it and returns
+// false.
+func (m *Mutex) TryLock() bool {
+	v := atomic.LoadInt32(&m.v)
+	if v <= 0 {
+		return false
+	}
+	return atomic.CompareAndSwapInt32(&m.v, 1, 0)
+}
+
+// Unlock releases the mutex.
+func (m *Mutex) Unlock() {
+	if atomic.SwapInt32(&m.v, 1) == 0 {
+		// There were no pending waiters.
+		return
+	}
+
+	// Wake some waiter up.
+	select {
+	case m.ch <- struct{}{}:
+	default:
+	}
+}
-- 
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