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// 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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