2019-04-29 21:25:05 +00:00
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// Copyright 2018 The gVisor Authors.
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2018-07-09 21:03:03 +00:00
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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2018-04-27 17:37:02 +00:00
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// Package tmutex provides the implementation of a mutex that implements an
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// efficient TryLock function in addition to Lock and Unlock.
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package tmutex
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import (
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"sync/atomic"
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)
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// Mutex is a mutual exclusion primitive that implements TryLock in addition
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// to Lock and Unlock.
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type Mutex struct {
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v int32
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ch chan struct{}
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}
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// Init initializes the mutex.
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func (m *Mutex) Init() {
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m.v = 1
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m.ch = make(chan struct{}, 1)
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}
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// Lock acquires the mutex. If it is currently held by another goroutine, Lock
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// will wait until it has a chance to acquire it.
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func (m *Mutex) Lock() {
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// Uncontended case.
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if atomic.AddInt32(&m.v, -1) == 0 {
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return
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}
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for {
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// Try to acquire the mutex again, at the same time making sure
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// that m.v is negative, which indicates to the owner of the
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// lock that it is contended, which will force it to try to wake
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// someone up when it releases the mutex.
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if v := atomic.LoadInt32(&m.v); v >= 0 && atomic.SwapInt32(&m.v, -1) == 1 {
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return
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}
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// Wait for the mutex to be released before trying again.
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<-m.ch
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}
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}
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// TryLock attempts to acquire the mutex without blocking. If the mutex is
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// currently held by another goroutine, it fails to acquire it and returns
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// false.
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func (m *Mutex) TryLock() bool {
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v := atomic.LoadInt32(&m.v)
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if v <= 0 {
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return false
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}
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return atomic.CompareAndSwapInt32(&m.v, 1, 0)
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}
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// Unlock releases the mutex.
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func (m *Mutex) Unlock() {
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if atomic.SwapInt32(&m.v, 1) == 0 {
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// There were no pending waiters.
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return
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}
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// Wake some waiter up.
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select {
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case m.ch <- struct{}{}:
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default:
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}
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}
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