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// Copyright 2018 The gVisor Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// Package amutex provides the implementation of an abortable mutex. It allows
// the Lock() function to be canceled while it waits to acquire the mutex.
package amutex
import (
"sync/atomic"
)
// Sleeper must be implemented by users of the abortable mutex to allow for
// cancelation of waits.
type Sleeper interface {
// SleepStart is called by the AbortableMutex.Lock() function when the
// mutex is contended and the goroutine is about to sleep.
//
// A channel can be returned that causes the sleep to be canceled if
// it's readable. If no cancelation is desired, nil can be returned.
SleepStart() <-chan struct{}
// SleepFinish is called by AbortableMutex.Lock() once a contended mutex
// is acquired or the wait is aborted.
SleepFinish(success bool)
}
// NoopSleeper is a stateless no-op implementation of Sleeper for anonymous
// embedding in other types that do not support cancelation.
type NoopSleeper struct{}
// SleepStart implements Sleeper.SleepStart.
func (NoopSleeper) SleepStart() <-chan struct{} {
return nil
}
// SleepFinish implements Sleeper.SleepFinish.
func (NoopSleeper) SleepFinish(success bool) {}
// AbortableMutex is an abortable mutex. It allows Lock() to be aborted while it
// waits to acquire the mutex.
type AbortableMutex struct {
v int32
ch chan struct{}
}
// Init initializes the abortable mutex.
func (m *AbortableMutex) Init() {
m.v = 1
m.ch = make(chan struct{}, 1)
}
// Lock attempts to acquire the mutex, returning true on success. If something
// is written to the "c" while Lock waits, the wait is aborted and false is
// returned instead.
func (m *AbortableMutex) Lock(s Sleeper) bool {
// Uncontended case.
if atomic.AddInt32(&m.v, -1) == 0 {
return true
}
var c <-chan struct{}
if s != nil {
c = s.SleepStart()
}
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 ill 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 {
if s != nil {
s.SleepFinish(true)
}
return true
}
// Wait for the owner to wake us up before trying again, or for
// the wait to be aborted by the provided channel.
select {
case <-m.ch:
case <-c:
// s must be non-nil, otherwise c would be nil and we'd
// never reach this path.
s.SleepFinish(false)
return false
}
}
}
// Unlock releases the mutex.
func (m *AbortableMutex) 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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