sync/rwmutex go读写锁学习

本文结构

背景
源码
思考
问题

背景

最近写go，看到学习一些go源码记录上，另外相关依赖代码这里简单说明一下

读写锁

概念去搜一下就好，可以上多个读锁，一个写锁
上写锁时要等之前的读锁释放

代码前提

信号量的获取与释放，看注释就够了，这个是runtime的

// Semacquire waits until *s > 0 and then atomically decrements it.
// It is intended as a simple sleep primitive for use by the synchronization
// library and should not be used directly.
func runtime_Semacquire(s *uint32)

// Semrelease atomically increments *s and notifies a waiting goroutine
// if one is blocked in Semacquire.
// It is intended as a simple wakeup primitive for use by the synchronization
// library and should not be used directly.
// If handoff is true, pass count directly to the first waiter.
func runtime_Semrelease(s *uint32, handoff bool)

源码

变量

    w           Mutex    互斥锁
    writerSem   uint32   写锁信号量
    readerSem   uint32   读锁信号量
    readerCount int32    还未释放读锁的reader数量(偏差2^30,有write就是原有reader数量-2^30)
    readerWait  int32    写锁要等待的reader数量(之前的reader释放了锁，writer才能跑)

源码

下面的源码没有分析锁竞争（非公平锁）相关代码逻辑

// Copyright 2009 The Go 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 sync

import (
    "internal/race"
    "sync/atomic"
    "unsafe"
)

// There is a modified copy of this file in runtime/rwmutex.go.
// If you make any changes here, see if you should make them there.

// A RWMutex is a reader/writer mutual exclusion lock.
// The lock can be held by an arbitrary number of readers or a single writer.
// The zero value for a RWMutex is an unlocked mutex.
//
// A RWMutex must not be copied after first use.
//
// If a goroutine holds a RWMutex for reading and another goroutine might
// call Lock, no goroutine should expect to be able to acquire a read lock
// until the initial read lock is released. In particular, this prohibits
// recursive read locking. This is to ensure that the lock eventually becomes
// available; a blocked Lock call excludes new readers from acquiring the
// lock.
type RWMutex struct {
    w           Mutex  // held if there are pending writers  互斥锁
    writerSem   uint32 // semaphore for writers to wait for completing readers  写锁信号量
    readerSem   uint32 // semaphore for readers to wait for completing writers  读锁信号量
    readerCount int32  // number of pending readers  还未释放读锁的reader数量(偏差2^30,有write就是原有reader数量-2^30)
    readerWait  int32  // number of departing readers  写锁要等待的reader数量(之前的reader释放了锁，writer才能跑)
}

const rwmutexMaxReaders = 1 << 30  //假定最多2^30个reader

// RLock locks rw for reading.
//
// It should not be used for recursive read locking; a blocked Lock
// call excludes new readers from acquiring the lock. See the
// documentation on the RWMutex type.
func (rw *RWMutex) RLock() { //读锁
    if race.Enabled {
        _ = rw.w.state
        race.Disable()
    }
    if atomic.AddInt32(&rw.readerCount, 1) < 0 { //当前有写锁了
        // A writer is pending, wait for it.
        runtime_Semacquire(&rw.readerSem) //等待reader信号量
    }
    if race.Enabled {
        race.Enable()
        race.Acquire(unsafe.Pointer(&rw.readerSem))
    }
}

// RUnlock undoes a single RLock call;
// it does not affect other simultaneous readers.
// It is a run-time error if rw is not locked for reading
// on entry to RUnlock.
func (rw *RWMutex) RUnlock() {  //释放写锁
    if race.Enabled {
        _ = rw.w.state
        race.ReleaseMerge(unsafe.Pointer(&rw.writerSem))
        race.Disable()
    }
    if r := atomic.AddInt32(&rw.readerCount, -1); r < 0 { //reader数量-1，如果<0的话
        if r+1 == 0 || r+1 == -rwmutexMaxReaders { //如果已经没有读锁的，还去释放（如释放多次）
            race.Enable()
            throw("sync: RUnlock of unlocked RWMutex")
        }
        // A writer is pending. 下面的情况代表有写锁
        if atomic.AddInt32(&rw.readerWait, -1) == 0 { //写锁的reader wait数量-1
            // The last reader unblocks the writer.
            runtime_Semrelease(&rw.writerSem, false)//如果wait数量到0，释放writer信号量
        }
    }
    if race.Enabled {
        race.Enable()
    }
}

// Lock locks rw for writing.
// If the lock is already locked for reading or writing,
// Lock blocks until the lock is available.
func (rw *RWMutex) Lock() {
    if race.Enabled {
        _ = rw.w.state
        race.Disable()
    }
    // First, resolve competition with other writers.
    rw.w.Lock() //上互斥锁
    // Announce to readers there is a pending writer.
    r := atomic.AddInt32(&rw.readerCount, -rwmutexMaxReaders) + rwmutexMaxReaders  //readerCount变成负数，代表有写锁，r就是计算前readCount的绝对值(存疑，atomic计算)
    // Wait for active readers.
    if r != 0 && atomic.AddInt32(&rw.readerWait, r) != 0 {//当前readCount不为0，并且上一步r计算之后RUnlock的次数和之前readerCount相同(上一步计算后可能有多次RUnlock,readerWait会变成负数)
        runtime_Semacquire(&rw.writerSem)//等待writer信号量
    }
    if race.Enabled {
        race.Enable()
        race.Acquire(unsafe.Pointer(&rw.readerSem))
        race.Acquire(unsafe.Pointer(&rw.writerSem))
    }
}

// Unlock unlocks rw for writing. It is a run-time error if rw is
// not locked for writing on entry to Unlock.
//
// As with Mutexes, a locked RWMutex is not associated with a particular
// goroutine. One goroutine may RLock (Lock) a RWMutex and then
// arrange for another goroutine to RUnlock (Unlock) it.
func (rw *RWMutex) Unlock() {
    if race.Enabled {
        _ = rw.w.state
        race.Release(unsafe.Pointer(&rw.readerSem))
        race.Release(unsafe.Pointer(&rw.writerSem))
        race.Disable()
    }

    // Announce to readers there is no active writer.
    r := atomic.AddInt32(&rw.readerCount, rwmutexMaxReaders)//readerCount + 2^30变成正数
    if r >= rwmutexMaxReaders {//释放写锁多次
        race.Enable()
        throw("sync: Unlock of unlocked RWMutex")
    }
    // Unblock blocked readers, if any.
    for i := 0; i < int(r); i++ {//发送多次reader信号量
        runtime_Semrelease(&rw.readerSem, false)
    }
    // Allow other writers to proceed.
    rw.w.Unlock()
    if race.Enabled {
        race.Enable()
    }
}

// RLocker returns a Locker interface that implements
// the Lock and Unlock methods by calling rw.RLock and rw.RUnlock.
func (rw *RWMutex) RLocker() Locker {
    return (*rlocker)(rw)
}

type rlocker RWMutex

func (r *rlocker) Lock()   { (*RWMutex)(r).RLock() }
func (r *rlocker) Unlock() { (*RWMutex)(r).RUnlock() }

思考

如何区分当前是被读锁还是写锁占有

readerCount>0则是读锁，<0则是写锁

整体思路

通过readerCount和readerWait进行计算
对读写锁的获取进行判断，是否等待reader或者writer信号量
对读写锁的释放也进行判断，释放reader以及writer相关信号量

多次调用写锁不会成功的原理

里面有rw.w.lock(),这个是互斥锁，释放写锁的时候释放这个互斥锁

获取写锁里面if的第二个判断逻辑何时生效

if条件第二句什么时候满足

在上面计算r之后，readerCount可能变化了，即再计算r之后，其他线程又执行了Rlock和Runlock，并且整体Runlock次数>Rlock次数，就有可能出现readerWait为负数了
考虑场景

1.计算r，当前readerCount为1，r为1,计算后readerCount为1-2^30
2.Runlock一次，readerCount为-2^30，满足下面条件

image.png

3.此时readerWait设置成-1
4.Lock里面，计算if条件，第一个条件r!=0(满足) 第二个条件readerWait=-1，r=1，相加为0，满足条件，不用处理

问题

代码假定最多有2^30个readers

所以代码里面readerCount在写模式会-2^30

如何和java读写锁比较

印象里面java里面是有AQS队列,公平非公平锁的概念更清晰，并且记录了队列结构