go中读写锁(rwmutex)源码解读实现原理

go读写锁的实现原理

1、RWMutex读写锁的概念

读写锁也就是我们所使用的RWMutex，其实是对于go本身的mutex做的一个拓展，当一个goroutine获得了读锁后，其他goroutine同样可以获得读锁，但是不能获得写锁。相反，当一个goroutine获得了写锁，其他goroutine既不能读也不能写，互斥的概念。

2、使用场景

适用于读多写少的情况

3、底层实现

读写锁实现的结构体位于src下的sync包下的rwmutex.go文件中

type RWMutex struct {
	w           Mutex  // held if there are pending writers
	writerSem   uint32 // semaphore for writers to wait for completing readers
	readerSem   uint32 // semaphore for readers to wait for completing writers
	readerCount int32  // number of pending readers
	readerWait  int32  // number of departing readers
}	

w字段代表着复用了互斥锁

writerSem代表写信号量，用于写等待读

readerSem代表读信号量，用于读等待写

readerCount代表当前执行读的goroutine数量

readerWait代表被阻塞的准备读的goroutine的数量

4、读锁的实现

加读锁

func (rw *RWMutex) RLock() {
	if atomic.AddInt32(&rw.readerCount, 1) < 0 {
		// A writer is pending, wait for it.
		runtime_SemacquireMutex(&rw.readerSem, false, 0)
	}
}

首先看这个if方法，为什么要判断小于0呢？

atomic.AddInt32(&rw.readerCount, 1) < 0调用的这个原子方法，目的就是当goroutine加读锁的时候读锁数量+1，如果返回的数量是负数，那么就代表了当前有其他写锁，这个时候就掉用runtime_SemacquireMutex方法休眠当前goroutine，readerSem就记录者这个goroutine。所以要判断是否小于0

释放读锁

// 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() {
	if r := atomic.AddInt32(&rw.readerCount, -1); r < 0 {
		// Outlined slow-path to allow the fast-path to be inlined
		rw.rUnlockSlow(r)
	}
}

释放读锁的时候就是对readerCount读数量-1即可，如果返回值小于0，就代表着当前有写的操作，这个时候就会调用rUnlockSlow进入慢速通道

什么是慢速通道

func (rw *RWMutex) rUnlockSlow(r int32) {
	// A writer is pending.
	if atomic.AddInt32(&rw.readerWait, -1) == 0 {
		// The last reader unblocks the writer.
		runtime_Semrelease(&rw.writerSem, false, 1)
	}
}

被阻塞的准备读的goroutine数量-1，如果readerWait为0，就表示当前没有goroutine正在准备读，这个时候去唤醒写操作

5、写锁的实现

加写锁

const rwmutexMaxReaders = 1 << 30
// 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() {
	// First, resolve competition with other writers.
	rw.w.Lock()
	// Announce to readers there is a pending writer.
	r := atomic.AddInt32(&rw.readerCount, -rwmutexMaxReaders) + rwmutexMaxReaders
	// Wait for active readers.
	if r != 0 && atomic.AddInt32(&rw.readerWait, r) != 0 {
		runtime_SemacquireMutex(&rw.writerSem, false, 0)
	}
}

写锁的调用就是调用互斥锁w的lock，如果计算之后还是有其他goroutine持有读锁，那么就调用runtime_SemacquireMutex休眠当前的goroutine等待所有的读操作完成， atomic.AddInt32(&rw.readerCount, -rwmutexMaxReaders) + rwmutexMaxReaders这个操作目的是防止后面的goroutine拿到读锁，阻塞读的作用。

释放写锁

// 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() {
	// Announce to readers there is no active writer.
	r := atomic.AddInt32(&rw.readerCount, rwmutexMaxReaders)
	if r >= rwmutexMaxReaders {
		race.Enable()
		fatal("sync: Unlock of unlocked RWMutex")
	}
	// Unblock blocked readers, if any.
	for i := 0; i < int(r); i++ {
		runtime_Semrelease(&rw.readerSem, false, 0)
	}
	// Allow other writers to proceed.
	rw.w.Unlock()
}

当释放写锁的时候，首先会通过atomic.AddInt32(&rw.readerCount, rwmutexMaxReaders)恢复之前的写入的很大的那个负数，然后看当前有多少个读操作在等待，循环唤醒等待读的goroutine

注意：go中的锁不支持可重入锁，若想实现可自定义实现

6、总结

读写锁区分读锁和写锁，而普通的互斥锁不区分，读写锁主要应用在读多写少的场景，既保证了并发读的执行效率，又保证了线程之间的安全。