基于Mutex，实现读共享，写互斥 - RWMutex的底层

背景

sync.Mutex提供了互斥锁，可以保证在同一时间段内，有且仅有一个goroutine持有锁和操作共享资源。其余goroutine只有在互斥锁被释放，成功获取到锁之后，才能操作共享资源

对共享资源的操作其实可以分为两种：

读操作，不会改变共享资源
写操作，会改变共享资源

在实际业务中，往往是读操作次数大于写操作次数，sync.Mutex提供的互斥锁，不能支持并发的读操作，所以就有了sync.RWMutex

sync.RWMutex有以下特点：

在同一时间段，可以有多个goroutine获取到读锁，即读共享
在同一时间段，只能有一个goroutine获取到写锁，即写互斥
在同一时间段，只能存在读锁或写锁，即读写互斥

快速入门

读共享，写互斥快速入门：

func TestRWMutexLock(t *testing.T) {
   var rw sync.RWMutex
   var wg sync.WaitGroup
   for i := 0; i < 10; i++ {
      go func() {
         wg.Add(1)
         defer wg.Done()
         // 读锁
         rw.RLock()
         defer rw.RUnlock()
         time.Sleep(1 * time.Second)
         fmt.Println("读操作共享")
      }()
   }

   for i := 0; i < 10; i++ {
      go func() {
         wg.Add(1)
         defer wg.Done()
         // 写锁
         rw.Lock()
         defer rw.Unlock()
         time.Sleep(1 * time.Second)
         fmt.Println("写操作互斥")
      }()
   }
   wg.Wait()
}

源码分析

源码路径：src/sync/rwmutex.go

golang中的锁接口：

// A Locker represents an object that can be locked and unlocked.
type Locker interface {
   Lock()
   Unlock()
}

Locker表示可以锁定和解锁的对象

RWMutex实现了Locker接口中的方法，RWMutex结构和方法如下：

const rwmutexMaxReaders = 1 << 30
// 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
   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
}

// RWMutex提供了以下几个核心方法
// 加读锁，没有成功时，会一直阻塞
func (rw *RWMutex) RLock() {}
// 尝试加读锁，没有成功时，会快速返回结果，不阻塞
func (rw *RWMutex) TryRLock() bool {}
// 解读锁
func (rw *RWMutex) RUnlock() {}
// 加写锁，没有成功时，会一直阻塞
func (rw *RWMutex) Lock() {}
// 尝试加写锁，没有成功时，会快速返回结果，不阻塞
func (rw *RWMutex) TryLock() bool {}
// 解写锁
func (rw *RWMutex) Unlock() {}
// 返回Locker接口的实现
func (rw *RWMutex) RLocker() Locker {}

rwmutexMaxReaders：表示RWMutex能接受的最大读操作数量，超过最大数量就会panic
w：互斥锁，用于实现互斥写操作
writerSem：写操作信号量，用于写操作的阻塞和唤醒。当存在正在执行的读操作时，写操作会被阻塞；当读操作全部完成后，通过writerSem写操作信号量来唤醒写操作
readerSem：读操作信号量，用于读操作的阻塞和唤醒。当存在正在执行的写操作时，读操作会被阻塞；当写操作完成后，通过readerSem读操作信号量唤醒读操作
readerCount：正在执行的读操作数量，当不存在写操作时，从0开始计数，通过正数来表示；当存在写操作时，从负的rwmutexMaxReaders开始计数，通过负数来表示
readerWait：写操作等待读操作的数量，当执行Lock方法时，如果当前存在正在执行的读操作，会将正在执行的读操作数量记录在readerWait中，并阻塞写操作；当读操作执行完成后，会更新readerWait；当readerWait为0时，会唤醒写操作
RWMutex具有写操作优先的特点，写操作发生时，只允许正在执行的读操作继续执行完成，后续新来的读操作都会被阻塞，直到写操作完成后进行唤醒

Lock()

// 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.
   // 加写锁，保证写操作互斥
   rw.w.Lock()
   // Announce to readers there is a pending writer.
   // 将readerCount更新为负值，表示当前有写操作
   // 当readerCount为负数时，新的读操作会被阻塞
   // r表示当前正在执行的读操作数量
   r := atomic.AddInt32(&rw.readerCount, -rwmutexMaxReaders) + rwmutexMaxReaders
   // Wait for active readers.
   // r != 0 表示当前存在正在执行的读操作
   // 把当前正在执行的读操作数量更新到readerWait中
   if r != 0 && atomic.AddInt32(&rw.readerWait, r) != 0 {
      // 阻塞写操作，等待正在执行的读操作执行完后唤醒，执行RUnlock()
      runtime_SemacquireMutex(&rw.writerSem, false, 0)
   }
   if race.Enabled {
      race.Enable()
      race.Acquire(unsafe.Pointer(&rw.readerSem))
      race.Acquire(unsafe.Pointer(&rw.writerSem))
   }
}

先通过Mutex进行加锁，保证写操作互斥

将readerCount更新为负值，表示当前有写操作。当readerCount为负数时，新的读操作调用RLock()尝试加读锁时会被阻塞，保证写操作优先

若当前存在正在执行的读操作，把当前正在执行的读操作数量更新到readerWait中

阻塞当前写操作，读操作执行完调用RUnlock()，如果存在写操作，会对readerWait进行-1，当readerWait==0时，唤醒写操作，保证写操作优先

Unlock()

// 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.Disable()
   }

   // Announce to readers there is no active writer.
   // 将readerCount更新为正数，表示当前没有写操作
   r := atomic.AddInt32(&rw.readerCount, rwmutexMaxReaders)
   // 判断是否超过最大读操作数量
   if r >= rwmutexMaxReaders {
      race.Enable()
      throw("sync: Unlock of unlocked RWMutex")
   }
   // Unblock blocked readers, if any.
   // 唤醒所有等待的读操作
   for i := 0; i < int(r); i++ {
      runtime_Semrelease(&rw.readerSem, false, 0)
   }
   // Allow other writers to proceed.
   // 释放写锁
   rw.w.Unlock()
   if race.Enabled {
      race.Enable()
   }
}

将readerCount更新为正数，表示当前没有写操作

若存在等待的读操作，则唤醒所有等待的读操作

释放互斥锁

TryLock()

// TryLock tries to lock rw for writing and reports whether it succeeded.
//
// Note that while correct uses of TryLock do exist, they are rare,
// and use of TryLock is often a sign of a deeper problem
// in a particular use of mutexes.
func (rw *RWMutex) TryLock() bool {
   if race.Enabled {
      _ = rw.w.state
      race.Disable()
   }
   // 尝试加写锁
   if !rw.w.TryLock() {
      if race.Enabled {
         race.Enable()
      }
      return false
   }
   // 加写锁成功
   // 通过CAS，判断是否存在读操作，如果不存在读操作，则加写锁成功；如果存在读操作，则释放写锁
   if !atomic.CompareAndSwapInt32(&rw.readerCount, 0, -rwmutexMaxReaders) {
      rw.w.Unlock()
      if race.Enabled {
         race.Enable()
      }
      return false
   }
   if race.Enabled {
      race.Enable()
      race.Acquire(unsafe.Pointer(&rw.readerSem))
      race.Acquire(unsafe.Pointer(&rw.writerSem))
   }
   return true
}

尝试加写锁，如果加写锁失败，则直接返回false

如果加写锁成功，通过CAS，判断是否存在读操作，如果不存在读操作，则加写锁成功，返回true；如果存在读操作，则释放写锁，返回false

RLock()

// Happens-before relationships are indicated to the race detector via:
// - Unlock  -> Lock:  readerSem
// - Unlock  -> RLock: readerSem
// - RUnlock -> Lock:  writerSem
//
// The methods below temporarily disable handling of race synchronization
// events in order to provide the more precise model above to the race
// detector.
//
// For example, atomic.AddInt32 in RLock should not appear to provide
// acquire-release semantics, which would incorrectly synchronize racing
// readers, thus potentially missing races.

// 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() {
   if race.Enabled {
      _ = rw.w.state
      race.Disable()
   }
   // 原子更新readerCount+1，表示读操作数量+1
   // 若readerCount+1为负数，表示当前存在写操作，读操作会被阻塞，等待写操作完成后被唤醒
   if atomic.AddInt32(&rw.readerCount, 1) < 0 {
      // A writer is pending, wait for it.
      // 阻塞读操作，执行Unlock()，唤醒所有阻塞等待的读操作，释放写锁
      runtime_SemacquireMutex(&rw.readerSem, false, 0)
   }
   if race.Enabled {
      race.Enable()
      race.Acquire(unsafe.Pointer(&rw.readerSem))
   }
}

原子更新readerCount+1，读操作数量+1

如果readerCount+1为负数，则表示当前存在写操作，此时需要加锁的读操作会被阻塞，保证写操作优先；等待写操作完成，执行Unlock()再唤醒读操作

RUnlock()

// 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 race.Enabled {
      _ = rw.w.state
      race.ReleaseMerge(unsafe.Pointer(&rw.writerSem))
      race.Disable()
   }
   // 原子更新readerCount-1，表示读操作数量-1
   // 若readerCount-1为负数，表示当前读操作阻塞了写操作，需要进行额外处理
   if r := atomic.AddInt32(&rw.readerCount, -1); r < 0 {
      // Outlined slow-path to allow the fast-path to be inlined
      rw.rUnlockSlow(r)
   }
   if race.Enabled {
      race.Enable()
   }
}

func (rw *RWMutex) rUnlockSlow(r int32) {
   // 判断是否超过最大读操作数量
   if r+1 == 0 || r+1 == -rwmutexMaxReaders {
      race.Enable()
      throw("sync: RUnlock of unlocked RWMutex")
   }
   // A writer is pending.
   // 原子更新readerWait-1，表示阻塞写操作的读操作数量-1
   // 当readerWait-1为0时，表示导致写操作阻塞的所有读操作都已经执行完成，此时需要把阻塞的写操作唤醒
   if atomic.AddInt32(&rw.readerWait, -1) == 0 {
      // The last reader unblocks the writer.
      // 唤醒写操作
      runtime_Semrelease(&rw.writerSem, false, 1)
   }
}

原子更新readerCount-1，表示读操作数量-1

若readerCount-1为负数，表示当前读操作阻塞了写操作，需要进行额外处理

原子更新readerWait-1，表示阻塞写操作的读操作数量-1

当readerWait-1为0时，表示导致写操作阻塞的所有读操作都已经执行完成，此时需要把阻塞的写操作唤醒

TryRLock()

// TryRLock tries to lock rw for reading and reports whether it succeeded.
//
// Note that while correct uses of TryRLock do exist, they are rare,
// and use of TryRLock is often a sign of a deeper problem
// in a particular use of mutexes.
func (rw *RWMutex) TryRLock() bool {
   if race.Enabled {
      _ = rw.w.state
      race.Disable()
   }
   for {
      c := atomic.LoadInt32(&rw.readerCount)
      // readerCount为负数，表示存在写操作，加读锁失败
      if c < 0 {
         if race.Enabled {
            race.Enable()
         }
         return false
      }
      // 尝试加读锁
      if atomic.CompareAndSwapInt32(&rw.readerCount, c, c+1) {
         if race.Enabled {
            race.Enable()
            race.Acquire(unsafe.Pointer(&rw.readerSem))
         }
         return true
      }
   }
}

readerCount为负数，表示存在写操作，加读锁失败

尝试加读锁，成功则返回，失败则继续下一次循环尝试

RLocker

// 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() }