談談自己對GO的Mutex的理解

func(m *Mutex)lockSlow{ varwaitStartTime int64starving := false// 默認是正常模式awoke := falseiter := 0old := m.statefor{ // 當前Mutex在飢餓模式下已經被鎖了的話，當前G不進入自旋// 只有Mutex在正常模式且被鎖了的情況下，並且滿足自旋的條件，才會進入到自旋邏輯裡面ifold&(mutexLocked|mutexStarving) == mutexLocked && runtime_canSpin(iter) { // Active spinning makes sense.// Try to set mutexWoken flag to inform Unlock// to not wake other blocked goroutines.if!awoke && old&mutexWoken == 0&& old>>mutexWaiterShift != 0&& atomic.CompareAndSwapInt32(&m.state, old, old|mutexWoken) {awoke = true}runtime_doSpiniter++old = m.statecontinue}new:= old // 如果當前不是飢餓模式ifold&mutexStarving == 0{ // 加鎖new|= mutexLocked }// 如果Mutex已經被鎖，或者是在飢餓模式ifold&(mutexLocked|mutexStarving) != 0{ // 等待的G數量+1new+= 1<< mutexWaiterShift }// The current goroutine switches mutex to starvation mode.// But if the mutex is currently unlocked, don't do the switch.// Unlock expects that starving mutex has waiters, which will not// be true in this case.// 如果已經是飢餓模式，並且Mutex是被鎖的狀態ifstarving && old&mutexLocked != 0{ // 切換成飢餓模式new|= mutexStarving }ifawoke { // The goroutine has been woken from sleep,// so we need to reset the flag in either case.ifnew&mutexWoken == 0{ throw( "sync: inconsistent mutex state") }new&^= mutexWoken }// 更新state值ifatomic.CompareAndSwapInt32(&m.state, old, new) { // 非飢餓模式下搶鎖成功ifold&(mutexLocked|mutexStarving) == 0{ // 退出break// locked the mutex with CAS}// If we were already waiting before, queue at the front of the queue.// 如果之前已經設置過waitStartTime的話，queueLifo就是true了queueLifo := waitStartTime != 0// 沒有設置過，獲取下運行時間ifwaitStartTime == 0{ waitStartTime = runtime_nanotime}// 阻塞，等待被喚醒runtime_SemacquireMutex(&m.sema, queueLifo, 1) // 如果等待時間超過1ms，設置starving = true，否則就是falsestarving = starving || runtime_nanotime-waitStartTime > starvationThresholdNsold = m.state// 如果Mutex已經是飢餓模式ifold&mutexStarving != 0{ // If this goroutine was woken and mutex is in starvation mode,// ownership was handed off to us but mutex is in somewhat// inconsistent state: mutexLocked is not set and we are still// accounted as waiter. Fix that.// 如果當前G是在飢餓模式下被喚醒的// 加個判斷state是否正確設置的邏輯ifold&(mutexLocked|mutexWoken) != 0|| old>>mutexWaiterShift == 0{ throw( "sync: inconsistent mutex state") }// delta = -7 （1..... 0111）delta := int32(mutexLocked - 1<if!starving || old>>mutexWaiterShift == 1{ // 退出飢餓模式delta -= mutexStarving}// 更新stateatomic.AddInt32(&m.state, delta)break}awoke = trueiter = 0} else{ old = m.state}}

ifrace.Enabled { race.Acquire(unsafe.Pointer(m))}}

Unlock方法改動就非常小了