[go-nuts] Why doesn't Go use atomic.LoadInt32() in sync.Mutex.lockSlow()?

[机智的小小帅]

Hi,all. I'm reading the source code of sync package recently.

Here is a code snippet in sync/mutex.go.

// src/sync/mutex.go
type Mutex struct {
    state int32
    sema  uint32
}

func (m *Mutex) lockSlow() {
    var waitStartTime int64
    starving := false
    awoke := false
    iter := 0
    old := m.state // Here!!! not use atomic.LoadInt32()
    for {
        // Don't spin in starvation mode, ownership is handed off to waiters
        // so we won't be able to acquire the mutex anyway.
        if old&(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
            }
            // ...
            old = m.state // And here!!! not use atomic.LoadInt32()
            continue
        }
    // ...
  }
  // ...
}


You can see full context in link1 
<https://github.com/golang/go/blob/master/src/sync/mutex.go#L113> and link2 
<https://github.com/golang/go/blob/master/src/sync/mutex.go#L127>

I have read The Go Memory Model <https://go.dev/ref/mem>, It says "Reads 
and writes of values larger than a single machine word behave as multiple 
machine-word-sized operations in an unspecified order."

Althought it is usually 32 or 64 bits on modern machine,I think if you want 
a operator is atomic,you should use atomic.LoadXXX() explicitly.

I think a ordinary read operator like old := m.state in concurrent 
environment is not safe,it may read a unexpected value.

I wonder whether the oridinary read operation is right? Could you help me? 
Thanks in advance.

I'm very sorry to disturb you...

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