Why is it not safe to read here? The real write operation is done through
CAS, assuming the read value is out of date, then the CAS operation will
fail.
在2022年4月2日星期六 UTC+8 11:41:00<Ian Lance Taylor> 写道：
> On Fri, Apr 1, 2022 at 8:31 PM 机智的小小帅 <liweifor...@gmail.com> wrote:
> >
> > 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 and link2
> >
> > I have read The Go Memory Model, 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...
>
> When reading or writing a value that is the size of a memory word or
> smaller and is aligned such that the value does not cross a word
> boundary in memory Go assumes that the entire value will always be
> read or written as a unit. This is true of all processors that Go
> currently supports.
>
> The purpose of an atomic operation is different: it is about when the
> value is visible to other cores on the same system.
>
> Ian
>
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