为什么在runtime.GC中未调用atomic.Load来加载gcphase？

Question

I wonder why gcphase is not protected with atomic.Load:

n := atomic.Load(&work.cycles)
if gcphase == _GCmark {
    // Wait until sweep termination, mark, and mark
    // termination of cycle N complete.
    gp.schedlink = work.sweepWaiters.head
    work.sweepWaiters.head.set(gp)
    goparkunlock(&work.sweepWaiters.lock, "wait for GC cycle", traceEvGoBlock, 1)
} else {
    // We're in sweep N already.
    unlock(&work.sweepWaiters.lock)
}

Anyone knows?

Answer 1

Code excerpt:

func setGCPhase(x uint32) {
    atomic.Store(&gcphase, x)
    writeBarrier.needed = gcphase == _GCmark || gcphase == _GCmarktermination
    writeBarrier.enabled = writeBarrier.needed || writeBarrier.cgo
}

while gcphase is a global variable, but all writes to gcphase are done through the above function.

There were several variables in runtime which aren't paired properly, but seems they have reason for it and were sure they were having the exclusive access to it.

Here is the issue https://github.com/golang/go/issues/21931 filed about the same and here https://go-review.googlesource.com/c/go/+/65210 GC developers had some discussions on changing the same.