Function NeedsLazyInstance

src/butil/lazy_instance.cc:18–39

TODO(joth): This function could be shared with Singleton, in place of its WaitForInstance() call.

// TODO(joth): This function could be shared with Singleton, in place of its
// WaitForInstance() call.
bool NeedsLazyInstance(subtle::AtomicWord* state) {
  // Try to create the instance, if we're the first, will go from 0 to
  // kLazyInstanceStateCreating, otherwise we've already been beaten here.
  // The memory access has no memory ordering as state 0 and
  // kLazyInstanceStateCreating have no associated data (memory barriers are
  // all about ordering of memory accesses to *associated* data).
  if (subtle::NoBarrier_CompareAndSwap(state, 0,
                                       kLazyInstanceStateCreating) == 0)
    // Caller must create instance
    return true;

  // It's either in the process of being created, or already created. Spin.
  // The load has acquire memory ordering as a thread which sees
  // state_ == STATE_CREATED needs to acquire visibility over
  // the associated data (buf_). Pairing Release_Store is in
  // CompleteLazyInstance().
  while (subtle::Acquire_Load(state) == kLazyInstanceStateCreating) {
    PlatformThread::YieldCurrentThread();
  }
  // Someone else created the instance.
  return false;
}

void CompleteLazyInstance(subtle::AtomicWord* state,
                          subtle::AtomicWord new_instance,