三重缓冲示例中使用的信号量是否会阻止主线程?

看看Apple为金属缓冲区和纹理资源的三重缓冲提供的代码,似乎很奇怪,使用信号量会阻止代码中的主线程,如下所示。这是来自 CPU and GPU Synchronization .

- (void)drawInMTKView:(nonnull MTKView *)view
{
  dispatch_semaphore_t inFlightSemaphore = self.inFlightSemaphore;
  dispatch_semaphore_wait(inFlightSemaphore, DISPATCH_TIME_FOREVER);

  id <MTLCommandBuffer> commandBuffer = [mrc.commandQueue commandBuffer];

  __block dispatch_semaphore_t block_sema = inFlightSemaphore;
  [commandBuffer addCompletedHandler:^(id<MTLCommandBuffer> buffer)
   {
     dispatch_semaphore_signal(block_sema);
   }];

  // Metal render commands
}

调用dispatch\u semaphore\u wait()如何才能在调用drawInMTKView时阻止主线程?