Do I need to treat the below scenario as critical?

Question

I have Android java service which will interact with HAL service using HIDL calls.

I have below scenario, I'm not sure to treat it as critical.

+----------+  (AIDL) +--------------+
|App thread|-------->|Java Service  | (HIDL) +-----------+
+----------+         |(SendFunction)|------->|CPP service|  
                     +--------------+        +-----------+
                     ^
+--------------+     |
|AnotherThread |-----|
+--------------+

Definition of SendFunction is as below.

 private void SendFunction(int status, DiagCommandDesc response) {
      try {
          server.executeCommandResponse(status, response);
         if (Log.isLoggable(TAG, Log.DEBUG)) {
              Log.d(TAG, "Response sent to HAL.");
          }
      } catch (Exception e) {
          if (Log.isLoggable(TAG, Log.DEBUG)) {
             Log.d(TAG, "HAL Server error.");
         }
      }
  } 

SendFunction is being called from two different threads. Where server is an instance to CPP Server using HIDL.

MY question.

      server.executeCommandResponse(status, response);

Do I need to treat above call as critical and synchronize it? as server object will be accessed from two different threads.

Answer 1

No, you do not have to guard the call to server.executeCommandResponse(status, response) in your Java service.

The Binder communication is already thread-safe. That concurrent calls to executeCommandResponse are safe inside the HAL service has to be ensured by the HAL itself. There is an easy way to make it thread-safe on the HAL side: Using a thread-pool with only a single thread. This will make all other threads wait on the first one to finish, though.

int main()
{
    ::android::hardware::configureRpcThreadpool(1, true);
    ::android::sp<MyHal> service = new MyHal;
    if (::android::OK != service->registerAsService())
        return EXIT_FAILURE;
    ::android::hardware::joinRpcThreadpool();
    return EXIT_SUCCESS;
}

You can find more information here: https://source.android.com/devices/architecture/hidl/threading