[libvirt] [PATCH] remote_daemon_stream: Hold an extra reference to stream in daemonStreamFilter

[Michal Privoznik]

On 11/28/19 4:09 AM, Lance Liu wrote:
> Hi, Michal:
>      I think  the daemonFreeClientStream(NULL, stream);  in your patch 
> should line above  virMutexUnlock(&stream->priv->lock)， or else there 
> is issue WAW.

I'm sorry, I don't know what WAW is.

> That is two threads may sub stream->
> 
> Lance Liu <liu.lance.89 at gmail.com <mailto:liu.lance.89 at gmail.com>> 于 
> 2019年11月26日周二 下午1:54写道：
> 
>     Yeah, your patch is perfectly fine for stream freed problem.
>     But I have reviewed code in daemonStreamEvent() and
>     daemonStreamFilter() again, and I think there still two issue need
>     to be reconsidered:
>     1. stream->ref only ++ in daemonStreamFilter, except for error
>     occurred call daemonRemoveClientStream() lead to ref--, like code as
>     follow. Though code won't be executed because of
>     no VIR_STREAM_EVENT_HANGUP event taken place,
>     but it is not good code:

Do you perhaps mean daemonStreamEvent() instead of daemonStreamFilter()? 
Because in daemonStreamFilter() we do both ref & unref - the unref is 
hidden in daemonFreeClientStream().

>     /* If we got HANGUP, we need to only send an empty
>           * packet so the client sees an EOF and cleans up
>           */
>          if (!stream->closed && !stream->recvEOF &&
>              (events & VIR_STREAM_EVENT_HANGUP)) {
>              virNetMessagePtr msg;
>              events &= ~(VIR_STREAM_EVENT_HANGUP);
>              stream->tx = false;
>              stream->recvEOF = true;
>              if (!(msg = virNetMessageNew(false))) {
>                  daemonRemoveClientStream(client, stream);
>                  virNetServerClientClose(client);
>                  goto cleanup;
>              }
>              msg->cb = daemonStreamMessageFinished;
>              msg->opaque = stream;
>     stream->refs++;
>              if (virNetServerProgramSendStreamData(stream->prog,
>                                                    client,
>                                                    msg,
>                                                    stream->procedure,
>                                                    stream->serial,
>                                                    "", 0) < 0) {
>                  virNetMessageFree(msg);
>                  daemonRemoveClientStream(client, stream);
>                  virNetServerClientClose(client);
>                  goto cleanup;
>              }

Again, the unref is hidden in daemonStreamMessageFinished() which calls 
daemonFreeClientStream(). The msg->cb is guaranteed to be called 
eventually upon successful return from 
virNetServerProgramSendStreamData(). That's why you don't see the direct 
unref in the success path.

>          }
>     2. call virNetServerClientClose() is still inappropriate in  because
>     the it may free the client resource which other threads need ref,
>     may be replace it with virNetServerClientImmediateClose() is better,
>     the daemon can process client resource release unified

I've seen the patch you send, but it was without any commit message so 
no one had any idea why the change is needed.
I understand your argument here, but I'm having hard time finding 
practical example in the code. I mean, have you seen such issue in real?

>     3. Segmentfault may still exists when another thread
>     call remoteClientCloseFunc in virNetServerClientClose()(for example,
>     when new force console session break down existed console session,
>     and existed console session
>     's client close the session simutaneously, but remoteClientCloseFunc
>     not lock(priv->lock), so the resource maybe released when
>     daemonStreamEvent ref)

Again, sounds plausible, but this is a subset of the previous problem 
since remoteClientCloseFunc() is called from 
virNetServerClientCloseLocked() which is called from 
virNetServerClientClose().

Michal