Re: [PATCH 2/6] libvhost-user: Use slave_mutex in all slave messages

[Vivek Goyal <vgoyal@redhat.com>]

On Thu, Jan 28, 2021 at 03:31:23PM +0100, Greg Kurz wrote:
> On Mon, 25 Jan 2021 13:01:11 -0500
> Vivek Goyal <vgoyal@redhat.com> wrote:
> 
> > dev->slave_mutex needs to be taken when sending messages on slave_fd.
> > Currently _vu_queue_notify() does not do that.
> > 
> > Introduce a helper vu_message_slave_send_receive() which sends as well
> > as receive response. Use this helper in all the paths which send
> > message on slave_fd channel.
> > 
> 
> Does this fix any known bug ?

I am not aware of any bug. This fix is based on code inspection.

Also I wanted a central place/function to send messages on slave channel
so that I can check state of slave channel (open/close) and act
accordingly. Otherwise I will have to do the check at every place
which is trying to send/receive message on slave channel.

Vivek

> 
> > Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> > ---
> 
> LGTM
> 
> Reviewed-by: Greg Kurz <groug@kaod.org>
> 
> >  subprojects/libvhost-user/libvhost-user.c | 50 ++++++++++++-----------
> >  1 file changed, 27 insertions(+), 23 deletions(-)
> > 
> > diff --git a/subprojects/libvhost-user/libvhost-user.c b/subprojects/libvhost-user/libvhost-user.c
> > index 4cf4aef63d..7a56c56dc8 100644
> > --- a/subprojects/libvhost-user/libvhost-user.c
> > +++ b/subprojects/libvhost-user/libvhost-user.c
> > @@ -403,7 +403,7 @@ vu_send_reply(VuDev *dev, int conn_fd, VhostUserMsg *vmsg)
> >   * Processes a reply on the slave channel.
> >   * Entered with slave_mutex held and releases it before exit.
> >   * Returns true on success.
> > - * *payload is written on success
> > + * *payload is written on success, if payload is not NULL.
> >   */
> >  static bool
> >  vu_process_message_reply(VuDev *dev, const VhostUserMsg *vmsg,
> > @@ -427,7 +427,9 @@ vu_process_message_reply(VuDev *dev, const VhostUserMsg *vmsg,
> >          goto out;
> >      }
> >  
> > -    *payload = msg_reply.payload.u64;
> > +    if (payload) {
> > +        *payload = msg_reply.payload.u64;
> > +    }
> >      result = true;
> >  
> >  out:
> > @@ -435,6 +437,25 @@ out:
> >      return result;
> >  }
> >  
> > +/* Returns true on success, false otherwise */
> > +static bool
> > +vu_message_slave_send_receive(VuDev *dev, VhostUserMsg *vmsg, uint64_t *payload)
> > +{
> > +    pthread_mutex_lock(&dev->slave_mutex);
> > +    if (!vu_message_write(dev, dev->slave_fd, vmsg)) {
> > +        pthread_mutex_unlock(&dev->slave_mutex);
> > +        return false;
> > +    }
> > +
> > +    if ((vmsg->flags & VHOST_USER_NEED_REPLY_MASK) == 0) {
> > +        pthread_mutex_unlock(&dev->slave_mutex);
> > +        return true;
> > +    }
> > +
> > +    /* Also unlocks the slave_mutex */
> > +    return vu_process_message_reply(dev, vmsg, payload);
> > +}
> > +
> >  /* Kick the log_call_fd if required. */
> >  static void
> >  vu_log_kick(VuDev *dev)
> > @@ -1340,16 +1361,8 @@ bool vu_set_queue_host_notifier(VuDev *dev, VuVirtq *vq, int fd,
> >          return false;
> >      }
> >  
> > -    pthread_mutex_lock(&dev->slave_mutex);
> > -    if (!vu_message_write(dev, dev->slave_fd, &vmsg)) {
> > -        pthread_mutex_unlock(&dev->slave_mutex);
> > -        return false;
> > -    }
> > -
> > -    /* Also unlocks the slave_mutex */
> > -    res = vu_process_message_reply(dev, &vmsg, &payload);
> > +    res = vu_message_slave_send_receive(dev, &vmsg, &payload);
> >      res = res && (payload == 0);
> > -
> >      return res;
> >  }
> >  
> > @@ -2395,10 +2408,7 @@ static void _vu_queue_notify(VuDev *dev, VuVirtq *vq, bool sync)
> >              vmsg.flags |= VHOST_USER_NEED_REPLY_MASK;
> >          }
> >  
> > -        vu_message_write(dev, dev->slave_fd, &vmsg);
> > -        if (ack) {
> > -            vu_message_read_default(dev, dev->slave_fd, &vmsg);
> > -        }
> > +        vu_message_slave_send_receive(dev, &vmsg, NULL);
> >          return;
> >      }
> >  
> > @@ -2942,17 +2952,11 @@ int64_t vu_fs_cache_request(VuDev *dev, VhostUserSlaveRequest req, int fd,
> >          return -EINVAL;
> >      }
> >  
> > -    pthread_mutex_lock(&dev->slave_mutex);
> > -    if (!vu_message_write(dev, dev->slave_fd, &vmsg)) {
> > -        pthread_mutex_unlock(&dev->slave_mutex);
> > -        return -EIO;
> > -    }
> > -
> > -    /* Also unlocks the slave_mutex */
> > -    res = vu_process_message_reply(dev, &vmsg, &payload);
> > +    res = vu_message_slave_send_receive(dev, &vmsg, &payload);
> >      if (!res) {
> >          return -EIO;
> >      }
> > +
> >      /*
> >       * Payload is delivered as uint64_t but is actually signed for
> >       * errors.
>