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SubjectRe: [PATCH RFC 1/4] vsock/virtio: reduce credit update messages
On Fri, Apr 05, 2019 at 10:16:48AM +0200, Stefano Garzarella wrote:
> On Thu, Apr 04, 2019 at 08:15:39PM +0100, Stefan Hajnoczi wrote:
> > On Thu, Apr 04, 2019 at 12:58:35PM +0200, Stefano Garzarella wrote:
> > > int err = -EFAULT;
> > >
> > > spin_lock_bh(&vvs->rx_lock);
> > > @@ -288,9 +290,15 @@ virtio_transport_stream_do_dequeue(struct vsock_sock *vsk,
> > > }
> > > spin_unlock_bh(&vvs->rx_lock);
> > >
> > > - /* Send a credit pkt to peer */
> > > - virtio_transport_send_credit_update(vsk, VIRTIO_VSOCK_TYPE_STREAM,
> > > - NULL);
> > > + /* We send a credit update only when the space available seen
> > > + * by the transmitter is less than VIRTIO_VSOCK_MAX_PKT_BUF_SIZE
> > > + */
> > > + free_space = vvs->buf_alloc - (vvs->fwd_cnt - vvs->last_fwd_cnt);
> >
> > Locking? These fields should be accessed under tx_lock.
> >
>
> Yes, we need a lock, but looking in the code, vvs->fwd_cnd is written
> taking rx_lock (virtio_transport_dec_rx_pkt) and it is read with the
> tx_lock (virtio_transport_inc_tx_pkt).
>
> Maybe we should use another spin_lock shared between RX and TX for those
> fields or use atomic variables.
>
> What do you suggest?

Or make vvs->fwd_cnt atomic if it's the only field that needs to be
accessed in this manner.

Stefan
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