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Date:   Mon, 8 Apr 2019 10:25:39 +0100
From:   Stefan Hajnoczi <stefanha@redhat.com>
To:     Stefano Garzarella <sgarzare@redhat.com>
Cc:     Stefan Hajnoczi <stefanha@gmail.com>, netdev@vger.kernel.org,
        Jason Wang <jasowang@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>, kvm@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org,
        "David S. Miller" <davem@davemloft.net>
Subject: Re: [PATCH RFC 1/4] vsock/virtio: reduce credit update messages
Message-ID: <20190408092539.GN15001@stefanha-x1.localdomain>
References: <20190404105838.101559-1-sgarzare@redhat.com>
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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 =3D -EFAULT;
> > > =20
> > >  	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);
> > > =20
> > > -	/* 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 =3D vvs->buf_alloc - (vvs->fwd_cnt - vvs->last_fwd_cnt);
> >=20
> > Locking?  These fields should be accessed under tx_lock.
> >=20
>=20
> 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).
>=20
> Maybe we should use another spin_lock shared between RX and TX for those
> fields or use atomic variables.
>=20
> 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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