linux-kernel - Re: [PATCH v4 16/18] xen/pvcalls: implement read

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Date:   Wed, 21 Jun 2017 18:37:03 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xen.org
Cc:     linux-kernel@...r.kernel.org, jgross@...e.com,
        Stefano Stabellini <stefano@...reto.com>
Subject: Re: [PATCH v4 16/18] xen/pvcalls: implement read

On 06/15/2017 03:09 PM, Stefano Stabellini wrote:
> When an active socket has data available, increment the io and read
> counters, and schedule the ioworker.
>
> Implement the read function by reading from the socket, writing the data
> to the data ring.
>
> Set in_error on error.
>
> Signed-off-by: Stefano Stabellini <stefano@...reto.com>
> CC: boris.ostrovsky@...cle.com
> CC: jgross@...e.com
> ---
>  drivers/xen/pvcalls-back.c | 85 ++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 85 insertions(+)
>
> diff --git a/drivers/xen/pvcalls-back.c b/drivers/xen/pvcalls-back.c
> index b9a10b9..65d9eba 100644
> --- a/drivers/xen/pvcalls-back.c
> +++ b/drivers/xen/pvcalls-back.c
> @@ -100,6 +100,81 @@ static int pvcalls_back_release_active(struct xenbus_device *dev,
>  
>  static void pvcalls_conn_back_read(void *opaque)
>  {
> +	struct sock_mapping *map = (struct sock_mapping *)opaque;
> +	struct msghdr msg;
> +	struct kvec vec[2];
> +	RING_IDX cons, prod, size, wanted, array_size, masked_prod, masked_cons;
> +	int32_t error;
> +	struct pvcalls_data_intf *intf = map->ring;
> +	struct pvcalls_data *data = &map->data;
> +	unsigned long flags;
> +	int ret;
> +
> +	array_size = XEN_FLEX_RING_SIZE(map->ring_order);

I noticed that in the next patch you call this 'ring_size. Can you make
those things consistent? (There may be more than just this variable and,
in fact, perhaps some things can be factored out? There are code
fragments that look similar)

> +	cons = intf->in_cons;
> +	prod = intf->in_prod;
> +	error = intf->in_error;
> +	/* read the indexes first, then deal with the data */
> +	virt_mb();
> +
> +	if (error)
> +		return;
> +
> +	size = pvcalls_queued(prod, cons, array_size);
> +	if (size >= array_size)
> +		return;
> +	spin_lock_irqsave(&map->sock->sk->sk_receive_queue.lock, flags);
> +	if (skb_queue_empty(&map->sock->sk->sk_receive_queue)) {
> +		atomic_set(&map->read, 0);
> +		spin_unlock_irqrestore(&map->sock->sk->sk_receive_queue.lock,
> +				flags);
> +		return;
> +	}
> +	spin_unlock_irqrestore(&map->sock->sk->sk_receive_queue.lock, flags);
> +	wanted = array_size - size;
> +	masked_prod = pvcalls_mask(prod, array_size);
> +	masked_cons = pvcalls_mask(cons, array_size);
> +
> +	memset(&msg, 0, sizeof(msg));
> +	msg.msg_iter.type = ITER_KVEC|WRITE;
> +	msg.msg_iter.count = wanted;
> +	if (masked_prod < masked_cons) {
> +		vec[0].iov_base = data->in + masked_prod;
> +		vec[0].iov_len = wanted;
> +		msg.msg_iter.kvec = vec;
> +		msg.msg_iter.nr_segs = 1;
> +	} else {
> +		vec[0].iov_base = data->in + masked_prod;
> +		vec[0].iov_len = array_size - masked_prod;
> +		vec[1].iov_base = data->in;
> +		vec[1].iov_len = wanted - vec[0].iov_len;
> +		msg.msg_iter.kvec = vec;
> +		msg.msg_iter.nr_segs = 2;
> +	}


This is probably obvious to everyone but me but can you explain what is
going on here? ;-)


> +
> +	atomic_set(&map->read, 0);

Is this not atomic_dec() by any chance?


-boris

> +	ret = inet_recvmsg(map->sock, &msg, wanted, MSG_DONTWAIT);
> +	WARN_ON(ret > wanted);
> +	if (ret == -EAGAIN) /* shouldn't happen */
> +		return;
> +	if (!ret)
> +		ret = -ENOTCONN;
> +	spin_lock_irqsave(&map->sock->sk->sk_receive_queue.lock, flags);
> +	if (ret > 0 && !skb_queue_empty(&map->sock->sk->sk_receive_queue))
> +		atomic_inc(&map->read);
> +	spin_unlock_irqrestore(&map->sock->sk->sk_receive_queue.lock, flags);
> +
> +	/* write the data, then modify the indexes */
> +	virt_wmb();
> +	if (ret < 0)
> +		intf->in_error = ret;
> +	else
> +		intf->in_prod = prod + ret;
> +	/* update the indexes, then notify the other end */
> +	virt_wmb();
> +	notify_remote_via_irq(map->irq);
> +
> +	return;
>  }
>  
>  static int pvcalls_conn_back_write(struct sock_mapping *map)
> @@ -172,6 +247,16 @@ static void pvcalls_sk_state_change(struct sock *sock)
>  
>  static void pvcalls_sk_data_ready(struct sock *sock)
>  {
> +	struct sock_mapping *map = sock->sk_user_data;
> +	struct pvcalls_ioworker *iow;
> +
> +	if (map == NULL)
> +		return;
> +
> +	iow = &map->ioworker;
> +	atomic_inc(&map->read);
> +	atomic_inc(&map->io);
> +	queue_work(iow->wq, &iow->register_work);
>  }
>  
>  static struct sock_mapping *pvcalls_new_active_socket(