Re: [PATCH v5 08/13] xen/pvcalls: implement accept command

[Boris Ostrovsky]

On 10/06/2017 08:30 PM, Stefano Stabellini wrote:
> Introduce a waitqueue to allow only one outstanding accept command at
> any given time and to implement polling on the passive socket. Introduce
> a flags field to keep track of in-flight accept and poll commands.
> 
> Send PVCALLS_ACCEPT to the backend. Allocate a new active socket. Make
> sure that only one accept command is executed at any given time by
> setting PVCALLS_FLAG_ACCEPT_INFLIGHT and waiting on the
> inflight_accept_req waitqueue.
> 
> Convert the new struct sock_mapping pointer into an uint64_t and use it
> as id for the new socket to pass to the backend.
> 
> Check if the accept call is non-blocking: in that case after sending the
> ACCEPT command to the backend store the sock_mapping pointer of the new
> struct and the inflight req_id then return -EAGAIN (which will respond
> only when there is something to accept). Next time accept is called,
> we'll check if the ACCEPT command has been answered, if so we'll pick up
> where we left off, otherwise we return -EAGAIN again.
> 
> Note that, differently from the other commands, we can use
> wait_event_interruptible (instead of wait_event) in the case of accept
> as we are able to track the req_id of the ACCEPT response that we are
> waiting.
> 
> Signed-off-by: Stefano Stabellini <stefano@xxxxxxxxxxx>
> CC: boris.ostrovsky@xxxxxxxxxx
> CC: jgross@xxxxxxxx
> ---
>  drivers/xen/pvcalls-front.c | 146 ++++++++++++++++++++++++++++++++++++++++++++
>  drivers/xen/pvcalls-front.h |   3 +
>  2 files changed, 149 insertions(+)
> 
> diff --git a/drivers/xen/pvcalls-front.c b/drivers/xen/pvcalls-front.c
> index 5433fae..8958e74 100644
> --- a/drivers/xen/pvcalls-front.c
> +++ b/drivers/xen/pvcalls-front.c
> @@ -77,6 +77,16 @@ struct sock_mapping {
>  #define PVCALLS_STATUS_BIND          1
>  #define PVCALLS_STATUS_LISTEN        2
>  			uint8_t status;
> +		/*
> +		 * Internal state-machine flags.
> +		 * Only one accept operation can be inflight for a socket.
> +		 * Only one poll operation can be inflight for a given socket.
> +		 */
> +#define PVCALLS_FLAG_ACCEPT_INFLIGHT 0
> +			uint8_t flags;
> +			uint32_t inflight_req_id;
> +			struct sock_mapping *accept_map;
> +			wait_queue_head_t inflight_accept_req;
>  		} passive;
>  	};
>  };
> @@ -392,6 +402,8 @@ int pvcalls_front_bind(struct socket *sock, struct sockaddr *addr, int addr_len)
>  	memcpy(req->u.bind.addr, addr, sizeof(*addr));
>  	req->u.bind.len = addr_len;
>  
> +	init_waitqueue_head(&map->passive.inflight_accept_req);
> +
>  	map->active_socket = false;
>  
>  	bedata->ring.req_prod_pvt++;
> @@ -470,6 +482,140 @@ int pvcalls_front_listen(struct socket *sock, int backlog)
>  	return ret;
>  }
>  
> +int pvcalls_front_accept(struct socket *sock, struct socket *newsock, int flags)
> +{
> +	struct pvcalls_bedata *bedata;
> +	struct sock_mapping *map;
> +	struct sock_mapping *map2 = NULL;
> +	struct xen_pvcalls_request *req;
> +	int notify, req_id, ret, evtchn, nonblock;
> +
> +	pvcalls_enter();
> +	if (!pvcalls_front_dev) {
> +		pvcalls_exit();
> +		return -ENOTCONN;
> +	}
> +	bedata = dev_get_drvdata(&pvcalls_front_dev->dev);
> +
> +	map = (struct sock_mapping *) sock->sk->sk_send_head;
> +	if (!map) {
> +		pvcalls_exit();
> +		return -ENOTSOCK;
> +	}
> +
> +	if (map->passive.status != PVCALLS_STATUS_LISTEN) {
> +		pvcalls_exit();
> +		return -EINVAL;
> +	}
> +
> +	nonblock = flags & SOCK_NONBLOCK;
> +	/*
> +	 * Backend only supports 1 inflight accept request, will return
> +	 * errors for the others
> +	 */
> +	if (test_and_set_bit(PVCALLS_FLAG_ACCEPT_INFLIGHT,
> +			     (void *)&map->passive.flags)) {
> +		req_id = READ_ONCE(map->passive.inflight_req_id);
> +		if (req_id != PVCALLS_INVALID_ID &&
> +		    READ_ONCE(bedata->rsp[req_id].req_id) == req_id) {


READ_ONCE (especially the second one)? I know I may sound fixated on
this but I really don't understand how compiler may do anything wrong if
straight reads were used.

For the first case, I guess, theoretically the compiler may decide to
re-fetch map->passive.inflight_req_id. But even if it did, would that be
a problem? Both of these READ_ONCE targets are updated below before
PVCALLS_FLAG_ACCEPT_INFLIGHT is cleared so there should not be any
change between re-fetching, I think. (The only exception is the noblock
case, which does WRITE_ONCE that don't understand either)


> +			map2 = map->passive.accept_map;
> +			goto received;
> +		}
> +		if (nonblock) {
> +			pvcalls_exit();
> +			return -EAGAIN;
> +		}
> +		if (wait_event_interruptible(map->passive.inflight_accept_req,
> +			!test_and_set_bit(PVCALLS_FLAG_ACCEPT_INFLIGHT,
> +					  (void *)&map->passive.flags))) {
> +			pvcalls_exit();
> +			return -EINTR;
> +		}
> +	}
> +
> +	spin_lock(&bedata->socket_lock);
> +	ret = get_request(bedata, &req_id);
> +	if (ret < 0) {
> +		clear_bit(PVCALLS_FLAG_ACCEPT_INFLIGHT,
> +			  (void *)&map->passive.flags);
> +		spin_unlock(&bedata->socket_lock);
> +		pvcalls_exit();
> +		return ret;
> +	}
> +	map2 = kzalloc(sizeof(*map2), GFP_KERNEL);
> +	if (map2 == NULL) {
> +		clear_bit(PVCALLS_FLAG_ACCEPT_INFLIGHT,
> +			  (void *)&map->passive.flags);
> +		spin_unlock(&bedata->socket_lock);
> +		pvcalls_exit();
> +		return -ENOMEM;
> +	}
> +	ret =  create_active(map2, &evtchn);
> +	if (ret < 0) {
> +		kfree(map2);
> +		clear_bit(PVCALLS_FLAG_ACCEPT_INFLIGHT,
> +			  (void *)&map->passive.flags);
> +		spin_unlock(&bedata->socket_lock);
> +		pvcalls_exit();
> +		return -ENOMEM;

Why not ret?

-boris


> +	}
> +	list_add_tail(&map2->list, &bedata->socket_mappings);
> +
> +	req = RING_GET_REQUEST(&bedata->ring, req_id);
> +	req->req_id = req_id;
> +	req->cmd = PVCALLS_ACCEPT;
> +	req->u.accept.id = (uint64_t) map;
> +	req->u.accept.ref = map2->active.ref;
> +	req->u.accept.id_new = (uint64_t) map2;
> +	req->u.accept.evtchn = evtchn;
> +	map->passive.accept_map = map2;
> +
> +	bedata->ring.req_prod_pvt++;
> +	RING_PUSH_REQUESTS_AND_CHECK_NOTIFY(&bedata->ring, notify);
> +	spin_unlock(&bedata->socket_lock);
> +	if (notify)
> +		notify_remote_via_irq(bedata->irq);
> +	/* We could check if we have received a response before returning. */
> +	if (nonblock) {
> +		WRITE_ONCE(map->passive.inflight_req_id, req_id);
> +		pvcalls_exit();
> +		return -EAGAIN;
> +	}
> +
> +	if (wait_event_interruptible(bedata->inflight_req,
> +		READ_ONCE(bedata->rsp[req_id].req_id) == req_id)) {
> +		pvcalls_exit();
> +		return -EINTR;
> +	}
> +	/* read req_id, then the content */
> +	smp_rmb();
> +
> +received:
> +	map2->sock = newsock;
> +	newsock->sk = kzalloc(sizeof(*newsock->sk), GFP_KERNEL);
> +	if (!newsock->sk) {
> +		bedata->rsp[req_id].req_id = PVCALLS_INVALID_ID;
> +		map->passive.inflight_req_id = PVCALLS_INVALID_ID;
> +		clear_bit(PVCALLS_FLAG_ACCEPT_INFLIGHT,
> +			  (void *)&map->passive.flags);
> +		pvcalls_front_free_map(bedata, map2);
> +		kfree(map2);
> +		pvcalls_exit();
> +		return -ENOMEM;
> +	}
> +	newsock->sk->sk_send_head = (void *)map2;
> +
> +	ret = bedata->rsp[req_id].ret;
> +	bedata->rsp[req_id].req_id = PVCALLS_INVALID_ID;
> +	map->passive.inflight_req_id = PVCALLS_INVALID_ID;
> +
> +	clear_bit(PVCALLS_FLAG_ACCEPT_INFLIGHT, (void *)&map->passive.flags);
> +	wake_up(&map->passive.inflight_accept_req);
> +
> +	pvcalls_exit();
> +	return ret;
> +}
> +
>  static const struct xenbus_device_id pvcalls_front_ids[] = {
>  	{ "pvcalls" },
>  	{ "" }
> diff --git a/drivers/xen/pvcalls-front.h b/drivers/xen/pvcalls-front.h
> index aa8fe10..ab4f1da 100644
> --- a/drivers/xen/pvcalls-front.h
> +++ b/drivers/xen/pvcalls-front.h
> @@ -10,5 +10,8 @@ int pvcalls_front_bind(struct socket *sock,
>  		       struct sockaddr *addr,
>  		       int addr_len);
>  int pvcalls_front_listen(struct socket *sock, int backlog);
> +int pvcalls_front_accept(struct socket *sock,
> +			 struct socket *newsock,
> +			 int flags);
>  
>  #endif
>