Re: [PATCH v3 11/18] xen/pvcalls: implement accept command

[Juergen Gross]

On 14/06/17 02:47, Stefano Stabellini wrote:
> On Tue, 13 Jun 2017, Juergen Gross wrote:
>> On 02/06/17 21:31, Stefano Stabellini wrote:
>>> Implement the accept command by calling inet_accept. To avoid blocking
>>> in the kernel, call inet_accept(O_NONBLOCK) from a workqueue, which get
>>> scheduled on sk_data_ready (for a passive socket, it means that there
>>> are connections to accept).
>>>
>>> Use the reqcopy field to store the request. Accept the new socket from
>>> the delayed work function, create a new sock_mapping for it, map
>>> the indexes page and data ring, and reply to the other end. Allocate an
>>> ioworker for the socket.
>>>
>>> Only support one outstanding blocking accept request for every socket at
>>> any time.
>>>
>>> Add a field to sock_mapping to remember the passive socket from which an
>>> active socket was created.
>>>
>>> Signed-off-by: Stefano Stabellini <stefano@xxxxxxxxxxx>
>>> CC: boris.ostrovsky@xxxxxxxxxx
>>> CC: jgross@xxxxxxxx
>>> ---
>>>  drivers/xen/pvcalls-back.c | 109 ++++++++++++++++++++++++++++++++++++++++++++-
>>>  1 file changed, 108 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/xen/pvcalls-back.c b/drivers/xen/pvcalls-back.c
>>> index a75586e..f1173f4 100644
>>> --- a/drivers/xen/pvcalls-back.c
>>> +++ b/drivers/xen/pvcalls-back.c
>>> @@ -65,6 +65,7 @@ struct pvcalls_ioworker {
>>>  struct sock_mapping {
>>>  	struct list_head list;
>>>  	struct pvcalls_fedata *priv;
>>> +	struct sockpass_mapping *sockpass;
>>>  	struct socket *sock;
>>>  	uint64_t id;
>>>  	grant_ref_t ref;
>>> @@ -275,10 +276,79 @@ static int pvcalls_back_release(struct xenbus_device *dev,
>>>  
>>>  static void __pvcalls_back_accept(struct work_struct *work)
>>>  {
>>> +	struct sockpass_mapping *mappass = container_of(
>>> +		work, struct sockpass_mapping, register_work);
>>> +	struct sock_mapping *map;
>>> +	struct pvcalls_ioworker *iow;
>>> +	struct pvcalls_fedata *priv;
>>> +	struct socket *sock;
>>> +	struct xen_pvcalls_response *rsp;
>>> +	struct xen_pvcalls_request *req;
>>> +	int notify;
>>> +	int ret = -EINVAL;
>>> +	unsigned long flags;
>>> +
>>> +	priv = mappass->priv;
>>> +	/* We only need to check the value of "cmd" atomically on read. */
>>> +	spin_lock_irqsave(&mappass->copy_lock, flags);
>>> +	req = &mappass->reqcopy;
>>> +	if (req->cmd != PVCALLS_ACCEPT) {
>>> +		spin_unlock_irqrestore(&mappass->copy_lock, flags);
>>> +		return;
>>> +	}
>>> +	spin_unlock_irqrestore(&mappass->copy_lock, flags);
>>
>> What about:
>> 	req = &mappass->reqcopy;
>> 	if (ACCESS_ONCE(req->cmd) != PVCALLS_ACCEPT)
>> 		return;
>>
>> I can't see the need for taking a lock here.
> 
> Sure, good idea
> 
> 
>>> +
>>> +	sock = sock_alloc();
>>> +	if (sock == NULL)
>>> +		goto out_error;
>>> +	sock->type = mappass->sock->type;
>>> +	sock->ops = mappass->sock->ops;
>>> +
>>> +	ret = inet_accept(mappass->sock, sock, O_NONBLOCK, true);
>>> +	if (ret == -EAGAIN) {
>>> +		sock_release(sock);
>>> +		goto out_error;
>>> +	}
>>> +
>>> +	map = pvcalls_new_active_socket(priv,
>>> +					req->u.accept.id_new,
>>> +					req->u.accept.ref,
>>> +					req->u.accept.evtchn,
>>> +					sock);
>>> +	if (!map) {
>>> +		sock_release(sock);
>>> +		goto out_error;
>>> +	}
>>> +
>>> +	map->sockpass = mappass;
>>> +	iow = &map->ioworker;
>>> +	atomic_inc(&map->read);
>>> +	atomic_inc(&map->io);
>>> +	queue_work_on(iow->cpu, iow->wq, &iow->register_work);
>>> +
>>> +out_error:
>>> +	rsp = RING_GET_RESPONSE(&priv->ring, priv->ring.rsp_prod_pvt++);
>>> +	rsp->req_id = req->req_id;
>>> +	rsp->cmd = req->cmd;
>>> +	rsp->u.accept.id = req->u.accept.id;
>>> +	rsp->ret = ret;
>>> +	RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(&priv->ring, notify);
>>> +	if (notify)
>>> +		notify_remote_via_irq(priv->irq);
>>> +
>>> +	spin_lock_irqsave(&mappass->copy_lock, flags);
>>> +	mappass->reqcopy.cmd = 0;
>>> +	spin_unlock_irqrestore(&mappass->copy_lock, flags);
>>
>> ACCESS_ONCE(mappass->reqcopy.cmd) = 0;
> 
> OK
> 
> 
>>>  }
>>>  
>>>  static void pvcalls_pass_sk_data_ready(struct sock *sock)
>>>  {
>>> +	struct sockpass_mapping *mappass = sock->sk_user_data;
>>> +
>>> +	if (mappass == NULL)
>>> +		return;
>>> +
>>> +	queue_work(mappass->wq, &mappass->register_work);
>>>  }
>>>  
>>>  static int pvcalls_back_bind(struct xenbus_device *dev,
>>> @@ -380,7 +450,44 @@ static int pvcalls_back_listen(struct xenbus_device *dev,
>>>  static int pvcalls_back_accept(struct xenbus_device *dev,
>>>  			       struct xen_pvcalls_request *req)
>>>  {
>>> -	return 0;
>>> +	struct pvcalls_fedata *priv;
>>> +	struct sockpass_mapping *mappass;
>>> +	int ret = -EINVAL;
>>> +	struct xen_pvcalls_response *rsp;
>>> +	unsigned long flags;
>>> +
>>> +	priv = dev_get_drvdata(&dev->dev);
>>> +
>>> +	mappass = radix_tree_lookup(&priv->socketpass_mappings,
>>> +		req->u.accept.id);
>>> +	if (mappass == NULL)
>>> +		goto out_error;
>>> +
>>> +	/* 
>>> +	 * Limitation of the current implementation: only support one
>>> +	 * concurrent accept or poll call on one socket.
>>> +	 */
>>> +	spin_lock_irqsave(&mappass->copy_lock, flags);
>>> +	if (mappass->reqcopy.cmd != 0) {
>>> +		spin_unlock_irqrestore(&mappass->copy_lock, flags);
>>> +		ret = -EINTR;
>>> +		goto out_error;
>>> +	}
>>> +
>>> +	mappass->reqcopy = *req;
>>
>> This time you need the lock, however you should use:
>>
>> ACCESS_ONCE(mappass->reqcopy) = *req;
> 
> I don't think that guarantees atomic accesses to the cmd field of the
> struct. Shouldn't this be:
> 
>   ACCESS_ONCE(mappass->reqcopy.cmd) = req->cmd;
>   mappass->reqcopy = *req;

Hmm, what if the frontend changes cmd between those two accesses?
You either need another local buffer or you have to copy cmd via
ACCESS_ONCE() and the rest of *req separately (seems not to be
that hard: its just cmd, req_id and u).

BTW: Maybe you should use READ_ONCE() and WRITE_ONCE() instead of
ACCESS_ONCE(), as those seem to be preferred nowadays.


Juergen