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

[Juergen Gross <jgross@xxxxxxxx>]

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

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