Date:   Thu, 15 Jun 2017 08:58:05 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Stefano Stabellini <sstabellini@...nel.org>
Cc:     xen-devel@...ts.xen.org, linux-kernel@...r.kernel.org,
        boris.ostrovsky@...cle.com,
        Stefano Stabellini <stefano@...reto.com>
Subject: Re: [PATCH v3 11/18] xen/pvcalls: implement accept command

On 14/06/17 21:27, Stefano Stabellini wrote:
> On Wed, 14 Jun 2017, Juergen Gross wrote:
>> 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@...reto.com>
>>>>> CC: boris.ostrovsky@...cle.com
>>>>> CC: jgross@...e.com
>>>>> ---
>>>>>  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?
> 
> This cannot happen because req is a copy of the guest request here.
> However, it is possible that __pvcalls_back_accept is racing against
> pvcalls_back_accept. In that case, I would need to make sure not only
> that cmd is written atomically, but now that I am thinking about this,
> that cmd is written *after* the rest of reqcopy: otherwise
> __pvcalls_back_accept could see a partially updated reqcopy.
> 
> It would be possible to do this with atomic accesses and barriers, but
> I am thinking that it is not worth the effort. I am tempted to roll back
> to the previous version with spinlocks.

Okay. Maybe add a comment mentioning this possible race.


Juergen

> 
> 
>> 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.
> 

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