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

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  Re: [PATCH v3 11/18] xen/pvcalls: implement accept command Juergen Gross <jgross@suse.com> - 2017-06-13 09:30 +0200
    Re: [PATCH v3 11/18] xen/pvcalls: implement accept command Stefano Stabellini <sstabellini@kernel.org> - 2017-06-14 02:50 +0200
      Re: [PATCH v3 11/18] xen/pvcalls: implement accept command Juergen Gross <jgross@suse.com> - 2017-06-14 08:40 +0200
        Re: [PATCH v3 11/18] xen/pvcalls: implement accept command Stefano Stabellini <sstabellini@kernel.org> - 2017-06-14 21:40 +0200
          Re: [PATCH v3 11/18] xen/pvcalls: implement accept command Juergen Gross <jgross@suse.com> - 2017-06-15 09:00 +0200
            Re: [PATCH v3 11/18] xen/pvcalls: implement accept command Stefano Stabellini <sstabellini@kernel.org> - 2017-06-15 20:00 +0200

#1664541 — Re: [PATCH v3 11/18] xen/pvcalls: implement accept command

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@aporeto.com>
> CC: boris.ostrovsky@oracle.com
> CC: jgross@suse.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.

> +
> +	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;

>  }
>  
>  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;

> +	spin_unlock_irqrestore(&mappass->copy_lock, flags);
> +	queue_work(mappass->wq, &mappass->register_work);
> +
> +	/* Tell the caller we don't need to send back a notification yet */
> +	return -1;
> +
> +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;
> +	return ret;

return 0?


Juergen

>  }
>  
>  static int pvcalls_back_poll(struct xenbus_device *dev,
> 

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#1665331

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@aporeto.com>
> > CC: boris.ostrovsky@oracle.com
> > CC: jgross@suse.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;


> > +	spin_unlock_irqrestore(&mappass->copy_lock, flags);
> > +	queue_work(mappass->wq, &mappass->register_work);
> > +
> > +	/* Tell the caller we don't need to send back a notification yet */
> > +	return -1;
> > +
> > +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;
> > +	return ret;
> 
> return 0?

Yes, thanks

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#1665481

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@aporeto.com>
>>> CC: boris.ostrovsky@oracle.com
>>> CC: jgross@suse.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?
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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#1666167

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@aporeto.com>
> >>> CC: boris.ostrovsky@oracle.com
> >>> CC: jgross@suse.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.


> 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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#1666494

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@aporeto.com>
>>>>> CC: boris.ostrovsky@oracle.com
>>>>> CC: jgross@suse.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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#1666949

On Thu, 15 Jun 2017, Juergen Gross wrote:
> 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@aporeto.com>
> >>>>> CC: boris.ostrovsky@oracle.com
> >>>>> CC: jgross@suse.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.

I'll do

 
> > 
> > 
> >> 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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