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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH v3 11/18] xen/pvcalls: implement accept command
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@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?
> >
> > 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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