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SubjectRe: [PATCH v3 11/13] xen/pvcalls: implement poll command
On Tue, 15 Aug 2017, Boris Ostrovsky wrote:
> > @@ -736,6 +755,104 @@ int pvcalls_front_accept(struct socket *sock, struct socket *newsock, int flags)
> > return ret;
> > }
> >
> > +static unsigned int pvcalls_front_poll_passive(struct file *file,
> > + struct pvcalls_bedata *bedata,
> > + struct sock_mapping *map,
> > + poll_table *wait)
> > +{
> > + int notify, req_id, ret;
> > + struct xen_pvcalls_request *req;
> > +
>
> I am a bit confused by the logic here.
>
> > + if (test_bit(PVCALLS_FLAG_ACCEPT_INFLIGHT,
> > + (void *)&map->passive.flags)) {
> > + uint32_t req_id = READ_ONCE(map->passive.inflight_req_id);
> > + if (req_id != PVCALLS_INVALID_ID &&
> > + READ_ONCE(bedata->rsp[req_id].req_id) == req_id)
> > + return POLLIN;
>
> This is successful accept?

Yes: if an accept reply is pending, there must be something to read on
the passive socket.


> Why is it returning POLLIN only and not
> POLLIN | POLLRDNORM (or POLLOUT, for that matter)?

Keep in mind that this is a passive socket. There is no writing to
them. But yes, POLLIN and POLLRDNORM are equivalent, so it is best to
set both. I'll do that.


> > +
> > + poll_wait(file, &map->passive.inflight_accept_req, wait);
> > + return 0;
> > + }
> > +
> > + if (test_and_clear_bit(PVCALLS_FLAG_POLL_RET,
> > + (void *)&map->passive.flags))
> > + return POLLIN;
>
> This is previous poll request completing?

Yes, a previous POLL request completing.


> > +
> > + /*
> > + * First check RET, then INFLIGHT. No barriers necessary to
> > + * ensure execution ordering because of the conditional
> > + * instructions creating control dependencies.
> > + */
> > +
> > + if (test_and_set_bit(PVCALLS_FLAG_POLL_INFLIGHT,
> > + (void *)&map->passive.flags)) {
> > + poll_wait(file, &bedata->inflight_req, wait);
> > + return 0;
> > + }
>
> This I don't understand, could you explain?

If none of the conditions above apply (there isn't an
accept reply ready, and there isn't a poll reply ready) then it means
that we actually need to tell the caller to wait. This is done by
calling poll_wait to add a wait_queue to the poll table.

Then we need to tell the backend that we are polling, that is done by
the code below that sends a PVCALLS_POLL message to the backend.

The check on PVCALLS_FLAG_POLL_INFLIGHT here is to avoid sending
multiple PVCALLS_POLL requests to the backend. Thus, if there is one
already in-flight, we can just call poll_wait and return immediately
without sending a second request.


> > +
> > + spin_lock(&bedata->pvcallss_lock);
> > + ret = get_request(bedata, &req_id);
> > + if (ret < 0) {
> > + spin_unlock(&bedata->pvcallss_lock);
> > + return ret;
> > + }
> > + req = RING_GET_REQUEST(&bedata->ring, req_id);
> > + req->req_id = req_id;
> > + req->cmd = PVCALLS_POLL;
> > + req->u.poll.id = (uint64_t) map;
> > +
> > + bedata->ring.req_prod_pvt++;
> > + RING_PUSH_REQUESTS_AND_CHECK_NOTIFY(&bedata->ring, notify);
> > + spin_unlock(&bedata->pvcallss_lock);
> > + if (notify)
> > + notify_remote_via_irq(bedata->irq);
> > +
> > + poll_wait(file, &bedata->inflight_req, wait);
> > + return 0;
> > +}
> > +
> > +static unsigned int pvcalls_front_poll_active(struct file *file,
> > + struct pvcalls_bedata *bedata,
> > + struct sock_mapping *map,
> > + poll_table *wait)
> > +{
> > + unsigned int mask = 0;
> > + int32_t in_error, out_error;
> > + struct pvcalls_data_intf *intf = map->active.ring;
> > +
> > + out_error = intf->out_error;
> > + in_error = intf->in_error;
> > +
> > + poll_wait(file, &map->active.inflight_conn_req, wait);
> > + if (pvcalls_front_write_todo(map))
> > + mask |= POLLOUT | POLLWRNORM;
> > + if (pvcalls_front_read_todo(map))
> > + mask |= POLLIN | POLLRDNORM;
> > + if (in_error != 0 || out_error != 0)
> > + mask |= POLLERR;
> > +
> > + return mask;
> > +}
> > +
> > +unsigned int pvcalls_front_poll(struct file *file, struct socket *sock,
> > + poll_table *wait)
> > +{
> > + struct pvcalls_bedata *bedata;
> > + struct sock_mapping *map;
> > +
> > + if (!pvcalls_front_dev)
> > + return POLLNVAL;
> > + bedata = dev_get_drvdata(&pvcalls_front_dev->dev);
> > +
> > + map = (struct sock_mapping *) READ_ONCE(sock->sk->sk_send_head);
>
> I just noticed this --- why is it READ_ONCE? Are you concerned that
> sk_send_head may change?

No, but I wanted to avoid partial reads. A caller could call
pvcalls_front_accept and pvcalls_front_poll on newsock almost at the
same time (it is probably not the correct way to use the API), I wanted
to make sure that "map" is either read correctly, or not read at all.


> > + if (!map)
> > + return POLLNVAL;
> > + if (map->active_socket)
> > + return pvcalls_front_poll_active(file, bedata, map, wait);
> > + else
> > + return pvcalls_front_poll_passive(file, bedata, map, wait);
> > +}
> > +
> > static const struct xenbus_device_id pvcalls_front_ids[] = {
> > { "pvcalls" },
> > { "" }
> > diff --git a/drivers/xen/pvcalls-front.h b/drivers/xen/pvcalls-front.h
> > index de24041..25e05b8 100644
> > --- a/drivers/xen/pvcalls-front.h
> > +++ b/drivers/xen/pvcalls-front.h
> > @@ -20,5 +20,8 @@ int pvcalls_front_recvmsg(struct socket *sock,
> > struct msghdr *msg,
> > size_t len,
> > int flags);
> > +unsigned int pvcalls_front_poll(struct file *file,
> > + struct socket *sock,
> > + poll_table *wait);
> >
> > #endif
>

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