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Date: Mon, 23 Oct 2017 16:03:53 -0700 (PDT)
From: Stefano Stabellini <sstabellini@...nel.org>
To: Boris Ostrovsky <boris.ostrovsky@...cle.com>
cc: Stefano Stabellini <sstabellini@...nel.org>,
xen-devel@...ts.xen.org, linux-kernel@...r.kernel.org,
jgross@...e.com, Stefano Stabellini <stefano@...reto.com>
Subject: Re: [PATCH v5 08/13] xen/pvcalls: implement accept command
On Tue, 17 Oct 2017, Boris Ostrovsky wrote:
> On 10/06/2017 08:30 PM, Stefano Stabellini wrote:
> > Introduce a waitqueue to allow only one outstanding accept command at
> > any given time and to implement polling on the passive socket. Introduce
> > a flags field to keep track of in-flight accept and poll commands.
> >
> > Send PVCALLS_ACCEPT to the backend. Allocate a new active socket. Make
> > sure that only one accept command is executed at any given time by
> > setting PVCALLS_FLAG_ACCEPT_INFLIGHT and waiting on the
> > inflight_accept_req waitqueue.
> >
> > Convert the new struct sock_mapping pointer into an uint64_t and use it
> > as id for the new socket to pass to the backend.
> >
> > Check if the accept call is non-blocking: in that case after sending the
> > ACCEPT command to the backend store the sock_mapping pointer of the new
> > struct and the inflight req_id then return -EAGAIN (which will respond
> > only when there is something to accept). Next time accept is called,
> > we'll check if the ACCEPT command has been answered, if so we'll pick up
> > where we left off, otherwise we return -EAGAIN again.
> >
> > Note that, differently from the other commands, we can use
> > wait_event_interruptible (instead of wait_event) in the case of accept
> > as we are able to track the req_id of the ACCEPT response that we are
> > waiting.
> >
> > Signed-off-by: Stefano Stabellini <stefano@...reto.com>
> > CC: boris.ostrovsky@...cle.com
> > CC: jgross@...e.com
> > ---
> > drivers/xen/pvcalls-front.c | 146 ++++++++++++++++++++++++++++++++++++++++++++
> > drivers/xen/pvcalls-front.h | 3 +
> > 2 files changed, 149 insertions(+)
> >
> > diff --git a/drivers/xen/pvcalls-front.c b/drivers/xen/pvcalls-front.c
> > index 5433fae..8958e74 100644
> > --- a/drivers/xen/pvcalls-front.c
> > +++ b/drivers/xen/pvcalls-front.c
> > @@ -77,6 +77,16 @@ struct sock_mapping {
> > #define PVCALLS_STATUS_BIND 1
> > #define PVCALLS_STATUS_LISTEN 2
> > uint8_t status;
> > + /*
> > + * Internal state-machine flags.
> > + * Only one accept operation can be inflight for a socket.
> > + * Only one poll operation can be inflight for a given socket.
> > + */
> > +#define PVCALLS_FLAG_ACCEPT_INFLIGHT 0
> > + uint8_t flags;
> > + uint32_t inflight_req_id;
> > + struct sock_mapping *accept_map;
> > + wait_queue_head_t inflight_accept_req;
> > } passive;
> > };
> > };
> > @@ -392,6 +402,8 @@ int pvcalls_front_bind(struct socket *sock, struct sockaddr *addr, int addr_len)
> > memcpy(req->u.bind.addr, addr, sizeof(*addr));
> > req->u.bind.len = addr_len;
> >
> > + init_waitqueue_head(&map->passive.inflight_accept_req);
> > +
> > map->active_socket = false;
> >
> > bedata->ring.req_prod_pvt++;
> > @@ -470,6 +482,140 @@ int pvcalls_front_listen(struct socket *sock, int backlog)
> > return ret;
> > }
> >
> > +int pvcalls_front_accept(struct socket *sock, struct socket *newsock, int flags)
> > +{
> > + struct pvcalls_bedata *bedata;
> > + struct sock_mapping *map;
> > + struct sock_mapping *map2 = NULL;
> > + struct xen_pvcalls_request *req;
> > + int notify, req_id, ret, evtchn, nonblock;
> > +
> > + pvcalls_enter();
> > + if (!pvcalls_front_dev) {
> > + pvcalls_exit();
> > + return -ENOTCONN;
> > + }
> > + bedata = dev_get_drvdata(&pvcalls_front_dev->dev);
> > +
> > + map = (struct sock_mapping *) sock->sk->sk_send_head;
> > + if (!map) {
> > + pvcalls_exit();
> > + return -ENOTSOCK;
> > + }
> > +
> > + if (map->passive.status != PVCALLS_STATUS_LISTEN) {
> > + pvcalls_exit();
> > + return -EINVAL;
> > + }
> > +
> > + nonblock = flags & SOCK_NONBLOCK;
> > + /*
> > + * Backend only supports 1 inflight accept request, will return
> > + * errors for the others
> > + */
> > + if (test_and_set_bit(PVCALLS_FLAG_ACCEPT_INFLIGHT,
> > + (void *)&map->passive.flags)) {
> > + 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) {
>
>
> READ_ONCE (especially the second one)? I know I may sound fixated on
> this but I really don't understand how compiler may do anything wrong if
> straight reads were used.
>
> For the first case, I guess, theoretically the compiler may decide to
> re-fetch map->passive.inflight_req_id. But even if it did, would that be
> a problem? Both of these READ_ONCE targets are updated below before
> PVCALLS_FLAG_ACCEPT_INFLIGHT is cleared so there should not be any
> change between re-fetching, I think. (The only exception is the noblock
> case, which does WRITE_ONCE that don't understand either)
READ_ONCE is reasonably cheap: do we really want to have this kind of
conversation every time we touch this code in the future? Personally, I
would have used READ/WRITE_ONCE everywhere for inflight_req_id and
req_id, because it makes the code easier to understand.
We have already limited their usage, but at least we have followed a set
of guidelines. Doing further optimizations on this code seems
unnecessary and prone to confuse the reader.
> > + map2 = map->passive.accept_map;
> > + goto received;
> > + }
> > + if (nonblock) {
> > + pvcalls_exit();
> > + return -EAGAIN;
> > + }
> > + if (wait_event_interruptible(map->passive.inflight_accept_req,
> > + !test_and_set_bit(PVCALLS_FLAG_ACCEPT_INFLIGHT,
> > + (void *)&map->passive.flags))) {
> > + pvcalls_exit();
> > + return -EINTR;
> > + }
> > + }
> > +
> > + spin_lock(&bedata->socket_lock);
> > + ret = get_request(bedata, &req_id);
> > + if (ret < 0) {
> > + clear_bit(PVCALLS_FLAG_ACCEPT_INFLIGHT,
> > + (void *)&map->passive.flags);
> > + spin_unlock(&bedata->socket_lock);
> > + pvcalls_exit();
> > + return ret;
> > + }
> > + map2 = kzalloc(sizeof(*map2), GFP_KERNEL);
> > + if (map2 == NULL) {
> > + clear_bit(PVCALLS_FLAG_ACCEPT_INFLIGHT,
> > + (void *)&map->passive.flags);
> > + spin_unlock(&bedata->socket_lock);
> > + pvcalls_exit();
> > + return -ENOMEM;
> > + }
> > + ret = create_active(map2, &evtchn);
> > + if (ret < 0) {
> > + kfree(map2);
> > + clear_bit(PVCALLS_FLAG_ACCEPT_INFLIGHT,
> > + (void *)&map->passive.flags);
> > + spin_unlock(&bedata->socket_lock);
> > + pvcalls_exit();
> > + return -ENOMEM;
>
> Why not ret?
yes, good idea.
>
> > + }
> > + list_add_tail(&map2->list, &bedata->socket_mappings);
> > +
> > + req = RING_GET_REQUEST(&bedata->ring, req_id);
> > + req->req_id = req_id;
> > + req->cmd = PVCALLS_ACCEPT;
> > + req->u.accept.id = (uint64_t) map;
> > + req->u.accept.ref = map2->active.ref;
> > + req->u.accept.id_new = (uint64_t) map2;
> > + req->u.accept.evtchn = evtchn;
> > + map->passive.accept_map = map2;
> > +
> > + bedata->ring.req_prod_pvt++;
> > + RING_PUSH_REQUESTS_AND_CHECK_NOTIFY(&bedata->ring, notify);
> > + spin_unlock(&bedata->socket_lock);
> > + if (notify)
> > + notify_remote_via_irq(bedata->irq);
> > + /* We could check if we have received a response before returning. */
> > + if (nonblock) {
> > + WRITE_ONCE(map->passive.inflight_req_id, req_id);
> > + pvcalls_exit();
> > + return -EAGAIN;
> > + }
> > +
> > + if (wait_event_interruptible(bedata->inflight_req,
> > + READ_ONCE(bedata->rsp[req_id].req_id) == req_id)) {
> > + pvcalls_exit();
> > + return -EINTR;
> > + }
> > + /* read req_id, then the content */
> > + smp_rmb();
> > +
> > +received:
> > + map2->sock = newsock;
> > + newsock->sk = kzalloc(sizeof(*newsock->sk), GFP_KERNEL);
> > + if (!newsock->sk) {
> > + bedata->rsp[req_id].req_id = PVCALLS_INVALID_ID;
> > + map->passive.inflight_req_id = PVCALLS_INVALID_ID;
> > + clear_bit(PVCALLS_FLAG_ACCEPT_INFLIGHT,
> > + (void *)&map->passive.flags);
> > + pvcalls_front_free_map(bedata, map2);
> > + kfree(map2);
> > + pvcalls_exit();
> > + return -ENOMEM;
> > + }
> > + newsock->sk->sk_send_head = (void *)map2;
> > +
> > + ret = bedata->rsp[req_id].ret;
> > + bedata->rsp[req_id].req_id = PVCALLS_INVALID_ID;
> > + map->passive.inflight_req_id = PVCALLS_INVALID_ID;
> > +
> > + clear_bit(PVCALLS_FLAG_ACCEPT_INFLIGHT, (void *)&map->passive.flags);
> > + wake_up(&map->passive.inflight_accept_req);
> > +
> > + pvcalls_exit();
> > + return ret;
> > +}
> > +
> > 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 aa8fe10..ab4f1da 100644
> > --- a/drivers/xen/pvcalls-front.h
> > +++ b/drivers/xen/pvcalls-front.h
> > @@ -10,5 +10,8 @@ int pvcalls_front_bind(struct socket *sock,
> > struct sockaddr *addr,
> > int addr_len);
> > int pvcalls_front_listen(struct socket *sock, int backlog);
> > +int pvcalls_front_accept(struct socket *sock,
> > + struct socket *newsock,
> > + int flags);
> >
> > #endif
> >
>
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