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Date: Wed, 14 Jun 2017 08:32:07 +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 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?
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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