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SubjectRe: [PATCH bpf v3] xdp: fix hang while unregistering device bound to xdp socket
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On 11.06.2019 15:13, Björn Töpel wrote:
> On Tue, 11 Jun 2019 at 10:42, Ilya Maximets <i.maximets@samsung.com> wrote:
>>
>> On 11.06.2019 11:09, Björn Töpel wrote:
>>> On Mon, 10 Jun 2019 at 22:49, Jonathan Lemon <jonathan.lemon@gmail.com> wrote:
>>>>
>>>> On 10 Jun 2019, at 9:15, Ilya Maximets wrote:
>>>>
>>>>> Device that bound to XDP socket will not have zero refcount until the
>>>>> userspace application will not close it. This leads to hang inside
>>>>> 'netdev_wait_allrefs()' if device unregistering requested:
>>>>>
>>>>> # ip link del p1
>>>>> < hang on recvmsg on netlink socket >
>>>>>
>>>>> # ps -x | grep ip
>>>>> 5126 pts/0 D+ 0:00 ip link del p1
>>>>>
>>>>> # journalctl -b
>>>>>
>>>>> Jun 05 07:19:16 kernel:
>>>>> unregister_netdevice: waiting for p1 to become free. Usage count = 1
>>>>>
>>>>> Jun 05 07:19:27 kernel:
>>>>> unregister_netdevice: waiting for p1 to become free. Usage count = 1
>>>>> ...
>>>>>
>>>>> Fix that by implementing NETDEV_UNREGISTER event notification handler
>>>>> to properly clean up all the resources and unref device.
>>>>>
>>>>> This should also allow socket killing via ss(8) utility.
>>>>>
>>>>> Fixes: 965a99098443 ("xsk: add support for bind for Rx")
>>>>> Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
>>>>> ---
>>>>>
>>>>> Version 3:
>>>>>
>>>>> * Declaration lines ordered from longest to shortest.
>>>>> * Checking of event type moved to the top to avoid unnecessary
>>>>> locking.
>>>>>
>>>>> Version 2:
>>>>>
>>>>> * Completely re-implemented using netdev event handler.
>>>>>
>>>>> net/xdp/xsk.c | 65
>>>>> ++++++++++++++++++++++++++++++++++++++++++++++++++-
>>>>> 1 file changed, 64 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
>>>>> index a14e8864e4fa..273a419a8c4d 100644
>>>>> --- a/net/xdp/xsk.c
>>>>> +++ b/net/xdp/xsk.c
>>>>> @@ -693,6 +693,57 @@ static int xsk_mmap(struct file *file, struct
>>>>> socket *sock,
>>>>> size, vma->vm_page_prot);
>>>>> }
>>>>>
>>>>> +static int xsk_notifier(struct notifier_block *this,
>>>>> + unsigned long msg, void *ptr)
>>>>> +{
>>>>> + struct net_device *dev = netdev_notifier_info_to_dev(ptr);
>>>>> + struct net *net = dev_net(dev);
>>>>> + int i, unregister_count = 0;
>>>>> + struct sock *sk;
>>>>> +
>>>>> + switch (msg) {
>>>>> + case NETDEV_UNREGISTER:
>>>>> + mutex_lock(&net->xdp.lock);
>>>>
>>>> The call is under the rtnl lock, and we're not modifying
>>>> the list, so this mutex shouldn't be needed.
>>>>
>>>
>>> The list can, however, be modified outside the rtnl lock (e.g. at
>>> socket creation). AFAIK the hlist cannot be traversed lock-less,
>>> right?
>>
>> We could use 'rcu_read_lock' instead and iterate with 'sk_for_each_rcu',
>> but we'll not be able to synchronize inside.
>>
>>>
>>>>
>>>>> + sk_for_each(sk, &net->xdp.list) {
>>>>> + struct xdp_sock *xs = xdp_sk(sk);
>>>>> +
>>>>> + mutex_lock(&xs->mutex);
>>>>> + if (dev != xs->dev) {
>>>>> + mutex_unlock(&xs->mutex);
>>>>> + continue;
>>>>> + }
>>>>> +
>>>>> + sk->sk_err = ENETDOWN;
>>>>> + if (!sock_flag(sk, SOCK_DEAD))
>>>>> + sk->sk_error_report(sk);
>>>>> +
>>>>> + /* Wait for driver to stop using the xdp socket. */
>>>>> + xdp_del_sk_umem(xs->umem, xs);
>>>>> + xs->dev = NULL;
>>>>> + synchronize_net();
>>>> Isn't this by handled by the unregister_count case below?
>>>>
>>>
>>> To clarify, setting dev to NULL and xdp_del_sk_umem() + sync makes
>>> sure that a driver doesn't touch the Tx and Rx rings. Nothing can be
>>> assumed about completion + fill ring (umem), until zero-copy has been
>>> disabled via ndo_bpf.
>>>
>>>>> +
>>>>> + /* Clear device references in umem. */
>>>>> + xdp_put_umem(xs->umem);
>>>>> + xs->umem = NULL;
>>>>
>>>> This makes me uneasy. We need to unregister the umem from
>>>> the device (xdp_umem_clear_dev()) but this can remove the umem
>>>> pages out from underneath the xsk.
>>>>
>>>
>>> Yes, this is scary. The socket is alive, and userland typically has
>>> the fill/completion rings mmapped. Then the umem refcount is decreased
>>> and can potentially free the umem (fill rings etc.), as Jonathan says,
>>> underneath the xsk. Also, setting the xs umem/dev to zero, while the
>>> socket is alive, would allow a user to re-setup the socket, which we
>>> don't want to allow.
>>>
>>>> Perhaps what's needed here is the equivalent of an unbind()
>>>> call that just detaches the umem/sk from the device, but does
>>>> not otherwise tear them down.
>>>>
>>>
>>> Yeah, I agree. A detached/zombie state is needed during the socket lifetime.
>>
>>
>> I could try to rip the 'xdp_umem_release()' apart so the 'xdp_umem_clear_dev()'
>> could be called separately. This will allow to not tear down the 'umem'.
>> However, it seems that it'll not be easy to synchronize all parts.
>> Any suggestions are welcome.
>>
>
> Thanks for continuing to work on this, Ilya.
>
> What need to be done is exactly an "unbind()", i.e. returning the
> socket to the state prior bind, but disallowing any changes from
> userland (e.g. setsockopt/bind). So, unbind() + track that we're in
> "unbound" mode. :-) I think breaking up xdp_umem_release() is good way
> to go.

Thanks, I'll move in this direction.

BTW, I'll be out of office from tomorrow until the end of the week.
So, I'll most probably return to this on Monday.

>
>> Also, there is no way to not clear the 'dev' as we have to put the reference.
>> Maybe we could add the additional check to 'xsk_bind()' for current device
>> state (dev->reg_state == NETREG_REGISTERED). This will allow us to avoid
>> re-setup of the socket.
>>
>
> Yes, and also make sure that the rest of the syscall implementations
> don't allow for re-setup.

OK.

>
>
> Björn
>
>>>
>>>>
>>>>> + mutex_unlock(&xs->mutex);
>>>>> + unregister_count++;
>>>>> + }
>>>>> + mutex_unlock(&net->xdp.lock);
>>>>> +
>>>>> + if (unregister_count) {
>>>>> + /* Wait for umem clearing completion. */
>>>>> + synchronize_net();
>>>>> + for (i = 0; i < unregister_count; i++)
>>>>> + dev_put(dev);
>>>>> + }
>>>>> +
>>>>> + break;
>>>>> + }
>>>>> +
>>>>> + return NOTIFY_DONE;
>>>>> +}
>>>>> +
>>>>> static struct proto xsk_proto = {
>>>>> .name = "XDP",
>>>>> .owner = THIS_MODULE,
>>>>> @@ -727,7 +778,8 @@ static void xsk_destruct(struct sock *sk)
>>>>> if (!sock_flag(sk, SOCK_DEAD))
>>>>> return;
>>>>>
>>>>> - xdp_put_umem(xs->umem);
>>>>> + if (xs->umem)
>>>>> + xdp_put_umem(xs->umem);
>>>> Not needed - xdp_put_umem() already does a null check.
>>
>> Indeed. Thanks.
>>
>>>> --
>>>> Jonathan
>>>>
>>>>
>>>>>
>>>>> sk_refcnt_debug_dec(sk);
>>>>> }
>>>>> @@ -784,6 +836,10 @@ static const struct net_proto_family
>>>>> xsk_family_ops = {
>>>>> .owner = THIS_MODULE,
>>>>> };
>>>>>
>>>>> +static struct notifier_block xsk_netdev_notifier = {
>>>>> + .notifier_call = xsk_notifier,
>>>>> +};
>>>>> +
>>>>> static int __net_init xsk_net_init(struct net *net)
>>>>> {
>>>>> mutex_init(&net->xdp.lock);
>>>>> @@ -816,8 +872,15 @@ static int __init xsk_init(void)
>>>>> err = register_pernet_subsys(&xsk_net_ops);
>>>>> if (err)
>>>>> goto out_sk;
>>>>> +
>>>>> + err = register_netdevice_notifier(&xsk_netdev_notifier);
>>>>> + if (err)
>>>>> + goto out_pernet;
>>>>> +
>>>>> return 0;
>>>>>
>>>>> +out_pernet:
>>>>> + unregister_pernet_subsys(&xsk_net_ops);
>>>>> out_sk:
>>>>> sock_unregister(PF_XDP);
>>>>> out_proto:
>>>>> --
>>>>> 2.17.1
>>>
>>>
>
>

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