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SubjectRe: [PATCH bpf v3] xdp: fix hang while unregistering device bound to xdp socket
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?

>
> > + 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.

>
> > + 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.
> --
> 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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