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SubjectRe: suspicious RCU usage (was: Re: [PATCHv3 next 1/3] loopback: create blackhole net device similar to loopack.)
On Tue, Jul 2, 2019 at 5:54 AM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>
> Hi Mahesh,
>
> On Mon, 1 Jul 2019, Mahesh Bandewar wrote:
> > Create a blackhole net device that can be used for "dead"
> > dst entries instead of loopback device. This blackhole device differs
> > from loopback in few aspects: (a) It's not per-ns. (b) MTU on this
> > device is ETH_MIN_MTU (c) The xmit function is essentially kfree_skb().
> > and (d) since it's not registered it won't have ifindex.
> >
> > Lower MTU effectively make the device not pass the MTU check during
> > the route check when a dst associated with the skb is dead.
> >
> > Signed-off-by: Mahesh Bandewar <maheshb@google.com>
>
> This is now commit 4de83b88c66a1e4d ("loopback: create blackhole net
> device similar to loopack.") in net-next, and causes the following
> warning on arm64:
>
> WARNING: suspicious RCU usage
> 5.2.0-rc6-arm64-renesas-01699-g4de83b88c66a1e4d #263 Not tainted
> -----------------------------
> include/linux/rtnetlink.h:85 suspicious rcu_dereference_protected() usage!
>
> other info that might help us debug this:
>
>
> rcu_scheduler_active = 2, debug_locks = 1
> no locks held by swapper/0/1.
>
thanks for the report. Let me take a look at this.

> stack backtrace:
> CPU: 2 PID: 1 Comm: swapper/0 Not tainted 5.2.0-rc6-arm64-renesas-01699-g4de83b88c66a1e4d #263
> Hardware name: Renesas Salvator-X 2nd version board based on r8a7795 ES2.0+ (DT)
> Call trace:
> dump_backtrace+0x0/0x148
> show_stack+0x14/0x20
> dump_stack+0xd4/0x11c
> lockdep_rcu_suspicious+0xcc/0x110
> dev_init_scheduler+0x114/0x150
> blackhole_netdev_init+0x40/0x80
> do_one_initcall+0x178/0x37c
> kernel_init_freeable+0x490/0x530
> kernel_init+0x10/0x100
> ret_from_fork+0x10/0x1c
>
>
> > ---
> > v1->v2->v3
> > no change
> >
> > drivers/net/loopback.c | 76 ++++++++++++++++++++++++++++++++++-----
> > include/linux/netdevice.h | 2 ++
> > 2 files changed, 69 insertions(+), 9 deletions(-)
> >
> > diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
> > index 87d361666cdd..3b39def5471e 100644
> > --- a/drivers/net/loopback.c
> > +++ b/drivers/net/loopback.c
> > @@ -55,6 +55,13 @@
> > #include <net/net_namespace.h>
> > #include <linux/u64_stats_sync.h>
> >
> > +/* blackhole_netdev - a device used for dsts that are marked expired!
> > + * This is global device (instead of per-net-ns) since it's not needed
> > + * to be per-ns and gets initialized at boot time.
> > + */
> > +struct net_device *blackhole_netdev;
> > +EXPORT_SYMBOL(blackhole_netdev);
> > +
> > /* The higher levels take care of making this non-reentrant (it's
> > * called with bh's disabled).
> > */
> > @@ -150,12 +157,14 @@ static const struct net_device_ops loopback_ops = {
> > .ndo_set_mac_address = eth_mac_addr,
> > };
> >
> > -/* The loopback device is special. There is only one instance
> > - * per network namespace.
> > - */
> > -static void loopback_setup(struct net_device *dev)
> > +static void gen_lo_setup(struct net_device *dev,
> > + unsigned int mtu,
> > + const struct ethtool_ops *eth_ops,
> > + const struct header_ops *hdr_ops,
> > + const struct net_device_ops *dev_ops,
> > + void (*dev_destructor)(struct net_device *dev))
> > {
> > - dev->mtu = 64 * 1024;
> > + dev->mtu = mtu;
> > dev->hard_header_len = ETH_HLEN; /* 14 */
> > dev->min_header_len = ETH_HLEN; /* 14 */
> > dev->addr_len = ETH_ALEN; /* 6 */
> > @@ -174,11 +183,20 @@ static void loopback_setup(struct net_device *dev)
> > | NETIF_F_NETNS_LOCAL
> > | NETIF_F_VLAN_CHALLENGED
> > | NETIF_F_LOOPBACK;
> > - dev->ethtool_ops = &loopback_ethtool_ops;
> > - dev->header_ops = &eth_header_ops;
> > - dev->netdev_ops = &loopback_ops;
> > + dev->ethtool_ops = eth_ops;
> > + dev->header_ops = hdr_ops;
> > + dev->netdev_ops = dev_ops;
> > dev->needs_free_netdev = true;
> > - dev->priv_destructor = loopback_dev_free;
> > + dev->priv_destructor = dev_destructor;
> > +}
> > +
> > +/* The loopback device is special. There is only one instance
> > + * per network namespace.
> > + */
> > +static void loopback_setup(struct net_device *dev)
> > +{
> > + gen_lo_setup(dev, (64 * 1024), &loopback_ethtool_ops, &eth_header_ops,
> > + &loopback_ops, loopback_dev_free);
> > }
> >
> > /* Setup and register the loopback device. */
> > @@ -213,3 +231,43 @@ static __net_init int loopback_net_init(struct net *net)
> > struct pernet_operations __net_initdata loopback_net_ops = {
> > .init = loopback_net_init,
> > };
> > +
> > +/* blackhole netdevice */
> > +static netdev_tx_t blackhole_netdev_xmit(struct sk_buff *skb,
> > + struct net_device *dev)
> > +{
> > + kfree_skb(skb);
> > + net_warn_ratelimited("%s(): Dropping skb.\n", __func__);
> > + return NETDEV_TX_OK;
> > +}
> > +
> > +static const struct net_device_ops blackhole_netdev_ops = {
> > + .ndo_start_xmit = blackhole_netdev_xmit,
> > +};
> > +
> > +/* This is a dst-dummy device used specifically for invalidated
> > + * DSTs and unlike loopback, this is not per-ns.
> > + */
> > +static void blackhole_netdev_setup(struct net_device *dev)
> > +{
> > + gen_lo_setup(dev, ETH_MIN_MTU, NULL, NULL, &blackhole_netdev_ops, NULL);
> > +}
> > +
> > +/* Setup and register the blackhole_netdev. */
> > +static int __init blackhole_netdev_init(void)
> > +{
> > + blackhole_netdev = alloc_netdev(0, "blackhole_dev", NET_NAME_UNKNOWN,
> > + blackhole_netdev_setup);
> > + if (!blackhole_netdev)
> > + return -ENOMEM;
> > +
> > + dev_init_scheduler(blackhole_netdev);
> > + dev_activate(blackhole_netdev);
> > +
> > + blackhole_netdev->flags |= IFF_UP | IFF_RUNNING;
> > + dev_net_set(blackhole_netdev, &init_net);
> > +
> > + return 0;
> > +}
> > +
> > +device_initcall(blackhole_netdev_init);
> > diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> > index eeacebd7debb..88292953aa6f 100644
> > --- a/include/linux/netdevice.h
> > +++ b/include/linux/netdevice.h
> > @@ -4870,4 +4870,6 @@ do { \
> > #define PTYPE_HASH_SIZE (16)
> > #define PTYPE_HASH_MASK (PTYPE_HASH_SIZE - 1)
> >
> > +extern struct net_device *blackhole_netdev;
> > +
> > #endif /* _LINUX_NETDEVICE_H */
> >
> Gr{oetje,eeting}s,
>
> Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
>
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
> -- Linus Torvalds
>

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