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Date: Tue, 2 Jul 2019 21:45:46 -0700 From: Mahesh Bandewar (महेश बंडेवार) <maheshb@...gle.com> To: Geert Uytterhoeven <geert@...ux-m68k.org> Cc: Netdev <netdev@...r.kernel.org>, Eric Dumazet <edumazet@...gle.com>, David Miller <davem@...emloft.net>, Michael Chan <michael.chan@...adcom.com>, Daniel Axtens <dja@...ens.net>, Mahesh Bandewar <mahesh@...dewar.net>, "Paul E. McKenney" <paulmck@...ux.ibm.com>, linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org Subject: Re: 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@...ux-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@...gle.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 = ð_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, ð_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@...ux-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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