[PATCH v3 net-next 0/6] ip: Batch flushing uncached routes per batched device unregistration.

[PATCH v3 net-next 0/6] ip: Batch flushing uncached routes per batched device unregistration.

[Kuniyuki Iwashima]

Chris J Arges reported high RTNL contention during cleanup_net()
caused by rt_flush_dev() and rt6_uncached_list_flush_dev() iterating
over the global per-cpu uncached route lists for every netdev in
dying netns [0].

This series resolves the issue by batching the uncached route
cleanup after __rtnl_unlock() in netdev_run_todo()

[0]: https://lore.kernel.org/netdev/20260917-hash-bucket-route-lists-v3-0-30493a37b6eb@cloudflare.com/


Changelog:
  v3:
    * Add patch 1 ~ 4
    * Patch 6
      * Unlink routes only when both devices are blackhole_netdev
      * Clarify netdev_run_todo() does not run before
        rt6_uncached_list is initialised
      * Fix CONFIG_IPV6=n build

  v2: https://lore.kernel.org/netdev/20260929201546.1533268-1-kuniyu@google.com/
    * Drop ->pre_exit_batch()
    * Do batching after setting NETDEV_UNREGISTERED in netdev_run_todo()

  v1: https://lore.kernel.org/netdev/20260927202429.2452589-1-kuniyu@google.com/


Kuniyuki Iwashima (6):
  net: Order blackhole_netdev_init(), inet_init(), and inet6_init().
  ipv4: Inline inet_blackhole_dev_init() to devinet_init().
  net: Rename dev_isalive() to netif_is_alive().
  xfrm: Check netif_is_alive() in xfrm_bundle_create() and
    xfrm_create_dummy_bundle().
  ipv4: Batch rt_flush_dev() in netdev_run_todo().
  ipv6: Batch rt6_uncached_list_flush_dev() in netdev_run_todo().

 drivers/net/loopback.c    |  2 +-
 include/linux/netdevice.h |  6 ++++++
 include/net/ip6_route.h   |  7 +++++++
 include/net/route.h       |  6 ++++++
 net/core/dev.c            |  7 +++++++
 net/core/dev.h            |  6 ------
 net/core/net-sysfs.c      | 16 ++++++++--------
 net/core/netdev_work.c    |  4 ++--
 net/core/rtnetlink.c      |  2 +-
 net/ipv4/devinet.c        | 17 +++++------------
 net/ipv4/route.c          | 11 +++++++++--
 net/ipv6/af_inet6.c       |  2 +-
 net/ipv6/route.c          | 24 ++++++++++++++++--------
 net/xfrm/xfrm_policy.c    |  4 ++--
 14 files changed, 71 insertions(+), 43 deletions(-)

-- 
2.56.0.rc1.315.gc6ed9934b7-goog

[PATCH v3 net-next 1/6] net: Order blackhole_netdev_init(), inet_init(), and inet6_init().

[Kuniyuki Iwashima]

Commit 309b905deee5 ("ipv6: convert CONFIG_IPV6 to built-in
only and clean up Kconfigs") started to call inet6_init() as
device_initcall().

It explains the reason as a race condition on IPv6 addrconf,
but this is not accurate.

The change initially used fs_initcall() but switched to
device_initcall() due to the null-ptr-deref stack trace in
the link below.

It happened because blackhole_netdev_init() uses device_initcall()
and blackhole_netdev was NULL when addrconf_init() called
ipv6_add_dev(blackhole_netdev).

Due to the order in Kbuild,

  $ cat -n Kbuild | grep -E "(drivers|net)"
     111	obj-y			+= drivers/
     114	obj-$(CONFIG_NET)	+= net/

using device_initcall() for IPv6 allows built-in drivers to
be initialised earlier, which requires unnecessary NULL checks.

For the same ordering reason, IPv4 uses late_initcall() to
initialise blackhole_netdev.

Given loopback_net_ops is registered in net_dev_init() at
subsys_initcall(), blackhole_netdev_init() can be called
after that.

In addition, some places assume that IPv4 must be initialised
before IPv6.  For example, mptcp_proto_v6_init() copies
mptcp_prot to mptcp_v6_prot, which would otherwise have NULL
mptcp_v6_prot.h.hashinfo.

Let's explicitly order blackhole_netdev_init() -> inet_init()
-> inet6_init() before device_initcall() with 3 different
initcall levels:

  subsys_initcall      : 4  : net_dev_init()
  subsys_initcall_sync : 4s : blackhole_netdev_init()
  fs_initcall          : 5  : inet_init()
  fs_initcall_sync     : 5s : inet6_init()
  device_initcall      : 6  : built-in drivers

Note that both IPv4 and IPv6 can still use fs_initcall() thanks
to the order in net/Makefile, but explicit ordering would be
less error-prone.

  $ cat -n net/Makefile | grep ipv
      17	obj-$(CONFIG_INET)		+= ipv4/
      22	obj-y				+= ipv6/

Link: https://lore.kernel.org/netdev/20260309074758.0ea95a18@kernel.org/
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
Cc: Fernando Fernandez Mancera <fmancera@suse.de>
---
 drivers/net/loopback.c | 2 +-
 net/ipv6/af_inet6.c    | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
index 1fb6ce6843ad..55ecd5ebac0f 100644
--- a/drivers/net/loopback.c
+++ b/drivers/net/loopback.c
@@ -289,4 +289,4 @@ static int __init blackhole_netdev_init(void)
 	return 0;
 }
 
-device_initcall(blackhole_netdev_init);
+subsys_initcall_sync(blackhole_netdev_init);
diff --git a/net/ipv6/af_inet6.c b/net/ipv6/af_inet6.c
index f0efdc13baf4..a9b8b4b42a1a 100644
--- a/net/ipv6/af_inet6.c
+++ b/net/ipv6/af_inet6.c
@@ -1217,4 +1217,4 @@ static int __init inet6_init(void)
 	proto_unregister(&tcpv6_prot);
 	goto out;
 }
-device_initcall(inet6_init);
+fs_initcall_sync(inet6_init);
-- 
2.56.0.rc1.315.gc6ed9934b7-goog

[PATCH v3 net-next 2/6] ipv4: Inline inet_blackhole_dev_init() to devinet_init().

[Kuniyuki Iwashima]

Commit 22600596b675 ("ipv4: give an IPv4 dev to blackhole_netdev")
started to allocate in_device for blackhole_netdev.

It used late_initcall() because blackhole_netdev was allocated
at device_initcall().

Now, it is allocated before inet_init(), and we no longer need
to use late_initcall().

Let's inline inet_blackhole_dev_init() to devinet_init().

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 net/ipv4/devinet.c | 17 +++++------------
 1 file changed, 5 insertions(+), 12 deletions(-)

diff --git a/net/ipv4/devinet.c b/net/ipv4/devinet.c
index e84d3cf92247..2ed4c706ca9e 100644
--- a/net/ipv4/devinet.c
+++ b/net/ipv4/devinet.c
@@ -340,18 +340,6 @@ static void inetdev_destroy(struct in_device *in_dev)
 	in_dev_put(in_dev);
 }
 
-static int __init inet_blackhole_dev_init(void)
-{
-	struct in_device *in_dev;
-
-	rtnl_lock();
-	in_dev = inetdev_init(blackhole_netdev);
-	rtnl_unlock();
-
-	return PTR_ERR_OR_ZERO(in_dev);
-}
-late_initcall(inet_blackhole_dev_init);
-
 int inet_addr_onlink(struct in_device *in_dev, __be32 a, __be32 b)
 {
 	const struct in_ifaddr *ifa;
@@ -2974,4 +2962,9 @@ void __init devinet_init(void)
 		panic("Unable to register inet_af_ops\n");
 
 	rtnl_register_many(devinet_rtnl_msg_handlers);
+
+	rtnl_lock();
+	if (IS_ERR(inetdev_init(blackhole_netdev)))
+		panic("IP: failed to allocate in_device for blackhole_netdev\n");
+	rtnl_unlock();
 }
-- 
2.56.0.rc1.315.gc6ed9934b7-goog

[PATCH v3 net-next 3/6] net: Rename dev_isalive() to netif_is_alive().

[Kuniyuki Iwashima]

The next patch needs to use dev_isalive() in xfrm.

We should try to avoid using the dev_ prefix on APIs
we export via include/linux/.

Let's move it to include/linux/netdevice.h and rename
to netif_is_alive().

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 include/linux/netdevice.h |  6 ++++++
 net/core/dev.h            |  6 ------
 net/core/net-sysfs.c      | 16 ++++++++--------
 net/core/netdev_work.c    |  4 ++--
 net/core/rtnetlink.c      |  2 +-
 5 files changed, 17 insertions(+), 17 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index d037faff7c44..971f1056aef0 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -5712,6 +5712,12 @@ static inline const char *netdev_name(const struct net_device *dev)
 	return dev->name;
 }
 
+/* Caller holds RTNL, netdev->lock or RCU */
+static inline bool netif_is_alive(const struct net_device *dev)
+{
+	return READ_ONCE(dev->reg_state) <= NETREG_REGISTERED;
+}
+
 static inline const char *netdev_reg_state(const struct net_device *dev)
 {
 	u8 reg_state = READ_ONCE(dev->reg_state);
diff --git a/net/core/dev.h b/net/core/dev.h
index 51921623fc0a..43beebddf22f 100644
--- a/net/core/dev.h
+++ b/net/core/dev.h
@@ -412,10 +412,4 @@ int dev_get_hwtstamp_phylib(struct net_device *dev,
 			    struct kernel_hwtstamp_config *cfg);
 int net_hwtstamp_validate(const struct kernel_hwtstamp_config *cfg);
 
-/* Caller holds RTNL, netdev->lock or RCU */
-static inline bool dev_isalive(const struct net_device *dev)
-{
-	return READ_ONCE(dev->reg_state) <= NETREG_REGISTERED;
-}
-
 #endif
diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
index fa790e442570..e24a5154b212 100644
--- a/net/core/net-sysfs.c
+++ b/net/core/net-sysfs.c
@@ -100,7 +100,7 @@ static int sysfs_rtnl_lock(struct kobject *kobj, struct attribute *attr,
 	/* Check dismantle on the device hasn't started, otherwise deny the
 	 * operation.
 	 */
-	if (!dev_isalive(ndev)) {
+	if (!netif_is_alive(ndev)) {
 		rtnl_unlock();
 		ret = -ENODEV;
 		goto unbreak;
@@ -143,7 +143,7 @@ static int sysfs_get_link_ksettings(struct device *dev,
 	}
 	netdev_lock_ops(netdev);
 
-	if (!dev_isalive(netdev)) {
+	if (!netif_is_alive(netdev)) {
 		ret = -ENODEV;
 		goto unlock;
 	}
@@ -170,7 +170,7 @@ static ssize_t netdev_show(const struct device *dev,
 	ssize_t ret = -EINVAL;
 
 	rcu_read_lock();
-	if (dev_isalive(ndev))
+	if (netif_is_alive(ndev))
 		ret = (*format)(ndev, buf);
 	rcu_read_unlock();
 
@@ -247,7 +247,7 @@ netdev_lock_store(struct device *dev, struct device_attribute *attr,
 
 	netdev_lock(netdev);
 
-	if (dev_isalive(netdev)) {
+	if (netif_is_alive(netdev)) {
 		ret = (*set)(netdev, new);
 		if (ret == 0)
 			ret = len;
@@ -303,7 +303,7 @@ static ssize_t address_show(struct device *dev, struct device_attribute *attr,
 	down_read(&dev_addr_sem);
 
 	rcu_read_lock();
-	if (dev_isalive(ndev))
+	if (netif_is_alive(ndev))
 		ret = sysfs_format_mac(buf, ndev->dev_addr, ndev->addr_len);
 	rcu_read_unlock();
 
@@ -319,7 +319,7 @@ static ssize_t broadcast_show(struct device *dev,
 	int ret = -EINVAL;
 
 	rcu_read_lock();
-	if (dev_isalive(ndev))
+	if (netif_is_alive(ndev))
 		ret = sysfs_format_mac(buf, ndev->broadcast, ndev->addr_len);
 	rcu_read_unlock();
 	return ret;
@@ -747,7 +747,7 @@ static ssize_t threaded_show(struct device *dev,
 
 	rcu_read_lock();
 
-	if (dev_isalive(netdev))
+	if (netif_is_alive(netdev))
 		ret = sysfs_emit(buf, fmt_dec, READ_ONCE(netdev->threaded));
 
 	rcu_read_unlock();
@@ -824,7 +824,7 @@ static ssize_t netstat_show(const struct device *d,
 		offset % sizeof(u64) != 0);
 
 	rcu_read_lock();
-	if (dev_isalive(dev)) {
+	if (netif_is_alive(dev)) {
 		struct rtnl_link_stats64 temp;
 		const struct rtnl_link_stats64 *stats = dev_get_stats(dev, &temp);
 
diff --git a/net/core/netdev_work.c b/net/core/netdev_work.c
index c99967907e18..1b11bcbeb196 100644
--- a/net/core/netdev_work.c
+++ b/net/core/netdev_work.c
@@ -30,7 +30,7 @@ static void netdev_work_enqueue(struct net_device *dev, unsigned long events,
 		return;
 
 	spin_lock_bh(&netdev_work_lock);
-	if (!dev_isalive(dev)) {
+	if (!netif_is_alive(dev)) {
 		spin_unlock_bh(&netdev_work_lock);
 		return;
 	}
@@ -154,7 +154,7 @@ static void netdev_work_proc(struct work_struct *work)
 			/* We took another ref above */
 			netdev_put(dev, &dev->work_tracker);
 
-			if (!dev_isalive(dev))
+			if (!netif_is_alive(dev))
 				core = events = 0;
 		}
 		spin_unlock_bh(&netdev_work_lock);
diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index e3444fd24061..62b4f18e37b8 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -4356,7 +4356,7 @@ static int rtnl_getlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 retry:
 	if (need_rtnl) {
 		rtnl_lock();
-		if (!dev_isalive(dev)) {
+		if (!netif_is_alive(dev)) {
 			err = -ENODEV;
 			nskb = NULL;
 			goto unlock;
-- 
2.56.0.rc1.315.gc6ed9934b7-goog

[PATCH v3 net-next 4/6] xfrm: Check netif_is_alive() in xfrm_bundle_create() and xfrm_create_dummy_bundle().

[Kuniyuki Iwashima]

struct dst_entry passed to xfrm_bundle_create() and
xfrm_create_dummy_bundle() could outlive synchronize_net() in
unregister_netdevice_many_notify().

For example, ip_route_output_flow() calls __ip_route_output_key()
to fetch a dst under RCU, but calls xfrm_lookup_route() outside of
that RCU critical section.

Thus, xfrm_fill_dst() could add a new uncached route after the
first NETDEV_UNREGISTER notification and rely on the rebroadcast
in netdev_wait_allrefs_any().

The following patches will move the uncached route flush for dying
netdev from the NETDEV_UNREGISTER handler to netdev_run_todo(),
where the rebroadcast is also skipped due to dev->dismantle,
making such late additions problematic.

Let's check netif_is_alive() after fetching dst_dev_rcu(dst)
under RCU.

This ensures that xfrm either finishes adding the uncached
route before synchronize_net() or fails with -ENODEV.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
---
 net/xfrm/xfrm_policy.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
index 513c9f228334..9ea60c7717ff 100644
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -2772,7 +2772,7 @@ static struct dst_entry *xfrm_bundle_create(struct xfrm_policy *policy,
 	err = -ENODEV;
 	rcu_read_lock();
 	dev = dst_dev_rcu(dst);
-	if (!dev) {
+	if (!dev || !netif_is_alive(dev)) {
 		rcu_read_unlock();
 		goto free_dst;
 	}
@@ -3066,7 +3066,7 @@ static struct xfrm_dst *xfrm_create_dummy_bundle(struct net *net,
 	err = -ENODEV;
 	rcu_read_lock();
 	dev = dst_dev_rcu(dst);
-	if (!dev) {
+	if (!dev || !netif_is_alive(dev)) {
 		rcu_read_unlock();
 		goto free_dst;
 	}
-- 
2.56.0.rc1.315.gc6ed9934b7-goog

[PATCH v3 net-next 5/6] ipv4: Batch rt_flush_dev() in netdev_run_todo().

[Kuniyuki Iwashima]

IPv4 uncached routes are linked to the global per-cpu lists,
rt_uncached_list.

When unregistering a netdev, rt_flush_dev() iterates over the
potentially long lists to find uncached routes tied to the device
and swap it with blackhole_netdev.

Since it is called for every device in dying netns under RTNL,
it adds O(N_dev x (N_cpu + N_route)) costs to any batched device
unregistration.

Let's call it once per batched device unregistration without RTNL.

Note that rt_flush_dev() must be called after setting dev->reg_state
to NETREG_UNREGISTERED.  Otherwise, because rt_flush_dev(NULL) runs
without RTNL, it could race with unregister_netdevice_many_notify()
and prematurely purge routes for NETREG_UNREGISTERING dev, for
which flush_all_backlogs() has not been called yet.

Reported-by: Chris J Arges <carges@cloudflare.com>
Closes: https://lore.kernel.org/netdev/20260917-hash-bucket-route-lists-v3-0-30493a37b6eb@cloudflare.com/
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 include/net/route.h |  6 ++++++
 net/core/dev.c      |  4 ++++
 net/ipv4/route.c    | 11 +++++++++--
 3 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/include/net/route.h b/include/net/route.h
index 6b55de2e4df8..3fccd31eb74d 100644
--- a/include/net/route.h
+++ b/include/net/route.h
@@ -129,7 +129,13 @@ struct in_device;
 
 int ip_rt_init(void);
 void rt_cache_flush(struct net *net);
+#ifdef CONFIG_INET
 void rt_flush_dev(struct net_device *dev);
+#else
+static inline void rt_flush_dev(struct net_device *dev)
+{
+}
+#endif
 
 static inline void inet_sk_init_flowi4(const struct inet_sock *inet,
 				       struct flowi4 *fl4)
diff --git a/net/core/dev.c b/net/core/dev.c
index a8eb382f40ca..7dba0292f052 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -129,6 +129,7 @@
 #include <linux/ip.h>
 #include <net/ip.h>
 #include <net/mpls.h>
+#include <net/route.h>
 #include <linux/ipv6.h>
 #include <linux/in.h>
 #include <linux/jhash.h>
@@ -11838,6 +11839,9 @@ void netdev_run_todo(void)
 		linkwatch_sync_dev(dev);
 	}
 
+	if (!list_empty(&list))
+		rt_flush_dev(NULL);
+
 	cnt = 0;
 	while (!list_empty(&list)) {
 		dev = netdev_wait_allrefs_any(&list);
diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index d7da2f1acbb5..1b641901f10d 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1587,6 +1587,9 @@ void rt_flush_dev(struct net_device *dev)
 	struct rtable *rt, *safe;
 	int cpu;
 
+	if (dev && dev->dismantle)
+		return;
+
 	for_each_possible_cpu(cpu) {
 		struct uncached_list *ul = &per_cpu(rt_uncached_list, cpu);
 
@@ -1595,10 +1598,14 @@ void rt_flush_dev(struct net_device *dev)
 
 		spin_lock_bh(&ul->lock);
 		list_for_each_entry_safe(rt, safe, &ul->head, dst.rt_uncached) {
-			if (rt->dst.dev != dev)
+			struct net_device *rt_dev = rt->dst.dev;
+
+			if (dev ? rt_dev != dev :
+			    READ_ONCE(rt_dev->reg_state) != NETREG_UNREGISTERED)
 				continue;
+
 			rcu_assign_pointer(rt->dst.dev_rcu, blackhole_netdev);
-			netdev_ref_replace(dev, blackhole_netdev,
+			netdev_ref_replace(rt_dev, blackhole_netdev,
 					   &rt->dst.dev_tracker, GFP_ATOMIC);
 			list_del_init(&rt->dst.rt_uncached);
 		}
-- 
2.56.0.rc1.315.gc6ed9934b7-goog

[PATCH v3 net-next 6/6] ipv6: Batch rt6_uncached_list_flush_dev() in netdev_run_todo().

[Kuniyuki Iwashima]

IPv6 uncached routes are linked to the global per-cpu lists,
rt6_uncached_list.

When unregistering a netdev, rt6_uncached_list_flush_dev() iterates
over the potentially long lists to find uncached routes tied to
the device and swap it with blackhole_netdev.

Since it is called for every device in dying netns under RTNL,
it adds O(N_dev x (N_cpu + N_route)) costs to any batched device
unregistration.

Let's call it once per batched device unregistration without RTNL.

Because it now runs without RTNL, rt->dst.dev and rt->rt6i_idev->dev
may transition to NETREG_UNREGISTERED at different times.  Thus, we
must check that both devices are blackhole_netdev before unlinking
the route; otherwise, the remaining device reference would not be
flushed when the other device is unregistered.

In addition, since netdev_run_todo() runs even when IPv6 is disabled,
rt6_uncached_list_flush_dev() must check ipv6_mod_enabled() before
touching rt6_uncached_list.

Note also that netdev_run_todo() cannot be called before
rt6_uncached_list is initialised because:

  1. unregister_netdevice_many_notify() panics when
     dev_boot_phase == 1 (net_dev_init() is subsys_initcall()).

  2. the only device registered before fs_initcall_sync() is
     loopback and loopback_net_init() panics in case of failure.

Reported-by: Chris J Arges <carges@cloudflare.com>
Closes: https://lore.kernel.org/netdev/20260917-hash-bucket-route-lists-v3-0-30493a37b6eb@cloudflare.com/
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
v3:
  * Unlink only when both devices are blackhole_netdev
  * Clarify netdev_run_todo() does not run before
    rt6_uncached_list is initialised
  * Fix CONFIG_IPV6=n build
---
 include/net/ip6_route.h |  7 +++++++
 net/core/dev.c          |  5 ++++-
 net/ipv6/route.c        | 24 ++++++++++++++++--------
 3 files changed, 27 insertions(+), 9 deletions(-)

diff --git a/include/net/ip6_route.h b/include/net/ip6_route.h
index 0f9b7a260d25..b9de335b639a 100644
--- a/include/net/ip6_route.h
+++ b/include/net/ip6_route.h
@@ -231,6 +231,13 @@ void rt6_multipath_rebalance(struct fib6_info *f6i);
 
 void rt6_uncached_list_add(struct rt6_info *rt);
 void rt6_uncached_list_del(struct rt6_info *rt);
+#ifdef CONFIG_IPV6
+void rt6_uncached_list_flush_dev(struct net_device *dev);
+#else
+static inline void rt6_uncached_list_flush_dev(struct net_device *dev)
+{
+}
+#endif
 
 static inline const struct rt6_info *skb_rt6_info(const struct sk_buff *skb)
 {
diff --git a/net/core/dev.c b/net/core/dev.c
index 7dba0292f052..12a0e7d6567b 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -131,6 +131,7 @@
 #include <net/mpls.h>
 #include <net/route.h>
 #include <linux/ipv6.h>
+#include <net/ip6_route.h>
 #include <linux/in.h>
 #include <linux/jhash.h>
 #include <linux/random.h>
@@ -11839,8 +11840,10 @@ void netdev_run_todo(void)
 		linkwatch_sync_dev(dev);
 	}
 
-	if (!list_empty(&list))
+	if (!list_empty(&list)) {
 		rt_flush_dev(NULL);
+		rt6_uncached_list_flush_dev(NULL);
+	}
 
 	cnt = 0;
 	while (!list_empty(&list)) {
diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index a76869ff87cd..b1dcb5e5da88 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -158,10 +158,16 @@ void rt6_uncached_list_del(struct rt6_info *rt)
 	}
 }
 
-static void rt6_uncached_list_flush_dev(struct net_device *dev)
+void rt6_uncached_list_flush_dev(struct net_device *dev)
 {
 	int cpu;
 
+	if (!ipv6_mod_enabled())
+		return;
+
+	if (dev && dev->dismantle)
+		return;
+
 	for_each_possible_cpu(cpu) {
 		struct uncached_list *ul = per_cpu_ptr(&rt6_uncached_list, cpu);
 		struct rt6_info *rt, *safe;
@@ -173,22 +179,24 @@ static void rt6_uncached_list_flush_dev(struct net_device *dev)
 		list_for_each_entry_safe(rt, safe, &ul->head, dst.rt_uncached) {
 			struct inet6_dev *rt_idev = rt->rt6i_idev;
 			struct net_device *rt_dev = rt->dst.dev;
-			bool handled = false;
 
-			if (rt_idev && rt_idev->dev == dev) {
+			if (rt_idev &&
+			    (dev ? rt_idev->dev == dev :
+			     READ_ONCE(rt_idev->dev->reg_state) == NETREG_UNREGISTERED)) {
 				rt->rt6i_idev = in6_dev_get(blackhole_netdev);
 				in6_dev_put(rt_idev);
-				handled = true;
 			}
 
-			if (rt_dev == dev) {
-				rt->dst.dev = blackhole_netdev;
+			if (dev ? rt_dev == dev :
+			    READ_ONCE(rt_dev->reg_state) == NETREG_UNREGISTERED) {
+				rcu_assign_pointer(rt->dst.dev_rcu, blackhole_netdev);
 				netdev_ref_replace(rt_dev, blackhole_netdev,
 						   &rt->dst.dev_tracker,
 						   GFP_ATOMIC);
-				handled = true;
 			}
-			if (handled)
+
+			if (rt->dst.dev == blackhole_netdev &&
+			    (!rt->rt6i_idev || rt->rt6i_idev->dev == blackhole_netdev))
 				list_del_init(&rt->dst.rt_uncached);
 		}
 		spin_unlock_bh(&ul->lock);
-- 
2.56.0.rc1.315.gc6ed9934b7-goog

Re: [PATCH v3 net-next 2/6] ipv4: Inline inet_blackhole_dev_init() to devinet_init().

[netdev-bot+sashiko]

Thank you for your contribution! Sashiko AI review found more severe
issue(s) in other patches of this series. This mail is only an FYI, in case
you want to address the nit picks below when you respin.

Low priority / nit picks for your consideration:

- [Low] In net/ipv4/devinet.c, devinet_init() now panics when
  inetdev_init(blackhole_netdev) returns an ERR_PTR.

Pre-existing issues:
- [Low] If alloc_netdev() fails in blackhole_netdev_init()
  (drivers/net/loopback.c, subsys_initcall_sync), the function returns…

--

ipv4: Inline inet_blackhole_dev_init() to devinet_init().

This commit removes the late_initcall inet_blackhole_dev_init(). The
in_device for blackhole_netdev is now allocated directly from
devinet_init(), because blackhole_netdev is allocated before inet_init().

> diff --git a/net/ipv4/devinet.c b/net/ipv4/devinet.c
> index e84d3cf922474..2ed4c706ca9e3 100644
> --- a/net/ipv4/devinet.c
> +++ b/net/ipv4/devinet.c
> @@ -340,18 +340,6 @@ static void inetdev_destroy(struct in_device *in_dev)
>  	in_dev_put(in_dev);
>  }
>  
> -static int __init inet_blackhole_dev_init(void)
> -{
> -	struct in_device *in_dev;
> -
> -	rtnl_lock();
> -	in_dev = inetdev_init(blackhole_netdev);
> -	rtnl_unlock();
> -
> -	return PTR_ERR_OR_ZERO(in_dev);
> -}
> -late_initcall(inet_blackhole_dev_init);
> -

[ ... ]

> @@ -2974,4 +2962,9 @@ void __init devinet_init(void)
>  		panic("Unable to register inet_af_ops\n");
>  
>  	rtnl_register_many(devinet_rtnl_msg_handlers);
> +
> +	rtnl_lock();
> +	if (IS_ERR(inetdev_init(blackhole_netdev)))
> +		panic("IP: failed to allocate in_device for blackhole_netdev\n");

[Severity: Low]
This isn't a bug, but should the commit message say that this failure is
now fatal?

The removed inet_blackhole_dev_init() returned PTR_ERR_OR_ZERO(in_dev).
On an inetdev_init() failure, the initcall core only logged the error,
and boot continued with blackhole_netdev->ip_ptr still NULL. After this
change, the same failure panics.

Panicking here matches how devinet_init() already handles a
rtnl_af_register() failure a few lines above. The only ways this call can
fail are the kzalloc_obj() and neigh_parms_alloc() allocations in
inetdev_init(). Even so, the commit message only says:

    Let's inline inet_blackhole_dev_init() to devinet_init().

Could a sentence be added noting that an allocation failure here now
panics instead of being ignored?

[Severity: Low]
This is a pre-existing issue and was not introduced by this patch, but
what happens if blackhole_netdev is NULL at this point?

drivers/net/loopback.c:blackhole_netdev_init() {
	blackhole_netdev = alloc_netdev(0, "blackhole_dev", NET_NAME_UNKNOWN,
					blackhole_netdev_setup);
	if (!blackhole_netdev)
		return -ENOMEM;
	...
}

The initcall core ignores that -ENOMEM, so blackhole_netdev stays NULL.
inetdev_init() then dereferences it through dev_net(dev) on its first
line. That would oops before the new panic message is printed.

The removed late_initcall had no NULL check either, and addrconf_init()
also calls ipv6_add_dev(blackhole_netdev) without one. This patch just
moves an existing early-boot crash from late_initcall time to fs_initcall
time.

> +	rtnl_unlock();
>  }

-- 
Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20261001204752.2572265-1-kuniyu%40google.com

Re: [PATCH v3 net-next 5/6] ipv4: Batch rt_flush_dev() in netdev_run_todo().

[netdev-bot+sashiko]

Thank you for your contribution! Sashiko AI review found 1 potential
issue(s) to consider.

Critical: 0 · High: 1 · Medium: 0 · Low: 0

If you believe none of the issues are legitimate, please reply to each
issue of Medium severity or higher and explain why the AI is wrong.
If any of them are legitimate and you plan to respin, please let patchwork
know by including "pw-bot: cr" as a separate line at the end of your reply
(one such reply per series is enough).

- [High] Missing RCU grace period between the device-pointer swap in
  rt_flush_dev(NULL) and freeing the net_device.

--

ipv4: Batch rt_flush_dev() in netdev_run_todo().

This patch takes the IPv4 uncached route flush out of the per-device
unregister path. netdev_run_todo() now calls rt_flush_dev(NULL) once per
batch, without RTNL. With a NULL dev, rt_flush_dev() moves every uncached
route whose device has reached NETREG_UNREGISTERED over to
blackhole_netdev.

Closes: https://lore.kernel.org/netdev/20260917-hash-bucket-route-lists-v3-0-30493a37b6eb@cloudflare.com/

> diff --git a/net/core/dev.c b/net/core/dev.c
> index 0f87fd8c68923..4658e4e16d9dd 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c

[ ... ]

> @@ -11871,6 +11872,9 @@ void netdev_run_todo(void)
>  		linkwatch_sync_dev(dev);
>  	}
>  
> +	if (!list_empty(&list))
> +		rt_flush_dev(NULL);
> +

[Severity: High]

Is there still an RCU grace period between the rt->dst.dev swap and
freeing the old net_device?

Before this patch, fib_netdev_event()->rt_flush_dev(dev) did the swap
during NETDEV_UNREGISTER. That was before the second synchronize_net() in
unregister_netdevice_many_notify(), and before the rcu_barrier() at the
top of netdev_run_todo(). So any RCU reader that had already loaded the
old rt->dst.dev was done before the device could be freed.

With this patch, the swap and the reference drop both happen after those
calls:

net/ipv4/route.c:rt_flush_dev() {
    ...
	rcu_assign_pointer(rt->dst.dev_rcu, blackhole_netdev);
	netdev_ref_replace(rt_dev, blackhole_netdev,
			   &rt->dst.dev_tracker, GFP_ATOMIC);
    ...
}

If that drops the last extra reference, netdev_wait_allrefs_any() returns
from its first check, before any rcu_barrier() or msleep():

	list_for_each_entry(dev, list, todo_list)
		if (netdev_refcnt_read(dev) == 1)
			return dev;

netdev_run_todo() then goes straight on to priv_destructor, free_netdev()
and kobject_put(). Some drivers instead call free_netdev() right after
unregister_netdev() returns. Either way, netdev_release() frees the
device without waiting:

	/* no need to wait for rcu grace period:
	 * device is dead and about to be freed.
	 */
	kfree(rcu_access_pointer(dev->ifalias));
	kvfree(dev);

The only synchronize_net() left on this path is in netdev_napi_exit(). It
runs only when dev->napi_list is not empty, so most virtual devices skip
it.

Meanwhile, a sender whose socket has an uncached IPv4 route to this device
in sk_dst_cache can be in sk_dst_check()->ipv4_dst_check()->rt_is_expired():

	rcu_read_lock();
	res = rth->rt_genid != rt_genid_ipv4(dev_net_rcu(rth->dst.dev));
	rcu_read_unlock();

rt_flush_dev() does not mark the dst obsolete, so this check still runs.
If the reader loads the old rth->dst.dev and is then preempted (under
PREEMPT_RCU) or delayed while unregistration finishes, can it read
dev->nd_net from freed memory? Other readers that use dst_dev_rcu() rely
on RCU to keep the device alive in the same way.

Deleting the device needs only CAP_NET_ADMIN in a user namespace's netns
(for example "ip link del"), so an unprivileged user with userns can
reach this path.

The later patch "ipv6: Batch rt6_uncached_list_flush_dev() in
netdev_run_todo()" seems to follow the same pattern for IPv6. At the end
of the series, netdev_run_todo() still calls rt_flush_dev(NULL) and
rt6_uncached_list_flush_dev(NULL) and goes directly into the free loop.

Would a synchronize_net() after the flush calls in netdev_run_todo() be
needed here, or is there something else that guarantees a grace period
before the device is freed?

-- 
Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20261001204752.2572265-1-kuniyu%40google.com

Re: [PATCH v3 net-next 6/6] ipv6: Batch rt6_uncached_list_flush_dev() in netdev_run_todo().

[netdev-bot+sashiko]

Thank you for your contribution! Sashiko AI review found 2 potential
issue(s) to consider.

Critical: 1 · High: 0 · Medium: 1 · Low: 0

If you believe none of the issues are legitimate, please reply to each
issue of Medium severity or higher and explain why the AI is wrong.
If any of them are legitimate and you plan to respin, please let patchwork
know by including "pw-bot: cr" as a separate line at the end of your reply
(one such reply per series is enough).

- [Critical] Use-after-free of the net_device because no RCU grace period
  separates the uncached-route swap from the device free.
- [Medium] Uninitialised per-CPU list access in
  rt6_uncached_list_flush_dev() when IPv6 init failed at boot.

--

ipv6: Batch rt6_uncached_list_flush_dev() in netdev_run_todo().

This commit stops flushing IPv6 uncached routes from the per-device
NETDEV_UNREGISTER path. It now does the flush once per batched
unregistration from netdev_run_todo(), without RTNL. Routes are matched by
NETREG_UNREGISTERED and are unlinked only after both dst.dev and
rt6i_idev->dev point at blackhole_netdev. An ipv6_mod_enabled() check is
added because netdev_run_todo() also runs when IPv6 is disabled.

Closes: https://lore.kernel.org/netdev/20260917-hash-bucket-route-lists-v3-0-30493a37b6eb@cloudflare.com/

> diff --git a/net/core/dev.c b/net/core/dev.c
> index 4658e4e16d9dd..4ce128864b8c8 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c

[ ... ]

> @@ -11872,8 +11873,10 @@ void netdev_run_todo(void)
>  		linkwatch_sync_dev(dev);
>  	}
>  
> -	if (!list_empty(&list))
> +	if (!list_empty(&list)) {
>  		rt_flush_dev(NULL);
> +		rt6_uncached_list_flush_dev(NULL);
> +	}
>  
>  	cnt = 0;
>  	while (!list_empty(&list)) {

[Severity: Critical]
Does this flush need an RCU grace period before the device is freed?

Before this patch, rt6_disable_ip() called rt6_uncached_list_flush_dev(dev)
during NETDEV_UNREGISTER processing. The synchronize_net() in
unregister_netdevice_many_notify() ran after that call. So any RCU reader
that had loaded the old dst.dev was done before the device could be freed.

Now the swap happens after the only rcu_barrier() in netdev_run_todo(), and
the old device reference is dropped right away:

rt6_uncached_list_flush_dev(NULL)
    rcu_assign_pointer(rt->dst.dev_rcu, blackhole_netdev);
    netdev_ref_replace(rt_dev, blackhole_netdev, ...)
        __dev_put(rt_dev)

The in6_dev_put(rt_idev) call can also reach in6_dev_finish_destroy(),
which drops a device reference synchronously.

netdev_wait_allrefs_any() then checks the refcount before any
rcu_barrier() or msleep():

	list_for_each_entry(dev, list, todo_list)
		if (netdev_refcnt_read(dev) == 1)
			return dev;

After that, netdev_run_todo() runs netdev_do_free_pcpu_stats(),
priv_destructor and free_netdev(). The final put reaches netdev_release(),
which calls kvfree(dev) under the comment "no need to wait for rcu grace
period".

Readers such as ip6_finish_output2() use the device under RCU without
taking a reference:

	struct net_device *dev = dst_dev_rcu(dst);
	struct inet6_dev *idev = ip6_dst_idev(dst);
	unsigned int hh_len = LL_RESERVED_SPACE(dev);

Take a sender holding an uncached rt6 on device X. It could come from an
IPV6_HDRINCL raw socket, an xfrm6 bundle, or icmp6_dst_alloc():

CPU1                           CPU0
ip6_finish_output2()
  dev = dst_dev_rcu(dst) /* X */
                               netdev_run_todo()
                                 rt6_uncached_list_flush_dev(NULL)
                                   swap to blackhole, put X
                                 netdev_wait_allrefs_any()
                                   refcnt == 1, return X
                                 ...
                                 kvfree(X)
  LL_RESERVED_SPACE(dev),
  dev_queue_xmit()

Can CPU1 then dereference the freed net_device and its tx queues? This
could happen while CPU1 is preempted inside its RCU section under
PREEMPT_RCU.

The previous patch in this series, "ipv4: Batch rt_flush_dev() in
netdev_run_todo().", seems to do the same for IPv4 uncached routes. Would
a synchronize_net() after the rt_flush_dev(NULL) and
rt6_uncached_list_flush_dev(NULL) calls, before netdev_wait_allrefs_any(),
close this window?

> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> index a76869ff87cd6..b1dcb5e5da88c 100644
> --- a/net/ipv6/route.c
> +++ b/net/ipv6/route.c
> @@ -158,10 +158,16 @@ void rt6_uncached_list_del(struct rt6_info *rt)
>  	}
>  }
>  
> -static void rt6_uncached_list_flush_dev(struct net_device *dev)
> +void rt6_uncached_list_flush_dev(struct net_device *dev)
>  {
>  	int cpu;
>  
> +	if (!ipv6_mod_enabled())
> +		return;
> +

[Severity: Medium]
Is ipv6_mod_enabled() enough to show that rt6_uncached_list was
initialised?

ipv6_mod_enabled() only reflects disable_ipv6_mod. The per-cpu list heads
and locks are set up at the very end of ip6_route_init(), after every step
that can fail:

net/ipv6/route.c:ip6_route_init() {
    ...
	ret = register_netdevice_notifier(&ip6_route_dev_notifier);
	if (ret)
		goto out_register_late_subsys;
    ...
	for_each_possible_cpu(cpu) {
		struct uncached_list *ul = per_cpu_ptr(&rt6_uncached_list, cpu);

		INIT_LIST_HEAD(&ul->head);
		spin_lock_init(&ul->lock);
	}
    ...
}

ip6_route_init() can fail, or inet6_init() can fail before it calls
ip6_route_init(). In either case the inet6_init() error paths leave
disable_ipv6_mod unset. Because CONFIG_IPV6 is bool, boot carries on.

The next device unregistration would then run:

rtnl_unlock()
  netdev_run_todo()
    rt6_uncached_list_flush_dev(NULL)
      list_empty(&ul->head)       /* head->next == NULL, seen as non-empty */
      spin_lock_bh(&ul->lock)     /* lock never initialised */
      list_for_each_entry_safe()  /* loads through NULL head->next */

Could this oops with BH disabled and the per-cpu lock held?

The commit message says:

    Note also that netdev_run_todo() cannot be called before
    rt6_uncached_list is initialised because:

That covers ordering, but not the case where initialisation never
finishes. Before this patch, the function was only reachable from
rt6_disable_ip() through the ip6_route/addrconf notifiers. Those are
registered only after ip6_route_init() has succeeded.

[ ... ]

-- 
Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20261001204752.2572265-1-kuniyu%40google.com

Re: [PATCH v3 net-next 6/6] ipv6: Batch rt6_uncached_list_flush_dev() in netdev_run_todo().

[Kuniyuki Iwashima]

On Sun, Oct 4, 2026 at 4:02 PM <netdev-bot+sashiko@kernel.org> wrote:
>
> Thank you for your contribution! Sashiko AI review found 2 potential
> issue(s) to consider.
>
> Critical: 1 · High: 0 · Medium: 1 · Low: 0
>
> If you believe none of the issues are legitimate, please reply to each
> issue of Medium severity or higher and explain why the AI is wrong.
> If any of them are legitimate and you plan to respin, please let patchwork
> know by including "pw-bot: cr" as a separate line at the end of your reply
> (one such reply per series is enough).
>
> - [Critical] Use-after-free of the net_device because no RCU grace period
>   separates the uncached-route swap from the device free.
> - [Medium] Uninitialised per-CPU list access in
>   rt6_uncached_list_flush_dev() when IPv6 init failed at boot.
>
> --
>
> ipv6: Batch rt6_uncached_list_flush_dev() in netdev_run_todo().
>
> This commit stops flushing IPv6 uncached routes from the per-device
> NETDEV_UNREGISTER path. It now does the flush once per batched
> unregistration from netdev_run_todo(), without RTNL. Routes are matched by
> NETREG_UNREGISTERED and are unlinked only after both dst.dev and
> rt6i_idev->dev point at blackhole_netdev. An ipv6_mod_enabled() check is
> added because netdev_run_todo() also runs when IPv6 is disabled.
>
> Closes: https://lore.kernel.org/netdev/20260917-hash-bucket-route-lists-v3-0-30493a37b6eb@cloudflare.com/
>
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index 4658e4e16d9dd..4ce128864b8c8 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
>
> [ ... ]
>
> > @@ -11872,8 +11873,10 @@ void netdev_run_todo(void)
> >               linkwatch_sync_dev(dev);
> >       }
> >
> > -     if (!list_empty(&list))
> > +     if (!list_empty(&list)) {
> >               rt_flush_dev(NULL);
> > +             rt6_uncached_list_flush_dev(NULL);
> > +     }
> >
> >       cnt = 0;
> >       while (!list_empty(&list)) {
>
> [Severity: Critical]
> Does this flush need an RCU grace period before the device is freed?

No.

ip6_finish_output2() mentioned below should not happen at this
point, and also nexthop does not wait RCU grace period
after dst_dev_put():

After calling fib6_info_release() at NETDEV_UNREGISTER,
synchronize_net() in unregister_netdevice_many_notify()
makes fib6_info_destroy_rcu() schedulable, and dst_dev_put()
is called there, and rcu_barrier() in netdev_run_todo() waits the
callback to finish, but there is not more synchornize_rcu() for
dst_dev_put() before free_netdev().

fib6_info_destroy_rcu() -> nexthop_free_rcu() -> fib6_nh_release()
-> fib6_nh_release_dsts() -> dst_dev_put()


>
> Before this patch, rt6_disable_ip() called rt6_uncached_list_flush_dev(dev)
> during NETDEV_UNREGISTER processing. The synchronize_net() in
> unregister_netdevice_many_notify() ran after that call. So any RCU reader
> that had loaded the old dst.dev was done before the device could be freed.
>
> Now the swap happens after the only rcu_barrier() in netdev_run_todo(), and
> the old device reference is dropped right away:
>
> rt6_uncached_list_flush_dev(NULL)
>     rcu_assign_pointer(rt->dst.dev_rcu, blackhole_netdev);
>     netdev_ref_replace(rt_dev, blackhole_netdev, ...)
>         __dev_put(rt_dev)
>
> The in6_dev_put(rt_idev) call can also reach in6_dev_finish_destroy(),
> which drops a device reference synchronously.
>
> netdev_wait_allrefs_any() then checks the refcount before any
> rcu_barrier() or msleep():
>
>         list_for_each_entry(dev, list, todo_list)
>                 if (netdev_refcnt_read(dev) == 1)
>                         return dev;
>
> After that, netdev_run_todo() runs netdev_do_free_pcpu_stats(),
> priv_destructor and free_netdev(). The final put reaches netdev_release(),
> which calls kvfree(dev) under the comment "no need to wait for rcu grace
> period".
>
> Readers such as ip6_finish_output2() use the device under RCU without
> taking a reference:
>
>         struct net_device *dev = dst_dev_rcu(dst);
>         struct inet6_dev *idev = ip6_dst_idev(dst);
>         unsigned int hh_len = LL_RESERVED_SPACE(dev);
>
> Take a sender holding an uncached rt6 on device X. It could come from an
> IPV6_HDRINCL raw socket, an xfrm6 bundle, or icmp6_dst_alloc():
>
> CPU1                           CPU0
> ip6_finish_output2()
>   dev = dst_dev_rcu(dst) /* X */
>                                netdev_run_todo()
>                                  rt6_uncached_list_flush_dev(NULL)
>                                    swap to blackhole, put X
>                                  netdev_wait_allrefs_any()
>                                    refcnt == 1, return X
>                                  ...
>                                  kvfree(X)
>   LL_RESERVED_SPACE(dev),
>   dev_queue_xmit()
>
> Can CPU1 then dereference the freed net_device and its tx queues? This
> could happen while CPU1 is preempted inside its RCU section under
> PREEMPT_RCU.
>
> The previous patch in this series, "ipv4: Batch rt_flush_dev() in
> netdev_run_todo().", seems to do the same for IPv4 uncached routes. Would
> a synchronize_net() after the rt_flush_dev(NULL) and
> rt6_uncached_list_flush_dev(NULL) calls, before netdev_wait_allrefs_any(),
> close this window?
>
> > diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> > index a76869ff87cd6..b1dcb5e5da88c 100644
> > --- a/net/ipv6/route.c
> > +++ b/net/ipv6/route.c
> > @@ -158,10 +158,16 @@ void rt6_uncached_list_del(struct rt6_info *rt)
> >       }
> >  }
> >
> > -static void rt6_uncached_list_flush_dev(struct net_device *dev)
> > +void rt6_uncached_list_flush_dev(struct net_device *dev)
> >  {
> >       int cpu;
> >
> > +     if (!ipv6_mod_enabled())
> > +             return;
> > +
>
> [Severity: Medium]
> Is ipv6_mod_enabled() enough to show that rt6_uncached_list was
> initialised?

Yes.

Now inet6_init() runs at fs_initcall_sync() during early boot, where
small GFP_KERNEL allocations do not fail.

The error handling is a leftover from when =m was possible.

Also, other ipv6_mod_enabled() users assume the same, if it
returns true, inet6_init() finished successfully.


>
> ipv6_mod_enabled() only reflects disable_ipv6_mod. The per-cpu list heads
> and locks are set up at the very end of ip6_route_init(), after every step
> that can fail:
>
> net/ipv6/route.c:ip6_route_init() {
>     ...
>         ret = register_netdevice_notifier(&ip6_route_dev_notifier);
>         if (ret)
>                 goto out_register_late_subsys;
>     ...
>         for_each_possible_cpu(cpu) {
>                 struct uncached_list *ul = per_cpu_ptr(&rt6_uncached_list, cpu);
>
>                 INIT_LIST_HEAD(&ul->head);
>                 spin_lock_init(&ul->lock);
>         }
>     ...
> }
>
> ip6_route_init() can fail, or inet6_init() can fail before it calls
> ip6_route_init(). In either case the inet6_init() error paths leave
> disable_ipv6_mod unset. Because CONFIG_IPV6 is bool, boot carries on.
>
> The next device unregistration would then run:
>
> rtnl_unlock()
>   netdev_run_todo()
>     rt6_uncached_list_flush_dev(NULL)
>       list_empty(&ul->head)       /* head->next == NULL, seen as non-empty */
>       spin_lock_bh(&ul->lock)     /* lock never initialised */
>       list_for_each_entry_safe()  /* loads through NULL head->next */
>
> Could this oops with BH disabled and the per-cpu lock held?
>
> The commit message says:
>
>     Note also that netdev_run_todo() cannot be called before
>     rt6_uncached_list is initialised because:
>
> That covers ordering, but not the case where initialisation never
> finishes. Before this patch, the function was only reachable from
> rt6_disable_ip() through the ip6_route/addrconf notifiers. Those are
> registered only after ip6_route_init() has succeeded.
>
> [ ... ]
>
> --
> Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20261001204752.2572265-1-kuniyu%40google.com

Re: [PATCH v3 net-next 5/6] ipv4: Batch rt_flush_dev() in netdev_run_todo().

[Kuniyuki Iwashima]

On Sun, Oct 4, 2026 at 4:02 PM <netdev-bot+sashiko@kernel.org> wrote:
>
> Thank you for your contribution! Sashiko AI review found 1 potential
> issue(s) to consider.
>
> Critical: 0 · High: 1 · Medium: 0 · Low: 0
>
> If you believe none of the issues are legitimate, please reply to each
> issue of Medium severity or higher and explain why the AI is wrong.
> If any of them are legitimate and you plan to respin, please let patchwork
> know by including "pw-bot: cr" as a separate line at the end of your reply
> (one such reply per series is enough).
>
> - [High] Missing RCU grace period between the device-pointer swap in
>   rt_flush_dev(NULL) and freeing the net_device.

Same with IPv6 comment.



>
> --
>
> ipv4: Batch rt_flush_dev() in netdev_run_todo().
>
> This patch takes the IPv4 uncached route flush out of the per-device
> unregister path. netdev_run_todo() now calls rt_flush_dev(NULL) once per
> batch, without RTNL. With a NULL dev, rt_flush_dev() moves every uncached
> route whose device has reached NETREG_UNREGISTERED over to
> blackhole_netdev.
>
> Closes: https://lore.kernel.org/netdev/20260917-hash-bucket-route-lists-v3-0-30493a37b6eb@cloudflare.com/
>
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index 0f87fd8c68923..4658e4e16d9dd 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
>
> [ ... ]
>
> > @@ -11871,6 +11872,9 @@ void netdev_run_todo(void)
> >               linkwatch_sync_dev(dev);
> >       }
> >
> > +     if (!list_empty(&list))
> > +             rt_flush_dev(NULL);
> > +
>
> [Severity: High]
>
> Is there still an RCU grace period between the rt->dst.dev swap and
> freeing the old net_device?
>
> Before this patch, fib_netdev_event()->rt_flush_dev(dev) did the swap
> during NETDEV_UNREGISTER. That was before the second synchronize_net() in
> unregister_netdevice_many_notify(), and before the rcu_barrier() at the
> top of netdev_run_todo(). So any RCU reader that had already loaded the
> old rt->dst.dev was done before the device could be freed.
>
> With this patch, the swap and the reference drop both happen after those
> calls:
>
> net/ipv4/route.c:rt_flush_dev() {
>     ...
>         rcu_assign_pointer(rt->dst.dev_rcu, blackhole_netdev);
>         netdev_ref_replace(rt_dev, blackhole_netdev,
>                            &rt->dst.dev_tracker, GFP_ATOMIC);
>     ...
> }
>
> If that drops the last extra reference, netdev_wait_allrefs_any() returns
> from its first check, before any rcu_barrier() or msleep():
>
>         list_for_each_entry(dev, list, todo_list)
>                 if (netdev_refcnt_read(dev) == 1)
>                         return dev;
>
> netdev_run_todo() then goes straight on to priv_destructor, free_netdev()
> and kobject_put(). Some drivers instead call free_netdev() right after
> unregister_netdev() returns. Either way, netdev_release() frees the
> device without waiting:
>
>         /* no need to wait for rcu grace period:
>          * device is dead and about to be freed.
>          */
>         kfree(rcu_access_pointer(dev->ifalias));
>         kvfree(dev);
>
> The only synchronize_net() left on this path is in netdev_napi_exit(). It
> runs only when dev->napi_list is not empty, so most virtual devices skip
> it.
>
> Meanwhile, a sender whose socket has an uncached IPv4 route to this device
> in sk_dst_cache can be in sk_dst_check()->ipv4_dst_check()->rt_is_expired():
>
>         rcu_read_lock();
>         res = rth->rt_genid != rt_genid_ipv4(dev_net_rcu(rth->dst.dev));
>         rcu_read_unlock();
>
> rt_flush_dev() does not mark the dst obsolete, so this check still runs.
> If the reader loads the old rth->dst.dev and is then preempted (under
> PREEMPT_RCU) or delayed while unregistration finishes, can it read
> dev->nd_net from freed memory? Other readers that use dst_dev_rcu() rely
> on RCU to keep the device alive in the same way.
>
> Deleting the device needs only CAP_NET_ADMIN in a user namespace's netns
> (for example "ip link del"), so an unprivileged user with userns can
> reach this path.
>
> The later patch "ipv6: Batch rt6_uncached_list_flush_dev() in
> netdev_run_todo()" seems to follow the same pattern for IPv6. At the end
> of the series, netdev_run_todo() still calls rt_flush_dev(NULL) and
> rt6_uncached_list_flush_dev(NULL) and goes directly into the free loop.
>
> Would a synchronize_net() after the flush calls in netdev_run_todo() be
> needed here, or is there something else that guarantees a grace period
> before the device is freed?
>
> --
> Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20261001204752.2572265-1-kuniyu%40google.com

Re: [PATCH v3 net-next 1/6] net: Order blackhole_netdev_init(), inet_init(), and inet6_init().

[Fernando Fernandez Mancera]

On 10/1/26 10:47 PM, Kuniyuki Iwashima wrote:
> Commit 309b905deee5 ("ipv6: convert CONFIG_IPV6 to built-in
> only and clean up Kconfigs") started to call inet6_init() as
> device_initcall().
> 
> It explains the reason as a race condition on IPv6 addrconf,
> but this is not accurate.
> 
> The change initially used fs_initcall() but switched to
> device_initcall() due to the null-ptr-deref stack trace in
> the link below.
> 
> It happened because blackhole_netdev_init() uses device_initcall()
> and blackhole_netdev was NULL when addrconf_init() called
> ipv6_add_dev(blackhole_netdev).
> 
> Due to the order in Kbuild,
> 
>    $ cat -n Kbuild | grep -E "(drivers|net)"
>       111	obj-y			+= drivers/
>       114	obj-$(CONFIG_NET)	+= net/
> 
> using device_initcall() for IPv6 allows built-in drivers to
> be initialised earlier, which requires unnecessary NULL checks.
> 
> For the same ordering reason, IPv4 uses late_initcall() to
> initialise blackhole_netdev.
> 
> Given loopback_net_ops is registered in net_dev_init() at
> subsys_initcall(), blackhole_netdev_init() can be called
> after that.
> 
> In addition, some places assume that IPv4 must be initialised
> before IPv6.  For example, mptcp_proto_v6_init() copies
> mptcp_prot to mptcp_v6_prot, which would otherwise have NULL
> mptcp_v6_prot.h.hashinfo.
> 
> Let's explicitly order blackhole_netdev_init() -> inet_init()
> -> inet6_init() before device_initcall() with 3 different
> initcall levels:
> 
>    subsys_initcall      : 4  : net_dev_init()
>    subsys_initcall_sync : 4s : blackhole_netdev_init()
>    fs_initcall          : 5  : inet_init()
>    fs_initcall_sync     : 5s : inet6_init()
>    device_initcall      : 6  : built-in drivers
> 
> Note that both IPv4 and IPv6 can still use fs_initcall() thanks
> to the order in net/Makefile, but explicit ordering would be
> less error-prone.
> 
>    $ cat -n net/Makefile | grep ipv
>        17	obj-$(CONFIG_INET)		+= ipv4/
>        22	obj-y				+= ipv6/
> 
> Link: https://lore.kernel.org/netdev/20260309074758.0ea95a18@kernel.org/
> Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>

Reviewed-by: Fernando Fernandez Mancera <fmancera@suse.de>

I have tested this in multiple environments with multiple Kconfigs and 
command line options. Thanks for looking into this Kuniyuki.

> ---
> Cc: Fernando Fernandez Mancera <fmancera@suse.de>
> ---
>   drivers/net/loopback.c | 2 +-
>   net/ipv6/af_inet6.c    | 2 +-
>   2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
> index 1fb6ce6843ad..55ecd5ebac0f 100644
> --- a/drivers/net/loopback.c
> +++ b/drivers/net/loopback.c
> @@ -289,4 +289,4 @@ static int __init blackhole_netdev_init(void)
>   	return 0;
>   }
>   
> -device_initcall(blackhole_netdev_init);
> +subsys_initcall_sync(blackhole_netdev_init);
> diff --git a/net/ipv6/af_inet6.c b/net/ipv6/af_inet6.c
> index f0efdc13baf4..a9b8b4b42a1a 100644
> --- a/net/ipv6/af_inet6.c
> +++ b/net/ipv6/af_inet6.c
> @@ -1217,4 +1217,4 @@ static int __init inet6_init(void)
>   	proto_unregister(&tcpv6_prot);
>   	goto out;
>   }
> -device_initcall(inet6_init);
> +fs_initcall_sync(inet6_init);

Re: [PATCH v3 net-next 0/6] ip: Batch flushing uncached routes per batched device unregistration.

[Ido Schimmel]

On Thu, Oct 01, 2026 at 08:47:12PM +0000, Kuniyuki Iwashima wrote:
> Chris J Arges reported high RTNL contention during cleanup_net()
> caused by rt_flush_dev() and rt6_uncached_list_flush_dev() iterating
> over the global per-cpu uncached route lists for every netdev in
> dying netns [0].
> 
> This series resolves the issue by batching the uncached route
> cleanup after __rtnl_unlock() in netdev_run_todo()
> 
> [0]: https://lore.kernel.org/netdev/20260917-hash-bucket-route-lists-v3-0-30493a37b6eb@cloudflare.com/
> 
> 
> Changelog:
>   v3:
>     * Add patch 1 ~ 4

Reviewed-by: Ido Schimmel <idosch@nvidia.com>

For next time:

The cover letter should explain how the patches fit together. Otherwise
it's up to the reviewer to figure it out. The dependency between patches
1 and 6 is not immediately clear. My understanding:

Patch 1 is needed so that the IPv6 uncached lists are initialized before
they can be traversed for devices that are unregistered due to failed
initialization.

Patch 2 is a cleanup that is made possible by patch 1.

Patches 3-4 are preparations for patches 5-6 that are going to walk the
uncached lists once per batch of unregistered devices. If a dst entry
that points to an unregistered device is put on the list after the walk,
nothing will clean it up after patches 5-6.

Patches 5-6 avoid walking the uncached lists for every device being
unregistered. Instead, walk the lists after one or more devices were
unregistered and delete from the lists entries that point to such
devices.

Re: [PATCH v3 net-next 0/6] ip: Batch flushing uncached routes per batched device unregistration.

[Kuniyuki Iwashima]

On Tue, Oct 6, 2026 at 1:12 AM Ido Schimmel <idosch@nvidia.com> wrote:
>
> On Thu, Oct 01, 2026 at 08:47:12PM +0000, Kuniyuki Iwashima wrote:
> > Chris J Arges reported high RTNL contention during cleanup_net()
> > caused by rt_flush_dev() and rt6_uncached_list_flush_dev() iterating
> > over the global per-cpu uncached route lists for every netdev in
> > dying netns [0].
> >
> > This series resolves the issue by batching the uncached route
> > cleanup after __rtnl_unlock() in netdev_run_todo()
> >
> > [0]: https://lore.kernel.org/netdev/20260917-hash-bucket-route-lists-v3-0-30493a37b6eb@cloudflare.com/
> >
> >
> > Changelog:
> >   v3:
> >     * Add patch 1 ~ 4
>
> Reviewed-by: Ido Schimmel <idosch@nvidia.com>
>
> For next time:
>
> The cover letter should explain how the patches fit together. Otherwise
> it's up to the reviewer to figure it out.

Sorry, I should have added it.

> The dependency between patches
> 1 and 6 is not immediately clear. My understanding:

Yes, correct.

Thanks !


>
> Patch 1 is needed so that the IPv6 uncached lists are initialized before
> they can be traversed for devices that are unregistered due to failed
> initialization.
>
> Patch 2 is a cleanup that is made possible by patch 1.
>
> Patches 3-4 are preparations for patches 5-6 that are going to walk the
> uncached lists once per batch of unregistered devices. If a dst entry
> that points to an unregistered device is put on the list after the walk,
> nothing will clean it up after patches 5-6.
>
> Patches 5-6 avoid walking the uncached lists for every device being
> unregistered. Instead, walk the lists after one or more devices were
> unregistered and delete from the lists entries that point to such
> devices.

Re: [PATCH v3 net-next 0/6] ip: Batch flushing uncached routes per batched device unregistration.

[Chris Arges]

On 2026-10-01 20:47:12, Kuniyuki Iwashima wrote:
> Chris J Arges reported high RTNL contention during cleanup_net()
> caused by rt_flush_dev() and rt6_uncached_list_flush_dev() iterating
> over the global per-cpu uncached route lists for every netdev in
> dying netns [0].
> 
> This series resolves the issue by batching the uncached route
> cleanup after __rtnl_unlock() in netdev_run_todo()
> 
> [0]: https://lore.kernel.org/netdev/20260917-hash-bucket-route-lists-v3-0-30493a37b6eb@cloudflare.com/
> 
> 
> Changelog:
>   v3:
>     * Add patch 1 ~ 4
>     * Patch 6
>       * Unlink routes only when both devices are blackhole_netdev
>       * Clarify netdev_run_todo() does not run before
>         rt6_uncached_list is initialised
>       * Fix CONFIG_IPV6=n build
> 
>   v2: https://lore.kernel.org/netdev/20260929201546.1533268-1-kuniyu@google.com/
>     * Drop ->pre_exit_batch()
>     * Do batching after setting NETDEV_UNREGISTERED in netdev_run_todo()
> 
>   v1: https://lore.kernel.org/netdev/20260927202429.2452589-1-kuniyu@google.com/
> 
> 
> Kuniyuki Iwashima (6):
>   net: Order blackhole_netdev_init(), inet_init(), and inet6_init().
>   ipv4: Inline inet_blackhole_dev_init() to devinet_init().
>   net: Rename dev_isalive() to netif_is_alive().
>   xfrm: Check netif_is_alive() in xfrm_bundle_create() and
>     xfrm_create_dummy_bundle().
>   ipv4: Batch rt_flush_dev() in netdev_run_todo().
>   ipv6: Batch rt6_uncached_list_flush_dev() in netdev_run_todo().
> 
>  drivers/net/loopback.c    |  2 +-
>  include/linux/netdevice.h |  6 ++++++
>  include/net/ip6_route.h   |  7 +++++++
>  include/net/route.h       |  6 ++++++
>  net/core/dev.c            |  7 +++++++
>  net/core/dev.h            |  6 ------
>  net/core/net-sysfs.c      | 16 ++++++++--------
>  net/core/netdev_work.c    |  4 ++--
>  net/core/rtnetlink.c      |  2 +-
>  net/ipv4/devinet.c        | 17 +++++------------
>  net/ipv4/route.c          | 11 +++++++++--
>  net/ipv6/af_inet6.c       |  2 +-
>  net/ipv6/route.c          | 24 ++++++++++++++++--------
>  net/xfrm/xfrm_policy.c    |  4 ++--
>  14 files changed, 71 insertions(+), 43 deletions(-)
> 
> -- 
> 2.56.0.rc1.315.gc6ed9934b7-goog
>

I was able to run the v3 patchset locally and confirm it addresses
the original issue.

Tested-By: Chris J Arges <carges@cloudflare.com>

--chris

Re: [PATCH v3 net-next 0/6] ip: Batch flushing uncached routes per batched device unregistration.

[patchwork-bot+netdevbpf]

Hello:

This series was applied to netdev/net-next.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Thu,  1 Oct 2026 20:47:12 +0000 you wrote:
> Chris J Arges reported high RTNL contention during cleanup_net()
> caused by rt_flush_dev() and rt6_uncached_list_flush_dev() iterating
> over the global per-cpu uncached route lists for every netdev in
> dying netns [0].
> 
> This series resolves the issue by batching the uncached route
> cleanup after __rtnl_unlock() in netdev_run_todo()
> 
> [...]

Here is the summary with links:
  - [v3,net-next,1/6] net: Order blackhole_netdev_init(), inet_init(), and inet6_init().
    https://git.kernel.org/netdev/net-next/c/20dff14006b1
  - [v3,net-next,2/6] ipv4: Inline inet_blackhole_dev_init() to devinet_init().
    https://git.kernel.org/netdev/net-next/c/3d51928a882b
  - [v3,net-next,3/6] net: Rename dev_isalive() to netif_is_alive().
    https://git.kernel.org/netdev/net-next/c/2e65372a2b64
  - [v3,net-next,4/6] xfrm: Check netif_is_alive() in xfrm_bundle_create() and xfrm_create_dummy_bundle().
    https://git.kernel.org/netdev/net-next/c/4eb2e2f358fc
  - [v3,net-next,5/6] ipv4: Batch rt_flush_dev() in netdev_run_todo().
    https://git.kernel.org/netdev/net-next/c/ca6854799069
  - [v3,net-next,6/6] ipv6: Batch rt6_uncached_list_flush_dev() in netdev_run_todo().
    https://git.kernel.org/netdev/net-next/c/f12d2907992d

You are awesome, thank you!
-- 
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