[PATCH net 1/2] ipv6: fix possible infinite loop in rt6_fill_node()

[Jiayuan Chen <jiayuan.chen@linux.dev>]

Sashiko reported this issue [1]. Apply the same fix as
commit f8d8ce1b515a ("ipv6: fix possible infinite loop in fib6_info_uses_dev()").

Writers holding tb6_lock can list_del_rcu(&rt->fib6_siblings)
without waiting for RCU readers; rt->fib6_siblings.next then still
points into the old ring and this softirq-side walker never reaches
&rt->fib6_siblings, causing a CPU stall. fib6_purge_rt() always
WRITE_ONCE()s rt->fib6_nsiblings to 0 before list_del_rcu(), so an
inside-loop check is a reliable detach signal.

[1] https://sashiko.dev/#/patchset/20260526020227.4857-1-jiayuan.chen%40linux.dev

Fixes: d9ccb18f83ea ("ipv6: Fix soft lockups in fib6_select_path under high next hop churn")
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 net/ipv6/route.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index b106e5fef9cb..dad416fdc585 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -5902,6 +5902,8 @@ static int rt6_fill_node(struct net *net, struct sk_buff *skb,
 
 				goto nla_put_failure;
 			}
+			if (!READ_ONCE(rt->fib6_nsiblings))
+				break;
 		}
 
 		rcu_read_unlock();
-- 
2.43.0