Subject: [PATCH] IPv6: Fix CPU contention on FIB6 GC

From: Oliver Herms <[email-masked]>
Date: Mon, 22 Jun 2020 22:53:55 +0200

When fib6_run_gc is called with parameter force=true the spinlock in
/net/ipv6/ip6_fib.c:2310 can lock all CPUs in softirq when
net.ipv6.route.max_size is exceeded (seen this multiple times).
One sotirq/CPU get's the lock. All others spin to get it. It takes
substantial time until all are done. Effectively it's a DOS vector.

As the splinlock is only enforcing that there is at most one GC running
at a time, it should IMHO be safe to use force=false here resulting
in spin_trylock_bh instead of spin_lock_bh, thus avoiding the lock
contention.

Finding a locked spinlock means some GC is going on already so it is
save to just skip another execution of the GC.

Signed-off-by: Oliver Herms <[email-masked]>
---
 net/ipv6/route.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 82cbb46a2a4f..7e6fbaf43549 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -3205,7 +3205,7 @@ static int ip6_dst_gc(struct dst_ops *ops)
 		goto out;
 
 	net->ipv6.ip6_rt_gc_expire++;
-	fib6_run_gc(net->ipv6.ip6_rt_gc_expire, net, true);
+	fib6_run_gc(net->ipv6.ip6_rt_gc_expire, net, false);
 	entries = dst_entries_get_slow(ops);
 	if (entries < ops->gc_thresh)
 		net->ipv6.ip6_rt_gc_expire = rt_gc_timeout>>1;
-- 
2.25.1