From: Eric Dumazet <edumazet@google.com>
Date: Wed, 7 Mar 2018 08:43:19 -0800
Subject: ip6mr: remove synchronize_rcu() in favor of SOCK_RCU_FREE
Patch-mainline: v4.17-rc1
Git-commit: a366e300ae9fc466d333e6d8f2bc5d58ed248041
References: bsc#1112374

Kirill found that recently added synchronize_rcu() call in
ip6mr_sk_done()
was slowing down netns dismantle and posted a patch to use it only if
the socket
was found.

I instead suggested to get rid of this call, and use instead
SOCK_RCU_FREE

We might later change IPv4 side to use the same technique and unify
both stacks. IPv4 does not use synchronize_rcu() but has a call_rcu()
that could be replaced by SOCK_RCU_FREE.

Tested:
 time for i in {1..1000}; do unshare -n /bin/false;done

 Before : real 7m18.911s
 After : real 10.187s

Fixes: 8571ab479a6e ("ip6mr: Make mroute_sk rcu-based")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Cc: Yuval Mintz <yuvalm@mellanox.com>
Reviewed-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
---
 net/ipv6/ip6mr.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

--- a/net/ipv6/ip6mr.c
+++ b/net/ipv6/ip6mr.c
@@ -1589,6 +1589,7 @@ static int ip6mr_sk_init(struct mr_table
 		err = -EADDRINUSE;
 	} else {
 		rcu_assign_pointer(mrt->mroute_sk, sk);
+		sock_set_flag(sk, SOCK_RCU_FREE);
 		net->ipv6.devconf_all->mc_forwarding++;
 	}
 	write_unlock_bh(&mrt_lock);
@@ -1614,6 +1615,10 @@ int ip6mr_sk_done(struct sock *sk)
 		if (sk == rtnl_dereference(mrt->mroute_sk)) {
 			write_lock_bh(&mrt_lock);
 			RCU_INIT_POINTER(mrt->mroute_sk, NULL);
+			/* Note that mroute_sk had SOCK_RCU_FREE set,
+			 * so the RCU grace period before sk freeing
+			 * is guaranteed by sk_destruct()
+			 */
 			net->ipv6.devconf_all->mc_forwarding--;
 			write_unlock_bh(&mrt_lock);
 			inet6_netconf_notify_devconf(net, RTM_NEWNETCONF,
@@ -1627,7 +1632,6 @@ int ip6mr_sk_done(struct sock *sk)
 		}
 	}
 	rtnl_unlock();
-	synchronize_rcu();
 
 	return err;
 }