netdev - [PATCH net-next] ip6mr: remove synchronize_rcu() in favor of SOCK_RCU

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Date:   Wed, 07 Mar 2018 08:43:19 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Yuval Mintz <yuvalm@...lanox.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     "davem@...emloft.net" <davem@...emloft.net>,
        "yoshfuji@...ux-ipv6.org" <yoshfuji@...ux-ipv6.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: [PATCH net-next] ip6mr: remove synchronize_rcu() in favor of
 SOCK_RCU_FREE

From: Eric Dumazet <edumazet@...gle.com>

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@...gle.com>
Reported-by: Kirill Tkhai <ktkhai@...tuozzo.com>
Cc: Yuval Mintz <yuvalm@...lanox.com>
---
 net/ipv6/ip6mr.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/ip6mr.c b/net/ipv6/ip6mr.c
index 2a38f9de45d399fafc9f4bcc662b44be17279e51..7345bd6c4b7dda39c0d73d542e9ca9a5366542ff 100644
--- a/net/ipv6/ip6mr.c
+++ b/net/ipv6/ip6mr.c
@@ -1443,6 +1443,7 @@ static int ip6mr_sk_init(struct mr_table *mrt, struct sock *sk)
 		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);
@@ -1472,6 +1473,10 @@ 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,
@@ -1485,7 +1490,6 @@ int ip6mr_sk_done(struct sock *sk)
 		}
 	}
 	rtnl_unlock();
-	synchronize_rcu();
 
 	return err;
 }