[PATCH RESEND net-next] net: Do synchronize_rcu() in ip6mr_sk_done() only if this is needed

[Kirill Tkhai]

After unshare test kworker hangs for ages:

    $ while :; do unshare -n true; done &

    $ perf report <kworker>
    -   88,82%     0,00%  kworker/u16:0  [kernel.vmlinux]  [k] cleanup_net
         cleanup_net
       - ops_exit_list.isra.9
          - 85,68% igmp6_net_exit
             - 53,31% sock_release
                - inet_release
                   - 25,33% rawv6_close
                      - ip6mr_sk_done
                         + 23,38% synchronize_rcu

Keep in mind, this perf report shows the time a function was executing, and
it does not show the time, it was sleeping. But it's easy to imagine, how
much it is sleeping, if synchronize_rcu() execution takes the most time.
Top shows the kworker R time is less then 1%.

This happen, because of in ip6mr_sk_done() we do too many synchronize_rcu(),
even for the sockets, that are not referenced in mr_table, and which are not
need it. So, the progress of kworker becomes very slow.

The patch introduces apparent solution, and it makes ip6mr_sk_done() to skip
synchronize_rcu() for sockets, that are not need that. After the patch,
kworker becomes able to warm the cpu up as expected.

Signed-off-by: Kirill Tkhai <ktk...@virtuozzo.com>
---
 net/ipv6/ip6mr.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/ip6mr.c b/net/ipv6/ip6mr.c
index 2a38f9de45d3..290a8d0d5eac 100644
--- a/net/ipv6/ip6mr.c
+++ b/net/ipv6/ip6mr.c
@@ -1485,7 +1485,9 @@ int ip6mr_sk_done(struct sock *sk)
                }
        }
        rtnl_unlock();
-       synchronize_rcu();
+
+       if (!err)
+               synchronize_rcu();
 
        return err;
 }