[PATCH v1 net-next 15/15] udp_tunnel: Remove synchronize_rcu() in udp_tunnel_sock_release().

[Kuniyuki Iwashima <kuniyu@google.com>]

Commit 3cf7203ca620 ("net/tunnel: wait until all sk_user_data
reader finish before releasing the sock") added synchronize_rcu()
in udp_tunnel_sock_release().

This was intended to protect the fast path of a dying vxlan device
from dereferencing vxlan_sock->sock->sk after sock_orphan() has set
sock->sk to NULL.

However, vxlan does not need to access struct socket itself
in the fast path; it only reads struct sock, and struct socket
is only used for tunnel setup and teardown.

This applies to all other UDP tunnel users, and they have been
converted to access struct sock directly.

In addition, each device-specific struct used in their fast paths
is freed after one RCU grace period.  Since this occurs after
udp_tunnel_sock_release(), the struct is guaranteed to be freed
after struct udp_sock.

Therefore, synchronize_rcu() in udp_tunnel_sock_release() is
now redundant.

Let's remove it.

Tested:

A script creating/upping vxlan devices in 4000 netns runs 10x
faster with this change.  We can see the same improvement with
other UDP tunnel devices as well.

  $ cat vxlan.sh
  for i in `seq 1 40`
  do
      (for j in `seq 1 100` ; do
            unshare -n bash -c "ip link add vxlan0 type vxlan id 100 local 127.0.0.1 dstport 4789 && ip link set vxlan0 up";
       done) &
  done
  wait

With bpftrace, we can see vxlan_stop() is significantly faster.

  bpftrace -e '
  kprobe:vxlan_stop {
          @start[tid] = nsecs;
  }

  kretprobe:vxlan_stop /@start[tid]/ {
          @duration_us = hist((nsecs - @start[tid]) / 1000);
          delete(@start[tid]);
  }

  END {
          printf("\nExecution time of vxlan_stop (us):\n");
  }'

Before:

  # time ./vxlan.sh // without bpftrace
  real	0m50.615s
  user	0m8.171s
  sys	1m45.101s

  @duration_us:
  [4K, 8K)            1266 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@                   |
  [8K, 16K)           1957 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
  [16K, 32K)           764 |@@@@@@@@@@@@@@@@@@@@                                |
  [32K, 64K)             6 |                                                    |
  [64K, 128K)            4 |                                                    |
  [128K, 256K)           3 |                                                    |

After:

  # time ./vxlan.sh // without bpftrace
  real	0m5.247s
  user	0m7.956s
  sys	1m47.404s

  @duration_us:
  [16, 32)            3411 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
  [32, 64)             383 |@@@@@                                               |
  [64, 128)            107 |@                                                   |
  [128, 256)            79 |@                                                   |
  [256, 512)            16 |                                                    |
  [512, 1K)              2 |                                                    |
  [1K, 2K)               2 |                                                    |

Next step is to remove another synchronize_net() in vxlan_stop()
and variants in other devices.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 net/ipv4/udp_tunnel_core.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/net/ipv4/udp_tunnel_core.c b/net/ipv4/udp_tunnel_core.c
index 44788b95c823..9ab3728f9630 100644
--- a/net/ipv4/udp_tunnel_core.c
+++ b/net/ipv4/udp_tunnel_core.c
@@ -194,7 +194,6 @@ void udp_tunnel_sock_release(struct sock *sk)
 	struct socket *sock = sk->sk_socket;
 
 	rcu_assign_sk_user_data(sk, NULL);
-	synchronize_rcu();
 	kernel_sock_shutdown(sock, SHUT_RDWR);
 	sock_release(sock);
 }
-- 
2.54.0.545.g6539524ca2-goog