net: sk_dst_cache RCUification

With latest CONFIG_PROVE_RCU stuff, I felt more comfortable to make this
work.

sk->sk_dst_cache is currently protected by a rwlock (sk_dst_lock)

This rwlock is readlocked for a very small amount of time, and dst
entries are already freed after RCU grace period. This calls for RCU
again :)

This patch converts sk_dst_lock to a spinlock, and use RCU for readers.

__sk_dst_get() is supposed to be called with rcu_read_lock() or if
socket locked by user, so use appropriate rcu_dereference_check()
condition (rcu_read_lock_held() || sock_owned_by_user(sk))

This patch avoids two atomic ops per tx packet on UDP connected sockets,
for example, and permits sk_dst_lock to be much less dirtied.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 4 deletions

diff --git a/net/core/sock.c b/net/core/sock.c
index c5812bbc2cc9..7effa1e689df 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -364,11 +364,11 @@ EXPORT_SYMBOL(sk_reset_txq);
 
 struct dst_entry *__sk_dst_check(struct sock *sk, u32 cookie)
 {
-        struct dst_entry *dst = sk->sk_dst_cache;
+        struct dst_entry *dst = __sk_dst_get(sk);
 
         if (dst && dst->obsolete && dst->ops->check(dst, cookie) == NULL) {
                 sk_tx_queue_clear(sk);
-                sk->sk_dst_cache = NULL;
+                rcu_assign_pointer(sk->sk_dst_cache, NULL);
                 dst_release(dst);
                 return NULL;
         }
@@ -1157,7 +1157,7 @@ struct sock *sk_clone(const struct sock *sk, const gfp_t priority)
                 skb_queue_head_init(&newsk->sk_async_wait_queue);
 #endif
 
-                rwlock_init(&newsk->sk_dst_lock);
+                spin_lock_init(&newsk->sk_dst_lock);
                 rwlock_init(&newsk->sk_callback_lock);
                 lockdep_set_class_and_name(&newsk->sk_callback_lock,
                                 af_callback_keys + newsk->sk_family,
@@ -1898,7 +1898,7 @@ void sock_init_data(struct socket *sock, struct sock *sk)
         } else
                 sk->sk_sleep    =       NULL;
 
-        rwlock_init(&sk->sk_dst_lock);
+        spin_lock_init(&sk->sk_dst_lock);
         rwlock_init(&sk->sk_callback_lock);
         lockdep_set_class_and_name(&sk->sk_callback_lock,
                         af_callback_keys + sk->sk_family,