sock: optimise sock_def_write_space barriers

Now we have a separate path for sock_def_write_space() and can go one
step further. When it's called from sock_wfree() we know that there is a
preceding atomic for putting down ->sk_wmem_alloc. We can use it to
replace to replace smb_mb() with a less expensive
smp_mb__after_atomic(). It also removes an extra RCU read lock/unlock as
a small bonus.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/sock.c

@@ -146,6 +146,7 @@
 static DEFINE_MUTEX(proto_list_mutex);
 static LIST_HEAD(proto_list);
 
+static void sock_def_write_space_wfree(struct sock *sk);
 static void sock_def_write_space(struct sock *sk);
 
 /**
@@ -2333,7 +2334,7 @@ void sock_wfree(struct sk_buff *skb)
 		    sk->sk_write_space == sock_def_write_space) {
 			rcu_read_lock();
 			free = refcount_sub_and_test(len, &sk->sk_wmem_alloc);
-			sock_def_write_space(sk);
+			sock_def_write_space_wfree(sk);
 			rcu_read_unlock();
 			if (unlikely(free))
 				__sk_free(sk);
@@ -3218,6 +3219,29 @@ static void sock_def_write_space(struct sock *sk)
 	rcu_read_unlock();
 }
 
+/* An optimised version of sock_def_write_space(), should only be called
+ * for SOCK_RCU_FREE sockets under RCU read section and after putting
+ * ->sk_wmem_alloc.
+ */
+static void sock_def_write_space_wfree(struct sock *sk)
+{
+	/* Do not wake up a writer until he can make "significant"
+	 * progress.  --DaveM
+	 */
+	if (sock_writeable(sk)) {
+		struct socket_wq *wq = rcu_dereference(sk->sk_wq);
+
+		/* rely on refcount_sub from sock_wfree() */
+		smp_mb__after_atomic();
+		if (wq && waitqueue_active(&wq->wait))
+			wake_up_interruptible_sync_poll(&wq->wait, EPOLLOUT |
+						EPOLLWRNORM | EPOLLWRBAND);
+
+		/* Should agree with poll, otherwise some programs break */
+		sk_wake_async(sk, SOCK_WAKE_SPACE, POLL_OUT);
+	}
+}
+
 static void sock_def_destruct(struct sock *sk)
 {
 }