tcp: add memory barriers to write space paths

Ensure that we either see that the buffer has write space
in tcp_poll() or that we perform a wakeup from the input
side. Did not run into any actual problem here, but thought
that we should make things explicit.

Signed-off-by: Jason Baron <jbaron@akamai.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -520,8 +520,10 @@ unsigned int tcp_poll(struct file *file, struct socket *sock, poll_table *wait)
 
 				/* Race breaker. If space is freed after
 				 * wspace test but before the flags are set,
-				 * IO signal will be lost.
+				 * IO signal will be lost. Memory barrier
+				 * pairs with the input side.
 				 */
+				smp_mb__after_atomic();
 				if (sk_stream_is_writeable(sk))
 					mask |= POLLOUT | POLLWRNORM;
 			}