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 <[email protected]> Signed-off-by: David S. Miller <[email protected]>
huntrar · Apr 21, 2015 · 3c71512 · 3c71512
1 parent 71cd26e
commit 3c71512
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diff --git a/net/ipv4/tcp.c b/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;
 			}

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
@@ -4845,6 +4845,8 @@ static void tcp_check_space(struct sock *sk)
 {
 	if (sock_flag(sk, SOCK_QUEUE_SHRUNK)) {
 		sock_reset_flag(sk, SOCK_QUEUE_SHRUNK);
+		/* pairs with tcp_poll() */
+		smp_mb__after_atomic();
 		if (sk->sk_socket &&
 		    test_bit(SOCK_NOSPACE, &sk->sk_socket->flags))
 			tcp_new_space(sk);