netdev - [PATCH] tcp: add memory barriers to write space paths

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Message-Id: <20150420200507.21414203F@prod-mail-relay06.akamai.com>
Date:	Mon, 20 Apr 2015 20:05:07 +0000 (GMT)
From:	Jason Baron <jbaron@...mai.com>
To:	davem@...emloft.net
Cc:	netdev@...r.kernel.org, eric.dumazet@...il.com
Subject: [PATCH] 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@...mai.com>
---
 net/ipv4/tcp.c       | 4 +++-
 net/ipv4/tcp_input.c | 2 ++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 094a682..726341e 100644
--- 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
index 031cf72..63b3d4e9 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -4847,6 +4847,8 @@ 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);
-- 
1.8.2.rc2

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