[PATCH net-next] tcp: make smp_rmb() conditional in tcp_poll()

[Eric Dumazet <edumazet@google.com>]

Commit a4d258036ed9 ("tcp: Fix race in tcp_poll") added smp_rmb() in
tcp_poll() and smp_wmb() in tcp_reset() (now tcp_done_with_error())
to ensure that if tcp_poll() observed socket closure, it would also
observe sk->sk_err.

Currently, tcp_poll() unconditionally executes smp_rmb() at the end
of every invocation, which on weakly-ordered architectures such as ARM64
emits a memory barrier instruction (dmb ishld) on the poll fast path,
even for healthy, active sockets.

However, tcp_poll() only needs this barrier if socket closure has been
observed, to ensure that the error code set by tcp_done_with_error()
before socket closure is visible before returning EPOLLERR.

Move smp_rmb() inside the conditional block handling socket closure
(shutdown == SHUTDOWN_MASK || state == TCP_CLOSE). For healthy
connected sockets in epoll, tcp_poll() avoids the barrier entirely.

Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/ipv4/tcp.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 562752352afe4d7ab864c00b8562c8b8489a76f9..3ac4856852794736c5d49f042ecd08e4246bdd6d 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -578,8 +578,13 @@ __poll_t tcp_poll(struct file *file, struct socket *sock, poll_table *wait)
 	 * blocking on fresh not-connected or disconnected socket. --ANK
 	 */
 	shutdown = READ_ONCE(sk->sk_shutdown);
-	if (shutdown == SHUTDOWN_MASK || state == TCP_CLOSE)
+	if (shutdown == SHUTDOWN_MASK || state == TCP_CLOSE) {
 		mask |= EPOLLHUP;
+		/* Coupled with smp_wmb() in tcp_done_with_error() to ensure
+		 * sk->sk_err is visible if socket closure was observed.
+		 */
+		smp_rmb();
+	}
 	if (shutdown & RCV_SHUTDOWN)
 		mask |= EPOLLIN | EPOLLRDNORM | EPOLLRDHUP;
 
@@ -626,8 +631,6 @@ __poll_t tcp_poll(struct file *file, struct socket *sock, poll_table *wait)
 		 */
 		mask |= EPOLLOUT | EPOLLWRNORM;
 	}
-	/* This barrier is coupled with smp_wmb() in tcp_done_with_error() */
-	smp_rmb();
 	if (READ_ONCE(sk->sk_err) ||
 	    !skb_queue_empty_lockless(&sk->sk_error_queue))
 		mask |= EPOLLERR;
-- 
2.55.0.1007.g17ff1f9808-goog