net: adding memory barrier to the poll and receive callbacks

Adding memory barrier after the poll_wait function, paired with
receive callbacks. Adding fuctions sock_poll_wait and sk_has_sleeper
to wrap the memory barrier.

Without the memory barrier, following race can happen.
The race fires, when following code paths meet, and the tp->rcv_nxt
and __add_wait_queue updates stay in CPU caches.

CPU1                         CPU2

sys_select                   receive packet
  ...                        ...
  __add_wait_queue           update tp->rcv_nxt
  ...                        ...
  tp->rcv_nxt check          sock_def_readable
  ...                        {
  schedule                      ...
                                if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
                                        wake_up_interruptible(sk->sk_sleep)
                                ...
                             }

If there was no cache the code would work ok, since the wait_queue and
rcv_nxt are opposit to each other.

Meaning that once tp->rcv_nxt is updated by CPU2, the CPU1 either already
passed the tp->rcv_nxt check and sleeps, or will get the new value for
tp->rcv_nxt and will return with new data mask.
In both cases the process (CPU1) is being added to the wait queue, so the
waitqueue_active (CPU2) call cannot miss and will wake up CPU1.

The bad case is when the __add_wait_queue changes done by CPU1 stay in its
cache, and so does the tp->rcv_nxt update on CPU2 side.  The CPU1 will then
endup calling schedule and sleep forever if there are no more data on the
socket.

Calls to poll_wait in following modules were ommited:
	net/bluetooth/af_bluetooth.c
	net/irda/af_irda.c
	net/irda/irnet/irnet_ppp.c
	net/mac80211/rc80211_pid_debugfs.c
	net/phonet/socket.c
	net/rds/af_rds.c
	net/rfkill/core.c
	net/sunrpc/cache.c
	net/sunrpc/rpc_pipe.c
	net/tipc/socket.c

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 2 insertions, 2 deletions

diff --git a/net/dccp/output.c b/net/dccp/output.c
index c0e88c1..c96119f 100644
--- a/net/dccp/output.c
+++ b/net/dccp/output.c
@@ -196,7 +196,7 @@ void dccp_write_space(struct sock *sk)
 {
 	read_lock(&sk->sk_callback_lock);
 
-	if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
+	if (sk_has_sleeper(sk))
 		wake_up_interruptible(sk->sk_sleep);
 	/* Should agree with poll, otherwise some programs break */
 	if (sock_writeable(sk))
diff --git a/net/dccp/proto.c b/net/dccp/proto.c
index 314a1b5..94ca8ea 100644
--- a/net/dccp/proto.c
+++ b/net/dccp/proto.c
@@ -311,7 +311,7 @@ unsigned int dccp_poll(struct file *file, struct socket *sock,
 	unsigned int mask;
 	struct sock *sk = sock->sk;
 
-	poll_wait(file, sk->sk_sleep, wait);
+	sock_poll_wait(file, sk->sk_sleep, wait);
 	if (sk->sk_state == DCCP_LISTEN)
 		return inet_csk_listen_poll(sk);