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>

1 files changed, 4 insertions, 4 deletions

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 36d4e44d6233..fc3ebb906911 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -315,7 +315,7 @@ static void unix_write_space(struct sock *sk)
 {
         read_lock(&sk->sk_callback_lock);
         if (unix_writable(sk)) {
-                if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
+                if (sk_has_sleeper(sk))
                         wake_up_interruptible_sync(sk->sk_sleep);
                 sk_wake_async(sk, SOCK_WAKE_SPACE, POLL_OUT);
         }
@@ -1985,7 +1985,7 @@ static unsigned int unix_poll(struct file *file, struct socket *sock, poll_table
         struct sock *sk = sock->sk;
         unsigned int mask;
 
-        poll_wait(file, sk->sk_sleep, wait);
+        sock_poll_wait(file, sk->sk_sleep, wait);
         mask = 0;
 
         /* exceptional events? */
@@ -2022,7 +2022,7 @@ static unsigned int unix_dgram_poll(struct file *file, struct socket *sock,
         struct sock *sk = sock->sk, *other;
         unsigned int mask, writable;
 
-        poll_wait(file, sk->sk_sleep, wait);
+        sock_poll_wait(file, sk->sk_sleep, wait);
         mask = 0;
 
         /* exceptional events? */
@@ -2053,7 +2053,7 @@ static unsigned int unix_dgram_poll(struct file *file, struct socket *sock,
                 other = unix_peer_get(sk);
                 if (other) {
                         if (unix_peer(other) != sk) {
-                                poll_wait(file, &unix_sk(other)->peer_wait,
+                                sock_poll_wait(file, &unix_sk(other)->peer_wait,
                                           wait);
                                 if (unix_recvq_full(other))
                                         writable = 0;