tipc: fix race condition causing hung sendto

When sending multicast messages via blocking socket,
if sending link is congested (tsk->cong_link_cnt is set to 1),
the sending thread will be put into sleeping state. However,
tipc_sk_filter_rcv() is called under socket spin lock but
tipc_wait_for_cond() is not. So, there is no guarantee that
the setting of tsk->cong_link_cnt to 0 in tipc_sk_proto_rcv() in
CPU-1 will be perceived by CPU-0. If that is the case, the sending
thread in CPU-0 after being waken up, will continue to see
tsk->cong_link_cnt as 1 and put the sending thread into sleeping
state again. The sending thread will sleep forever.

CPU-0                                | CPU-1
tipc_wait_for_cond()                 |
{                                    |
 // condition_ = !tsk->cong_link_cnt |
 while ((rc_ = !(condition_))) {     |
  ...                                |
  release_sock(sk_);                 |
  wait_woken();                      |
                                     | if (!sock_owned_by_user(sk))
                                     |  tipc_sk_filter_rcv()
                                     |  {
                                     |   ...
                                     |   tipc_sk_proto_rcv()
                                     |   {
                                     |    ...
                                     |    tsk->cong_link_cnt--;
                                     |    ...
                                     |    sk->sk_write_space(sk);
                                     |    ...
                                     |   }
                                     |   ...
                                     |  }
  sched_annotate_sleep();            |
  lock_sock(sk_);                    |
  remove_wait_queue();               |
 }                                   |
}                                    |

This commit fixes it by adding memory barrier to tipc_sk_proto_rcv()
and tipc_wait_for_cond().
Acked-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: Tung Nguyen <tung.q.nguyen@dektech.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tipc/socket.c

@@ -379,11 +379,13 @@ static int tipc_sk_sock_err(struct socket *sock, long *timeout)
 
 #define tipc_wait_for_cond(sock_, timeo_, condition_)			       \
 ({                                                                             \
+	DEFINE_WAIT_FUNC(wait_, woken_wake_function);                          \
 	struct sock *sk_;						       \
 	int rc_;							       \
 									       \
 	while ((rc_ = !(condition_))) {					       \
-		DEFINE_WAIT_FUNC(wait_, woken_wake_function);	               \
+		/* coupled with smp_wmb() in tipc_sk_proto_rcv() */            \
+		smp_rmb();                                                     \
 		sk_ = (sock_)->sk;					       \
 		rc_ = tipc_sk_sock_err((sock_), timeo_);		       \
 		if (rc_)						       \
@@ -1983,6 +1985,8 @@ static void tipc_sk_proto_rcv(struct sock *sk,
 		return;
 	case SOCK_WAKEUP:
 		tipc_dest_del(&tsk->cong_links, msg_orignode(hdr), 0);
+		/* coupled with smp_rmb() in tipc_wait_for_cond() */
+		smp_wmb();
 		tsk->cong_link_cnt--;
 		wakeup = true;
 		break;