tipc: fix race condition causing hung sendto

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>