[tipc-discussion] send_msg() may not be waken up by tipc_recv_msg() when port congestion happen

[jason <huz...@gm...>]

Hi All,

When I studying tipc-1.7.6 code. I see a potaintial issue that when a user
thread which calls send_msg() and is scheduled out because port congestion,
it may not be wake up any more by tipc_recv_msg().

Let's see send_msg related pseudo code:
01 add self to link.sleep_queue
02 if port.congest == 1 {
03      schedule();
04 }

and see tipc_recv_msg related code:

01 if link.sleep_queue empty {
02     port.congest = 0;
03     wakeup;
04 }
Say we are SMP. Right after CPU0 runs at tipc_recv_msg related code line 01
and see link.sleep_queue IS empty, CPU1 execute  send_msg related code from
line 01 to 04. In this conditon, send_msg will be schedule out by CPU1 and
CPU0 loss the opptunity to wake it up. If there is no incoming message any
more, then send_msg thread will not be waken any more.

If my analysis is right, I think one way to solve whis issue is to swap
tipc_recv_msg related code line 01 and 02, and smp_wmb should also be placed
between them.  But real code change is not as easy as a code line swapping.

Thanks!

-- 
Yours,
jason