Scheduler: Spinning until tasks are STOPPED

[Yuly Finkelberg <liquidicecube@gmail.com>]

                                                                      
                                                                      
                                                                      
                                          Hi,

I sent a message regarding this issue earlier, but after re-reading
it, I realized that it wasn't very clear.  Hopefully, this will
clarify things a little bit:

I have a strange scheduling issue: a bunch of worker tasks are all waiting 
on a wait queue. Each task is woken up by the preceeding, does some work, 
wakes up the next one, and then sends a SIGSTOP to itself. The last task however
does not stop itself, but instead yield()s until all tasks have reached state
TASK_STOPPED.

The code looks like this (irrelevant parts cut out):
	...
        ret = wait_event_interruptible(waitq, next_in_line == myself);
	...
	(some work)
	...
	next_in_line = next;	
        ret = wakeup_next_one();
	if (!last_one)
		send_sig(SIGSTOP, current, 1);
	else
		spin_until_all_stopped()

When run with 50 tasks, normally this works well. However sometimes one of the
tasks (never the last one) gets stuck between calling wakeup_next_one() and 
between sending the signal. It accumulates system time, and its stack looks
like (no pending signals, ti_flags is clear):

c55e7ad0 00000086 c55e6000 c55e7a94 00000046 c55e6000 c55e7ad0 c0109c2d
         00000000 c0497800 00000001 d38da344 0013bc9c c5632840 00071931 d3d93161
         0013bc9c c55d546c c05d3960 0000270f c05d3960 c55e6000 c0106f25 c05d3960

Call Trace:
[<c0106f25>] need_resched+0x27/0x32

(yes, this is not a mistake: this is ALL the stack reported by show_stack())

Normally the spinning task will magically get released after "a while", where 
few seconds < "a while" < 10 minutes and sometimes even longer. 
So the mystery is -
1. Why does the task spin for so long ?
2. Where does it spin ?  (the kernel stack doesn't hint on anything...)
3. How can I find out #2 ?
4. How to fix it ?
5. Is there a better way to make sure a specific task is STOPPED ?

Currently running 2.6.8.1 and 2.6.9 (UP, PREEMPT).  I'd appreciate any
help here...