3.9.x: Possible race related to stop_machine leads to lockup.

[Ben Greear <greearb@candelatech.com>]

I've been trying to figure out why I see the migration/* processes
hang in a busy loop....

While reading the stop_machine.c file, I think I might have an
answer.

The set_state() method sets the thread_ack to the current number
of threads.  Each thread's state machine then decrements it down to
zero where it bumps the state to the next level.  This lets each
cpu stop in lock-step it seems.

But, from what I can tell, the __stop_machine() method can
(re)set the state to STOPMACHINE_PREPARE while the migration
processes are in their loop.  That would explain why they sometimes
loop forever.

Does this make sense?

Any ideas on how to fix this properly?

Thanks,
Ben

-- 
Ben Greear <greearb@candelatech.com>
Candela Technologies Inc  http://www.candelatech.com