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

[Rusty Russell <rusty@rustcorp.com.au>]

Ben Greear <greearb@candelatech.com> writes:
> On 06/04/2013 02:18 PM, Ben Greear wrote:
>> 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?
>
> Err, no..that doesn't make sense.  'smdata' is on the stack.
>
> More printk debugging makes it look like one thread just
> never notices that smdata->state has been updated by another
> thread.
>
> There is this comment..maybe cpu_relax only does the chill out part
> and we need something else to make sure smdata->state is freshly
> read from the other CPU's cache?
>
> 		/* Chill out and ensure we re-read stopmachine_state. */
> 		cpu_relax();
> 		if (smdata->state != curstate) {
>
> Gah..way out of my league :P

What architecture?  Maybe someone didn't get the memo; cpu_relax()
should be a read barrier.

Cheers,
Rusty.