lkml.org 
[lkml]   [2013]   [Jun]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: 3.9.x: Possible race related to stop_machine leads to lockup.
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

Ben


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



\
 
 \ /
  Last update: 2013-06-05 00:41    [W:0.073 / U:0.444 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site