Re: [PATCH] stop_machine: Disable preemption after queueing stopper threads

[Sodagudi Prasad]

On 2018-07-30 14:07, Peter Zijlstra wrote:
> On Mon, Jul 30, 2018 at 10:12:43AM -0700, Sodagudi Prasad wrote:
> > How about including below change as well?  Currently, there is no way 
> > to
> > identify thread migrations completed or not.  When we observe this 
> > issue,
> > the symptom was work queue lock up. It is better to have some timeout 
> > here
> > and induce the bug_on.
>
> You'd trigger the soft-lockup or hung-task detector I think. And if 
> not,
> we ought to look at making it trigger at least one of those.
>
> > There is no way to identify the migration threads stuck or not.
>
> Should be pretty obvious from the splat generated by the above, no?
Hi Peter and Thomas,

Thanks for your support.
I have another question on this flow and retry mechanism used in this 
cpu_stop_queue_two_works() function using the global variable 
stop_cpus_in_progress.

This variable is getting used in various paths, such as task migration, 
set task affinity, and CPU hotplug.

For example cpu hotplug path, stop_cpus_in_progress variable getting set 
with true with out checking.
takedown_cpu()
--stop_machine_cpuslocked()
---stop_cpus()
---__stop_cpus()
----queue_stop_cpus_work()
setting stop_cpus_in_progress to true directly.

But in the task migration path only, the stop_cpus_in_progress  variable 
is used for retry.

I am thinking that stop_cpus_in_progress variable lead race conditions, 
where CPU hotplug and task migration happening simultaneously. Please 
correct me If my understanding wrong.

-Thanks, Prasad

> > --- a/kernel/stop_machine.c
> > +++ b/kernel/stop_machine.c
> > @@ -290,6 +290,7 @@ int stop_two_cpus(unsigned int cpu1, unsigned int 
> > cpu2,
> > cpu_stop_fn_t fn, void *
> >         struct cpu_stop_done done;
> >         struct cpu_stop_work work1, work2;
> >         struct multi_stop_data msdata;
> > +       int ret;
> >
> >         msdata = (struct multi_stop_data){
> >                 .fn = fn,
> > @@ -312,7 +313,10 @@ int stop_two_cpus(unsigned int cpu1, unsigned int 
> > cpu2,
> > cpu_stop_fn_t fn, void *
> >         if (cpu_stop_queue_two_works(cpu1, &work1, cpu2, &work2))
> >                 return -ENOENT;
> >
> > -       wait_for_completion(&done.completion);
> > +       ret = wait_for_completion_timeout(&done.completion,
> > msecs_to_jiffies(1000));
> > +       if (!ret)
> > +               BUG_ON(1);
> > +
>
> That's a random timeout, which if you spuriously trigger it, will take
> down your machine. That seems like a cure worse than the disease.

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