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

[Peter Zijlstra]

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?

> --- 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.