Re: [PATCH 4/4] kthread: Simplify kthread_park() completion

[Oleg Nesterov <oleg@redhat.com>]

Peter,

I am travelling till the end of the next week, unlikely I will be able
to reply to emails or even read them.

But I want very much to comment this change,

On 06/07, Peter Zijlstra wrote:
>
> Now that smpboot_update_cpumask_percpu_thread() is gone, we no longer
> have anybody calling kthread_park() on already parked threads. So
> revert commit:
>
>   b1f5b378e126 ("kthread: Allow kthread_park() on a parked kthread")

Great, I obviously like this patch but the changelog should be fixed ;)

smpboot_update_cpumask_percpu_thread() was actually fine. And we can
(should) revert this commit in any case. Unless I am totally confused.

So how this code

	for_each_cpu_and(cpu, &tmp, cpu_online_mask)
		smpboot_park_thread(plug_thread, cpu);

in smpboot_update_cpumask_percpu_thread() can hit a KTHREAD_SHOULD_PARK
thread? Lets look into kernel test robot's .config:

	CONFIG_NR_CPUS=1

Now look at NR_CPUS==1 version of for_each_cpu* helpers:

	#define for_each_cpu(cpu, mask)			\
		for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask)
	#define for_each_cpu_not(cpu, mask)		\
		for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask)
	#define for_each_cpu_wrap(cpu, mask, start)	\
		for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask, (void)(start))
	#define for_each_cpu_and(cpu, mask, and)	\
		for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask, (void)and)

See? They all ignore the "mask" argument, and this is obviously wrong.

So even if the "tmp" cpumask is empty the code above always does

		smpboot_park_thread(plug_thread, 0);

and hits the already parked kthread.

Oleg.