Messages in this thread | | | Date | Wed, 6 Jun 2018 15:51:16 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH v1] kthread/smpboot: Serialize kthread parking against wakeup |
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On 06/05, Peter Zijlstra wrote: > > Also, I think we still need TASK_PARKED as a special state for that.
I think it would be nice to kill the TASK_PARKED state altogether. But I don't know how. I'll try to look at this code later, but I am not sure I will find a way to cleanup it...
> --- a/kernel/kthread.c > +++ b/kernel/kthread.c > @@ -177,12 +177,24 @@ void *kthread_probe_data(struct task_struct *task) > static void __kthread_parkme(struct kthread *self) > { > for (;;) { > - set_current_state(TASK_PARKED); > + /* > + * TASK_PARKED is a special state; we must serialize against > + * possible pending wakeups to avoid store-store collisions on > + * task->state. > + * > + * Such a collision might possibly result in the task state > + * changin from TASK_PARKED and us failing the > + * wait_task_inactive() in kthread_park(). > + */ > + set_special_state(TASK_PARKED);
Agreed,
> if (!test_bit(KTHREAD_SHOULD_PARK, &self->flags)) > break; > + > + complete_all(&self->parked); > schedule(); > } > __set_current_state(TASK_RUNNING); > + reinit_completion(&self->parked);
But how can we know that all the callers of kthread_park() have already returned from wait_for_completion() ?
Oh. The very fact that __kthread_parkme() does complete_all() proves that we need some serious cleanups. In particular, I think that kthread_park() on a parked kthread must not be possible.
Just look at this code. It looks as if __kthread_parkme() can race with _unpark() and thus we need this wait-event-like loop.
But if it can race with _unpark() then kthread_park() can block forever.
For the start, can't we change kthread_park()
- set_bit(KTHREAD_SHOULD_PARK, &kthread->flags); + if (test_and_set_bit(...)) + return -EAGAIN;
and s/complete_all/complete/ in __kthread_parkme() ?
IIUC, this will only affect smpboot_update_cpumask_percpu_thread() which can hit an already parked thread, but it doesn't need to wait.
And it seems that smpboot_update_cpumask_percpu_thread() in turn needs some cleanups. Hmm. and its single user: kernel/watchdog.c.
And speaking of watchdog.c, can't we simply kill the "watchdog/%u" threads? This is off-topic, but can't watchdog_timer_fn() use stop_one_cpu_nowait(watchdog) ?
And I really think we should unexport kthread_park/unpark(), only smpboot_thread_fn() should use them. kthread() should not play with __kthread_parkme(). And even KTHREAD_SHOULD_PARK must die, I mean it should live in struct smp_hotplug_thread, not in struct kthread.
OK, this is off-topic too.
In short, I think this patch is fine but I didn't read it carefully, will try tomorrow.
And, let me repeat, can't we avoid complete_all() ?
Oleg.
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