From: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@alien8.de>, Dave Hansen <dave@sr71.net>,
LKML <linux-kernel@vger.kernel.org>,
Dave Jones <davej@redhat.com>,
dhillf@gmail.com, Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH] kthread: Prevent unpark race which puts threads on the wrong cpu
Date: Fri, 12 Apr 2013 02:49:50 +0530 [thread overview]
Message-ID: <516728F6.4090701@linux.vnet.ibm.com> (raw)
In-Reply-To: <alpine.LFD.2.02.1304112158040.21884@ionos>
On 04/12/2013 02:17 AM, Thomas Gleixner wrote:
> Srivatsa,
>
> On Fri, 12 Apr 2013, Srivatsa S. Bhat wrote:
>> On 04/09/2013 08:08 PM, Thomas Gleixner wrote:
>>> Add a new task state (TASK_PARKED) which prevents other wakeups and
>>> use this state explicitely for the unpark wakeup.
>>>
>>
>> Again, I think this is unnecessary. We are good as long as no one other
>> than the unpark code can kick the kthreads out of the loop in the park
>> code. Now that I understand the race you explained above, why not just
>> fix that race itself by reversing the ordering of clear(SHOULD_PARK)
>> and bind_to(CPU2)? That way, even if someone else wakes up the per-cpu
>> kthread, it will just remain confined to the park code, as intended.
>
> In theory.
>
>> A patch like below should do it IMHO. I guess I'm being a little too
>> persistent, sorry!
>
> No it's not about being persistent, you're JUST too much into voodoo
> programming instead of going for the straight forward and robust
> solutions.
>
> Darn, I hate it as much as everyone else to introduce a new task
> state, but that state allows us to make guarantees and gives us
> semantical clarity. A parked thread is parked and can only be woken up
> by the unpark code. That's clear semantics and not a magic construct
> which will work in most cases and for the remaining ones (See below)
> it will give us problems which are way harder to decode than the ones
> we tried to fix with that magic.
>
>> diff --git a/kernel/kthread.c b/kernel/kthread.c
>> index 691dc2e..9512fc5 100644
>> --- a/kernel/kthread.c
>> +++ b/kernel/kthread.c
>> @@ -308,6 +308,15 @@ struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data),
>> to_kthread(p)->cpu = cpu;
>> /* Park the thread to get it out of TASK_UNINTERRUPTIBLE state */
>> kthread_park(p);
>> +
>> + /*
>> + * Wait for p->on_rq to be reset to 0, to ensure that the per-cpu
>> + * migration thread (which belongs to the stop_task sched class)
>> + * doesn't run until the cpu is actually onlined and the thread is
>> + * unparked.
>> + */
>> + if (!wait_task_inactive(p, TASK_INTERRUPTIBLE))
>> + WARN_ON(1);
>
> Yay, we rely on TASK_INTERRUPTIBLE state with a task which already has
> references outside the creation code.
I doubt that. We have not even onlined the CPU, how would any else even
_know_ that we created this kthread??
*per_cpu_ptr(ht->store, cpu) = tsk; is executed _after_ returning from
this function.
The problem with ksoftirqd is very clear - we unpark threads _after_ we
online the CPU. So, in between the 2 steps, somebody on that CPU can call
__do_softirq(), leading to the race you described in your cover-letter.
That's why I tried to fix that race.
> And then we _HOPE_ that nothing
> wakes it up _BEFORE_ we do something else.
>
Nothing can wake it up, because no one is aware of the newly created
kthread.
> Aside of that, you are still insisting to enforce that for every per
> cpu thread even if the only one which needs that at this point are
> thos which have a create() callback (i.e. the migration thread). And
> next week you figure out that this is a performance impact on bringing
> up large machines....
>
Making this wait call specific to those kthreads with the ->create callback
won't be that much of a big deal, IMHO. But see below, I'm not going to
insist on going with my suggestions.
>> /**
>> * kthread_unpark - unpark a thread created by kthread_create().
>> * @k: thread created by kthread_create().
>> @@ -337,18 +357,29 @@ void kthread_unpark(struct task_struct *k)
>> struct kthread *kthread = task_get_live_kthread(k);
>>
>> if (kthread) {
>> + /*
>> + * Per-cpu kthreads such as ksoftirqd can get woken up by
>> + * other events. So after binding the thread, ensure that
>> + * it goes off the CPU atleast once, by parking it again.
>> + * This way, we can ensure that it will run on the correct
>> + * CPU on subsequent wakeup.
>> + */
>> + if (test_bit(KTHREAD_IS_PER_CPU, &kthread->flags)) {
>> + __kthread_bind(k, kthread->cpu);
>> + clear_bit(KTHREAD_IS_PARKED, &kthread->flags);
>
> And how is that f*cking different from the previous code?
>
> CPU0 CPU1 CPU2
> wakeup(T) -> run on CPU1 (last cpu)
>
> switch_to(T)
>
> __kthread_bind(T, CPU2)
>
> clear(KTHREAD_IS_PARKED)
>
> leave loop due to !KTHREAD_IS_PARKED
How?? The task will leave the loop only when we clear
SHOULD_PARK, not when we clear IS_PARKED. So it won't
leave the loop here. It will cause the kthread to
perform a fresh complete() for the waiting kthread_park()
on CPU0.
>
> BUG(wrong cpu) <--- VOODOO FAILURE
>
> kthread_park(T) <-- VOODOO TOO LATE
>
No, the purpose of clear(IS_PARKED) followed by __kthread_park() is to
ensure that the task gets *descheduled* atleast once after we did the
kthread_bind(). And that's because we can't use set_cpus_allowed_ptr() to
migrate a running kthread (because the kthread could be the migration
thread). So instead, we use kthread_bind() and depend on sleep->wakeup
to put the task on the right CPU.
> You can turn around the order of clearing/setting the flags as much as
> you want, I'm going to punch an hole in it.
>
> TASK_PARKED is the very obvious and robust solution which fixes _ALL_
> of the corner cases, at least as far as I can imagine them. And
> robustness rules at least in my world.
>
Yes, I agree that it is robust and has clear semantics. No doubt about
that. So I won't insist on going with my suggestions.
Regards,
Srivatsa S. Bhat
next prev parent reply other threads:[~2013-04-11 21:22 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-04-05 21:43 kernel BUG at kernel/smpboot.c:134! Dave Hansen
2013-04-06 7:12 ` Srivatsa S. Bhat
2013-04-06 8:31 ` Thomas Gleixner
2013-04-07 9:20 ` Thomas Gleixner
2013-04-07 9:50 ` Borislav Petkov
2013-04-08 9:24 ` Thomas Gleixner
2013-04-08 11:55 ` Borislav Petkov
2013-04-08 12:17 ` Thomas Gleixner
2013-04-09 14:38 ` [PATCH] kthread: Prevent unpark race which puts threads on the wrong cpu Thomas Gleixner
2013-04-09 15:55 ` Dave Hansen
2013-04-09 18:43 ` Thomas Gleixner
2013-04-09 19:30 ` Thomas Gleixner
2013-04-09 20:38 ` Dave Hansen
2013-04-09 20:54 ` Dave Hansen
2013-04-10 8:29 ` Thomas Gleixner
2013-04-10 10:51 ` Thomas Gleixner
2013-04-10 19:41 ` Dave Hansen
2013-04-11 10:19 ` Thomas Gleixner
2013-04-11 10:48 ` Srivatsa S. Bhat
2013-04-11 11:43 ` Srivatsa S. Bhat
2013-04-11 11:59 ` Srivatsa S. Bhat
2013-04-11 12:51 ` Thomas Gleixner
2013-04-11 12:54 ` Thomas Gleixner
2013-04-11 13:46 ` Thomas Gleixner
2013-04-11 18:07 ` Dave Hansen
2013-04-11 19:48 ` Thomas Gleixner
2013-04-10 14:03 ` [PATCH] CPU hotplug, smpboot: Fix crash in smpboot_thread_fn() Srivatsa S. Bhat
2013-04-11 8:10 ` Thomas Gleixner
2013-04-11 10:19 ` Srivatsa S. Bhat
2013-04-11 19:16 ` [PATCH] kthread: Prevent unpark race which puts threads on the wrong cpu Srivatsa S. Bhat
2013-04-11 20:47 ` Thomas Gleixner
2013-04-11 21:19 ` Srivatsa S. Bhat [this message]
2013-04-12 10:59 ` Thomas Gleixner
2013-04-12 11:26 ` Srivatsa S. Bhat
2013-04-15 19:49 ` Dave Hansen
2013-04-12 10:41 ` Peter Zijlstra
2013-04-12 12:32 ` [tip:core/urgent] " tip-bot for Thomas Gleixner
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