Re: [PATCH v3] sched/numa: fix unsafe get_task_struct() in task_numa_assign()

From: Kirill Tkhai
Date: Mon Oct 20 2014 - 17:05:38 EST


On 21.10.2014 00:50, Oleg Nesterov wrote:
> On 10/21, Kirill Tkhai wrote:
>>
>> I think generic helper is a good idea. The prototype looks OK.
>>
>> But I'm a little doubt about retry loop. If this helper is generic and
>> one day it may move to ./include directory,
>
> Well, if we add a generic helper I think it should be exported even if
> it has a single caller. But I agree this probably needs a justification
> too.
>
>> isn't there a probability
>> people will use it wrong? This loop may bring delays or something bad.
>
> Yes, I thought about livelock too. OK, we can remove it, just
> s/goto retry/return NULL/. Or, perhaps better, return ERR_PTR(-EAGAIN)
> so that the caller can know that retry is possible.
>
> I do not really mind, and we can reconsider this later. And I will not
> argue if you prefer to add the rq->curr specific hack (like your patch
> does).
>
>> And since we still depends on RCU, I'd suggest to add its lockdep assert.
>
> Agreed.
>
> Let me explain what I personally dislike in v3:
>
> - I think that we do not have enough reasons for
> SLAB_DESTROY_BY_RCU. This is the serious change.
>
> probe_kernel_read() looks better to me, and hopefully
> IS_ENABLED(DEBUG_PAGEALLOC) can make it conditional.
>
> - PF_EXITING was fine in task_numa_compare(), but if we
> move this logic into a helper (even if it is not exported)
> then I think we need a more specific check. sighand == NULL
> looks better to me because it clearly connects to
> release_task() which makes this task_struct "rcu-unsafe".
>
> - Again, perhaps we should start we a simple and stupid fix.
> We can do get_task_struct() under rq->lock or, if nothing
> else, just
>
> raw_spin_lock_irq(&rq->lock);
> cur = rq->curr;
> if (is_idle_task(cur) || (cur->flags & PF_EXITING))
> cur = NULL;
> raw_spin_unlock_irq(&rq->lock);
>
> Either way, I hope you will send v4 ;)

No, I won't send. Please do this. Your idea and your patch is almost
ready. Thanks :)

> But probably you should wait for for Peter's opinion first.

Yeah.

Kirill
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