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Date:	Thu, 23 Oct 2014 20:15:56 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Kirill Tkhai <ktkhai@...allels.com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Kirill Tkhai <tkhai@...dex.ru>
Subject: Re: introduce task_rcu_dereference?

On 10/23, Oleg Nesterov wrote:
>
> Damn.

Yes.

> On 10/22, Oleg Nesterov wrote:
> >
> > +struct task_struct *task_rcu_dereference(struct task_struct **ptask)
> > +{
> > +	struct task_struct *task;
> > +	struct sighand_struct *sighand;
> > +
> > +	task = rcu_dereference(*ptask);
> > +	if (!task)
> > +		return NULL;
> > +
> > +	/* If it fails the check below must fail too */
> > +	probe_slab_address(&task->sighand, sighand);
> > +	/*
> > +	 * Pairs with atomic_dec_and_test() in put_task_struct(task).
> > +	 * If we have read the freed/reused memory, we must see that
> > +	 * the pointer was updated. The caller might want to retry in
> > +	 * this case.
> > +	 */
> > +	smp_rmb();
> > +	if (unlikely(task != ACCESS_ONCE(*ptask)))
> > +		return ERR_PTR(-EAGAIN);
>
> This is not exactly right. task == *ptask can be false positive.
>
> It can be freed, then resused (so that sighand != NULL can be false
> positive), then freed again, and then reused again as task_struct.
>
> This is not that bad, we still can safely use this task_struct, but
> the comment should be updated. Plus -EINVAL below can be wrong in
> this case although this minor.

Yes.

> Yeees, SLAB_DESTTROY_BY_RCU closes this race. Not sure why I'd like
> to avoid it, but I do ;)

Argh. I only meant that SLAB_DESTTROY_BY_RCU can make the comments
simpler. "closes this race" applies too "check below must fail too"
too. Sorry if I confused you.

"task == *ptask can be false positive" is true with or without
SLAB_DESTTROY_BY_RCU, and this needs a good comment. Yes, it can't
be reused twice, but still we can't 100% trust the "sighand != NULL"
check.

So let me repeat, SDBR can only turn probe_slab_address() into a plain
load.

But I can't think properly today, will try to recheck tomorrow and send
v2.

Oleg.

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