linux-kernel - Re: introduce task_rcu

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Message-ID: <20141023181818.GB2740@redhat.com>
Date:	Thu, 23 Oct 2014 20:18:18 +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, Kirill Tkhai wrote:
>
> I'm agree generic helper is better. But probe_slab_address() has a sence
> if we know that SDBR is worse in our subject area.

And I still think it is worse.

> Less of code is
> easier to support :)

Sure, but ignoring the comments, SDBR needs the same code in
task_rcu_dereference() ? Except, of course

	-	probe_slab_address(&task->sighand, sighand);
	+	sighand = task->sighand;

or how do you think we can simplify it?

> probe_slab_address() it's not a trivial logic.

But it already has a user. And probably it can have more.

To me the usage of SDBR is not trivial (and confusing) in this case.
Once again, ignoring the CONFIG_DEBUG_PAGEALLOC problems it does not
help at all.

With or without SDBR rq->curr can be reused and we need to avoid this
race. The fact that with SDBR it can be reused only as another instance
of task_struct is absolutely immaterial imo.

Not to mention that SDBR still adds some overhead while probe_slab()
is free unless CONFIG_DEBUG_PAGEALLOC, but this option adds a large
slowdown anyway.

But again, I can't really work today, perhaps I missed something.
Perhaps you can show a better code which relies on SDBR?

Oleg.

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