linux-kernel - Re: introduce task_rcu

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Date:	Fri, 24 Oct 2014 11:51:58 +0400
From:	Kirill Tkhai <ktkhai@...allels.com>
To:	Oleg Nesterov <oleg@...hat.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?

В Чт, 23/10/2014 в 20:18 +0200, Oleg Nesterov пишет:
> 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?

Ok, really, not big simplification there. Your variant is good.

> > 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?

No, it would be the same except probe_slab_address(). So, let's stay
on probe_slab_address() variant and fix the bug this way.

Kirill

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