linux-kernel - Re: [tip:locking/core] locking/rwsem: Fix lock optimistic spinning when owner is not running

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Date:	Tue, 10 Mar 2015 19:33:12 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Peter Anvin <hpa@...or.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jason Low <jason.low2@...com>,
	Michel Lespinasse <walken@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Dave Jones <davej@...emonkey.org.uk>,
	Ming Lei <ming.lei@...onical.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Kirill Tkhai <tkhai@...dex.ru>
Subject: Re: [tip:locking/core] locking/rwsem: Fix lock optimistic spinning
	when owner is not running

On 03/10, Linus Torvalds wrote:
>
> So I'm not a huge fan of SLAB_DESTROY_BY_RCU in general, but for
> really core data structures like this, I think it's worth it.

I agree, but we have other users which can't be fixed if we just add
SLAB_DESTROY_BY_RCU to task_struct_cachep. So we need something else
anyway.

> The
> overhead of "call_rcu()" can be quite noticeable,

Yes, this was my concern too,

> and the other
> alternative solutions (like that suggested task_rcu_dereference()) are
> even *more* complex and subtle

Yes, but at least the ugliness is hidden inside task_rcu_dereference().
The usage is simple.

> and generally perform worse.

Well, task_rcu_dereference() should be cheap enough. probe_slab_address()
is a plain LOAD unless CONFIG_DEBUG_PAGEALLOC.

But!

> I'm not claiming it's perfect,

Same here, it is not that I think task_rcu_dereference() is very nice.


So I leave this to you and Peter ;)

Oleg.

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