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Date: Tue, 10 Mar 2015 19:33:12 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
        "linux-tip-commits@vger.kernel.org" 
	<linux-tip-commits@vger.kernel.org>,
        Davidlohr Bueso <dave@stgolabs.net>, Peter Anvin <hpa@zytor.com>,
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        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Dave Jones <davej@codemonkey.org.uk>,
        Ming Lei <ming.lei@canonical.com>,
        Tim Chen <tim.c.chen@linux.intel.com>, Kirill Tkhai <tkhai@yandex.ru>
Subject: Re: [tip:locking/core] locking/rwsem: Fix lock optimistic spinning
	when owner is not running
Message-ID: <20150310183312.GA13665@redhat.com>
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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.