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Date: Mon, 28 Mar 2011 15:36:02 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Matt Fleming <matt@console-pimps.org>
Cc: linux-kernel@vger.kernel.org
Subject: Re: __lock_task_sighand() and RCU
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References: <20110328085654.26a75db6@mfleming-mobl1.ger.corp.intel.com>
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Hi Matt,

On 03/28, Matt Fleming wrote:
>
> I've been reading __lock_task_sighand() and I can't quite wrap my head
> around what the rcu_read_lock()/rcu_read_unlock() is protecting in
> there.

It protects ->sighand.

> Initially I assumed that it was protecting the 'tsk' argument,

No. the caller must have a reference.

> Then I wondered if it was protecting the sighand from being freed from
> under us while we're trying to lock it.

Yes,

> But sighands aren't freed via
> rcu,

it is ;)

> it's just a simple refcnt and kmem_cache_free()

Yes, but please note that sighand_cachep was created with
SLAB_DESTROY_BY_RCU.

Oleg.