linux-kernel - Re: [PATCH] TOMOYO: Add garbage collector support. (v2)

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Date:	Wed, 27 May 2009 10:25:28 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:	paulmck@...ux.vnet.ibm.com
Cc:	jmorris@...ei.org, linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] TOMOYO: Add garbage collector support. (v2)

Hello.

Paul E. McKenney wrote:
> The list_del_rcu() primitive is designed for removing elements from
> lists that have concurrent readers, and it therefore avoids changing the
> ->next pointer.  That said, I don't immediately see whether or not there
> is some other reason you need to keep it on the list.  And do you need
> the ->prev pointer to stay valid?  If not, you might be able to use it
> in place of the ->is_deleted flag.

I don't need ->prev pointer.

> > Use of RCU does not help here because we need to keep the item valid as long as
> > the item is referred by ->read_var1 or ->read_var2 or ->write_var1 .
> 
> You would indeed need some way of tracking those references.  One
> approach is to make the RCU callback check to see if any of them
> reference the to-be-deleted object, reposting itself if so.  This
> approach assumes that such a reference is short-lived, of course.
> 
> If the ->read_var1 and other references are long-lived, could you
> post an RCU callback when the last such reference was removed?

The reference stored in ->read_var1 / ->read_var2 / ->write_var1 are long-lived
(it varies from less than one second to more than many hours).

Well, it's time for me to read all Documentation/RCU/* .

Thanks.
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