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

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Message-Id: <200906020116.n521GsHk097628@www262.sakura.ne.jp>
Date:	Tue, 02 Jun 2009 10:16:54 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:	serge@...lyn.com
Cc:	serue@...ibm.com, linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] TOMOYO: Add garbage collector support. (v2)

Serge E. Hallyn wrote:
> All readers should walk the list under rcu_read_lock, so if the manage
> to reach ->entry, then entry.list->next will be a valid entry on the live
> list until the next rcu cycle.

I have to tell you one thing.
Reader traverses the list via read(2) system call.
It means that reader can't read all elements in the list within a single
read(2) system call.
Thus, reader grabs a reference on an element (i.e. p->entry.users) before
leaving reader section and drops that reference after entering reader section.

Yes, it would be possible for readers to walk the list under rcu_read_lock(),
but it is not sufficient for GC to simply call synchronize_rcu(). GC has to
wait for p->entry.users to become 0 before modifying p->entry.list.next .

> BTW I'm not telling you have to do this, so if you just don't want to, then
> just say so.  But so long as you come back to me with technical reasons
> why you can't, then I take it you're interested in a technical solution :)
> and can't help but respond...

I see. I'll post only (1) and (2).

Thank you for your suggestion and time.
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