linux-kernel - Re: [PATCH 2/2] CRED: Fix __task_cred()'s lockdep check and banner comment

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Message-ID: <alpine.LFD.2.00.1008031943400.3225@localhost.localdomain>
Date:	Tue, 3 Aug 2010 19:48:20 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	David Howells <dhowells@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	paulmck@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org, Jiri Olsa <jolsa@...hat.com>
Subject: Re: [PATCH 2/2] CRED: Fix __task_cred()'s lockdep check and banner
 comment

On Tue, 3 Aug 2010, Linus Torvalds wrote:

> Added Oleg and Thomas to the participants.
> 
> Oleg/Thomas: the whole thread is on lkml, but I'm quoting most of the
> relevant parts..
> 
> On Tue, Aug 3, 2010 at 2:34 AM, David Howells <dhowells@...hat.com> wrote:
> >
> > A previous patch:
> >
> >        commit 8f92054e7ca1d3a3ae50fb42d2253ac8730d9b2a
> >        Author: David Howells <dhowells@...hat.com>
> >        Date:   Thu Jul 29 12:45:55 2010 +0100
> >        Subject: CRED: Fix __task_cred()'s lockdep check and banner comment
> >
> > fixed the lockdep checks on __task_cred().  This has shown up a place in the
> > signalling code where a lock should be held - namely that
> > check_kill_permission() requires its callers to hold the RCU lock.
> 
> It's not just check_kill_permission(), is it? I thought we could do
> the "for_each_process()" loops with just RCU, rather than holding the
> whole tasklist_lock? So I _think_ that getting the RCU read-lock would
> make it possible to get rid of the tasklist_lock in there too? At
> least in kill_something_info().
> 
> Yes/no? What am I missing? This is an Oleg question, mainly.

Yes, almost all places in the kernel which hold tasklist_lock read
locked are safe with RCU. I started to work on that, but got
distracted.
 
Thanks,

	tglx