linux-kernel - Re: lib/klist.c: bit 0 in pointer can't be used as flag

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Date:	Wed, 14 Jan 2009 11:19:08 +0100
From:	Jesper Nilsson <Jesper.Nilsson@...s.com>
To:	Bastien ROUCARIES <roucaries.bastien@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>, tj@...nel.org,
	gregkh@...e.de, stern@...land.harvard.edu, jens.axboe@...cle.com,
	hinko.kocevar@...rtapot.si
Subject: Re: lib/klist.c: bit 0 in pointer can't be used as flag

On Wed, Jan 14, 2009 at 12:11:32AM +0100, Bastien ROUCARIES wrote:
> On Tue, Jan 13, 2009 at 11:45 PM, David Miller <davem@...emloft.net> wrote:
> > From: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> > Date: Wed, 14 Jan 2009 07:40:19 +0900
> >
> >> > It may be that we've worked around the other spots, although I haven't
> >> > seen anything like that, we might just have been lucky until now.
> >> >
> >> > Can you recall another place where this trick is used?
> >>
> >> rmap.
> >> Don't CRIS use mmu?
> >
> > I'm beginning to suspect the issue is only with objects
> > in the kernel image itself.  Dynamically allocated memory
> > is properly aligned and therefore the "low bit status bits
> > in pointer" trick works.
> Perhaps using a pointerhackalign trick on this structure where
> #define pointerhackalign(x) __attribute__ ((aligned (x)))
> and declare
> struct klist_node {
> ...
> }  pointerhackalign(2);
> 
> Because  __attribute__ ((aligned (x))) could only increase alignment
> it will safe to do that and serve as documentation purpose :)

That works, but we need to do it not for the struct klist_node,
but for the struct we insert into the void * in klist_node,
which is struct klist.

The following patch works for CRIS, and is less intrusive than
my earlier patch. If this the way to go I can resubmit a proper patch.

diff --git a/include/linux/klist.h b/include/linux/klist.h
index 8ea98db..a21cd7a 100644
--- a/include/linux/klist.h
+++ b/include/linux/klist.h
@@ -23,7 +23,7 @@ struct klist {
 	struct list_head	k_list;
 	void			(*get)(struct klist_node *);
 	void			(*put)(struct klist_node *);
-};
+} __attribute__ ((aligned (4)));
 
 #define KLIST_INIT(_name, _get, _put)					\
 	{ .k_lock	= __SPIN_LOCK_UNLOCKED(_name.k_lock),		\

> Regards
> 
> Bastien

/^JN - Jesper Nilsson
-- 
               Jesper Nilsson -- jesper.nilsson@...s.com
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