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

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Message-ID: <20090114181705.GC16505@suse.de>
Date:	Wed, 14 Jan 2009 10:17:05 -0800
From:	Greg KH <gregkh@...e.de>
To:	Jesper Nilsson <Jesper.Nilsson@...s.com>
Cc:	David Miller <davem@...emloft.net>,
	"roucaries.bastien@...il.com" <roucaries.bastien@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"tj@...nel.org" <tj@...nel.org>,
	"stern@...land.harvard.edu" <stern@...land.harvard.edu>,
	"jens.axboe@...cle.com" <jens.axboe@...cle.com>,
	"hinko.kocevar@...rtapot.si" <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 04:12:21PM +0100, Jesper Nilsson wrote:
> On Wed, Jan 14, 2009 at 11:45:28AM +0100, David Miller wrote:
> > From: Jesper Nilsson <Jesper.Nilsson@...s.com>
> > Date: Wed, 14 Jan 2009 11:36:24 +0100
> > 
> > > On Wed, Jan 14, 2009 at 11:21:40AM +0100, David Miller wrote:
> > > > From: Jesper Nilsson <Jesper.Nilsson@...s.com>
> > > > Date: Wed, 14 Jan 2009 11:19:08 +0100
> > > > > 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.
> > > > 
> > > > Out of curiosity, is there a way to get gcc to output code such
> > > > that data objects are aligned more naturally?  Some option or
> > > > similar?
> > > 
> > > There's flags for alignment of objects, but no flags for changing
> > > structure layout or size, which is probably what we run into here.
> > 
> > Really?
> 
> Yes, after some more research, I found that the bug is indeed triggered by
> the struct klist being aligned to odd bytes inside other structs.
> 
> Since CRIS uses packed structs, having a char * inside the kobjects
> shifts the alignment for all data after any struct that contains
> a kobject (and indeed inside the kobject itself)
> 
> In this case it was the class pointer of the struct device
> that had a klist in the private pointer (struct class_private),
> after a struct kset (which contains a kobject).

So, in order to make things smaller, your "use an external flag" patch
would probably be better than forcing the alignment on a 4 byte boundry,
right?

thanks,

greg k-h
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