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

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Date:	Tue, 13 Jan 2009 23:12:35 +0100
From:	Jesper Nilsson <Jesper.Nilsson@...s.com>
To:	David Miller <davem@...emloft.net>
Cc:	"tj@...nel.org" <tj@...nel.org>, "gregkh@...e.de" <gregkh@...e.de>,
	"stern@...land.harvard.edu" <stern@...land.harvard.edu>,
	"jens.axboe@...cle.com" <jens.axboe@...cle.com>,
	"hinko.kocevar@...rtapot.si" <hinko.kocevar@...rtapot.si>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: lib/klist.c: bit 0 in pointer can't be used as flag

On Tue, Jan 13, 2009 at 10:10:30PM +0100, David Miller wrote:
> From: Jesper Nilsson <jesper.nilsson@...s.com>
> Date: Tue, 13 Jan 2009 16:14:06 +0100
> 
> > The commit a1ed5b0cffe4b16a93a6a3390e8cee0fbef94f86
> > (klist: don't iterate over deleted entries) introduces use of the
> > low bit in a pointer to indicate if the knode is dead or not,
> > assuming that this bit is always free.
> > 
> > This is not true for all architectures, CRIS for example may align data
> > on byte borders.
> 
> There are many other spots in the kernel where the low bits of a
> pointer are hijacked as status bits.  Lots of other things cannot
> possible be working on CRIS because of this.

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?

It seems that rt_mutex uses the same trick (with the lowest 2bits)
but AFAICT that's something we don't use on CRIS.

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