linux-kernel - Re: + futex-runtime-enable-pi-and-robust-functionality.patch added to -mm tree

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Date:	Sat, 16 Feb 2008 15:04:44 +0100
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	linux-kernel@...r.kernel.org, mm-commits@...r.kernel.org,
	buytenh@...tstofly.org, mingo@...e.hu, riku.voipio@...ial.fi,
	stable@...nel.org, schwidefsky@...ibm.com
Subject: Re: + futex-runtime-enable-pi-and-robust-functionality.patch added
	to -mm tree

On Sat, Feb 16, 2008 at 02:48:17PM +0100, Thomas Gleixner wrote:
> On Sat, 16 Feb 2008, Heiko Carstens wrote:
> 
> > > Well, NULL pointer dereferencing is supposed to fail, isn't it ?
> >
> > I wasn't sure that this is true for all architectures, but...
> 
> It's an requirement for futex support.

To be more precise: dereferencing alone won't cause an exception for
NULL pointers on s390. Only writes will do so.
That is very architecture specific since we cannot unmap page 0,
it contains per-cpu data -- like exception pointers and all such stuff
that the cpu needs.
Just in case there is any code that relies on the fact that also reads
via a NULL pointer are supposed to failed.
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