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Date: Fri, 9 Oct 2015 13:35:52 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Hans Zuidam <h.zuidam@computer.org>,
        Jaccon Bastiaansen <jaccon.bastiaansen@gmail.com>, x86@kernel.org,
        mingo@redhat.com, "H. Peter Anvin" <hpa@zytor.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        stable@vger.kernel.org, Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [RFC]: Possible race condition in kernel futex code
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On Fri, Oct 09, 2015 at 11:25:09AM +0100, Thomas Gleixner wrote:
> Hans,
> 
> On Fri, 9 Oct 2015, Hans Zuidam wrote:
> > On 9 okt. 2015, at 11:06, Thomas Gleixner <tglx@linutronix.de> wrote:
> > > You cannot use an explicit 32bit read. We need an access which
> > > handles the fault gracefully.
> >
> > The reason for the explicit read suggestion is to avoid the
> > _builtin_constant_p() in __copy_from_user_nocheck().  The GCC manual
> > says that there may be situations where it returns 0 even though the
> > argument is a constant.
> 
> That's insane at best.

Right, but I bet that is for cases where constant propagation completely
fails, and this is a trivial case, I have no problem relying on it.