Re: [RFC]: Possible race condition in kernel futex code

[Peter Zijlstra]

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@xxxxxxxxxxxxx> 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.
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