Re: [PATCH v2] introduce atomic_pointer to fix a race condition in cancelable mcs spinlocks

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jun 02, 2014 at 01:33:34PM -0400, Waiman Long wrote:
> On 06/02/2014 12:30 PM, Peter Zijlstra wrote:
> >On Mon, Jun 02, 2014 at 06:25:25PM +0200, Peter Zijlstra wrote:
> >>I'm almost inclined to just exclude parisc from using opt spinning.
> >>
> >>That said, this patch still doesn't address the far more interesting
> >>problem of actually finding these issues for these few weird archs.
> >So why do these archs provide xchg() and cmpxchg() at all? Wouldn't it
> >be much simpler if archs that cannot sanely do this, not provide these
> >primitives at all?
> 
> I believe xchg() and cmpxchg() are used in quite a number of places within
> the generic kernel code. So kernel compilation will fail if those APIs
> aren't provided by an architecture.

Yep.. so this is going to be painful for a while. But given their
(parisc, sparc32, metag-lock1) constraints, who knows how many of those
uses are actually broken.

So the question is, do you prefer subtly broken code or hard compile
fails? Me, I go for the compile fail.

In any case, this all goes towards what hpa said, what are the minimal
requirements we have for running Linux.