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

[Linus Torvalds]

On Mon, Jun 2, 2014 at 9:25 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> And I can't say I'm a particular fan of these ops either, as alternative
> I'm almost inclined to just exclude parisc from using opt spinning.

Please do.

There is no way in hell that we should introduce a magic new
atomic_pointer thing for parisc. And the idea somebody had to change
ACCESS_ONCE() for PA-RISC (I'm not going to go back to find who to
blame) is just horribly wrong too, since it's not even necessary for
any normal use: the special "load-and-store-zero" instruction isn't
actually used for "real" data, it's used only for the special
spinlocks afaik, so doing it for all ACCESS_ONCE() users would be
wrong even on PA-RISC. For any normal data, the usual "just load the
value, making sure the compiler doesn't reload it" is perfectly fine -
even on PA-RISC.

Now, if PA-RISC was a major architecture, we'd have to figure this
out. But as it is, PA-RISC is just about the shittiest RISC ever
invented (with original sparc being a strong contender), and let's
face it, nobody really uses it.  It's a "fun project", but it is not
something that we should use to mess up either ACCESS_ONCE() or the
MCS locks.

Just make PA-RISC use its own locks, not any of the new fancy ones.

              Linus
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