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

Mikulas Patocka
2014-06-02T20:47:47Z

On Mon, 2 Jun 2014, Linus Torvalds wrote:

> On Mon, Jun 2, 2014 at 9:25 AM, Peter Zijlstra <peterz@infradead.org> 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

And what else do you want to do?

Peter Zijlstra said "I've been using xchg() and cmpxchg() without such
consideration for quite a while." - so it basically implies that the
kernel is full of such races, mcs_spinlock is just the most visible one
that crashes the kernel first.

It's not only parisc - tile32, arc, metag (maybe hexagon) are broken too,
because they don't have cmpxchg in hardware.

We have atomic_t, atomic64_t, atomic_long_t that can be sanely used even
on architectures without hardware cmpxchg - so I ask - why can't we have
atomic_pointer_t with the same semantics? (pointer type conversion issues
can be solved, as it is done in the PATCH v2)

Mikulas