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

Paul E. McKenney
2014-06-03T15:00:02Z

On Tue, Jun 03, 2014 at 09:36:13AM +0200, Peter Zijlstra wrote:
> On Mon, Jun 02, 2014 at 05:12:16PM -0400, Mikulas Patocka wrote:
> > The patch adds atomic_pointer_t for all architectures - it is in the
> > common code and it is backed by atomic_long_t (that already exists for all
> > architectures). There is no new arch-specific code at all.
> >
> > When we have atomic_pointer_t, we can find the instances of xchg() and
> > cmpxchg() and convert them to atomic_pointer_t (or to other atomic*_t
> > types).
> >
> > When we convert them all, we can drop xchg() and cmpxchg() at all (at
> > least from architecture-neutral code).
> >
> > The problem with xchg() and cmpxchg() is that they are very easy to
> > misuse. Peter Zijlstra didn't know that they are not atomic w.r.t. normal
> > stores, a lot of other people don't know it too - and if we allow these
> > functions to be used, this race condition will reappear in the future
> > again and again.
> >
> > That's why I'm proposing atomic_pointer_t - it guarantees that this race
> > condition can't be made.
>
> But its horrible, and doesn't have any benefit afaict.
>
> So if we really want to keep supporting these platforms; I would propose
> something like:
>
> #ifdef __CHECKER__
> #define __atomic __attribute__((address_space(5)))
> #else
> #define __atomic
> #endif
>
> #define store(p, v) (*(p) = (typeof(*(p)) __force __atomic)(v))
> #define load(p) ((typeof(*p) __force)ACCESS_ONCE(*(p)))
>
> Along with changes to xchg() and cmpxchg() that require them to take
> pointers to __atomic.
>
> That way we keep the flexibility of xchg() and cmpxchg() for being
> (mostly) type and size invariant, and get sparse to find wrong usage.
>
> Then parisc, sparc32, tile32, metag-lock1 and arc-!llsc can go implement
> store() however they like.

Should be fun interacting with atomic operations on __rcu variables
(address space 4). Of course, that is already fun...

Thanx, Paul