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

Paul E. McKenney
2014-06-03T15:56:25Z

On Tue, Jun 03, 2014 at 05:09:08PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 03, 2014 at 07:07:27AM -0700, Paul E. McKenney wrote:
> > On Tue, Jun 03, 2014 at 09:36:13AM +0200, Peter Zijlstra wrote:
> > >
> > > #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...
> >
>
> Hmm, good point, I suppose sparse doesn't like two different
> address_space annotations on the same variable ?
>
> /me adds Christpoher Li to the CC list.
>
> ISTR Mikulas actually listing one such, me digs in recent email..
>
> > $ grep -w "fdt->fd" */*.c
> > fs/file.c: free_fdmem(fdt->fd);
> > fs/file.c: fdt->fd = data;
> > fs/file.c: free_fdmem(fdt->fd);
> > fs/file.c: struct file * file = xchg(&fdt->fd[i], NULL);
>
> So yes, that's going to be fun, mostly because rcu_assign_pointer()
> doesn't actually do the right magic for this to be safe on their
> platform(s).

Maybe at some point sparse needs to keep a bit mask for the address
spaces, so that you caould say somthing like:

struct foo __atomic __rcu *p;

Thanx, Paul