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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Jun 02, 2014 at 06:25:25PM +0200, Peter Zijlstra wrote:
> On Mon, Jun 02, 2014 at 12:00:45PM -0400, Mikulas Patocka wrote:
> >  struct optimistic_spin_queue {
> > -	struct optimistic_spin_queue *next, *prev;
> > +	atomic_pointer(struct optimistic_spin_queue *) next;
> > +	struct optimistic_spin_queue *prev;
> >  	int locked; /* 1 if lock acquired */
> >  };
> >  
> > Index: linux-3.15-rc8/include/asm-generic/atomic-long.h
> > ===================================================================
> > --- linux-3.15-rc8.orig/include/asm-generic/atomic-long.h	2014-06-02 17:11:17.000000000 +0200
> > +++ linux-3.15-rc8/include/asm-generic/atomic-long.h	2014-06-02 17:11:50.000000000 +0200
> > @@ -255,4 +255,31 @@ static inline long atomic_long_add_unles
> >  
> >  #endif  /*  BITS_PER_LONG == 64  */
> >  
> > +#define atomic_pointer(type)						\
> > +union {									\
> > +	atomic_long_t __a;						\
> > +	type __t;							\
> > +	char __check_sizeof[sizeof(type) == sizeof(long) ? 1 : -1];	\
> > +}
> 
> That's still entirely disgusting, and afaict entirely redundant. You can
> do that test in the operators below just fine.
> 
> > +#define ATOMIC_POINTER_INIT(i)	{ .__t = (i) }
> > +
> > +#define atomic_pointer_read(v)	((typeof((v)->__t))atomic_long_read(&(v)->__a))
> > +
> > +#define atomic_pointer_set(v, i)		({			\
> > +	typeof((v)->__t) __i = (i);					\
> > +	atomic_long_set(&(v)->__a, (long)(__i));			\
> > +})
> > +
> > +#define atomic_pointer_xchg(v, i)		({			\
> > +	typeof((v)->__t) __i = (i);					\
> > +	(typeof((v)->__t))atomic_long_xchg(&(v)->__a, (long)(__i));	\
> > +})
> > +
> > +#define atomic_pointer_cmpxchg(v, old, new)	({			\
> > +	typeof((v)->__t) __old = (old);					\
> > +	typeof((v)->__t) __new = (new);					\
> > +	(typeof((v)->__t))atomic_long_cmpxchg(&(v)->__a, (long)(__old), (long)(__new));\
> > +})
> 
> 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.

That is an excellent point for this particular issue.  Do parisc systems
really support enough CPUs to make queued spinlocks worthwhile?  If not,
maybe we should just have parisc stick with traditional spinlocks.

> That said, this patch still doesn't address the far more interesting
> problem of actually finding these issues for these few weird archs.

Indeed.  And finding other lower-probability failures due to other
atomic manipulations of pointers that are also accessed with normal
loads and stores.

							Thanx, Paul