Re: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove smp_read_barrier_depends()

[David Howells]

David Howells <dhowells@xxxxxxxxxx> wrote:

> Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> 
> >  static inline void *assoc_array_ptr_to_leaf(const struct assoc_array_ptr *x)
> >  {
> > -	return (void *)((unsigned long)x & ~ASSOC_ARRAY_PTR_TYPE_MASK);
> > +	return (void *)((unsigned long)READ_ONCE(x) & /* Address dependency. */
> > +		~ASSOC_ARRAY_PTR_TYPE_MASK);
> >  }
> 
> This is the wrong place to do this.  assoc_array_ptr_to_leaf() is effectively
> no more than a special cast; it removes a metadata bit from a pointer.  x is
> the value we're modifying, not *x, and x was read by the caller.

Also, x is not a pointer you can read from, so if READ_ONCE(x) ever effects a
memory access, you might get an oops.

David