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

David Howells [off-list ref] wrote:

Paul E. McKenney [off-list ref] wrote:

quoted

 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