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    SubjectRe: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove smp_read_barrier_depends()
    Date
    Paul E. McKenney <paulmck@linux.vnet.ibm.com> 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.

    > static inline
    > unsigned long __assoc_array_ptr_to_meta(const struct assoc_array_ptr *x)
    > {
    > - return (unsigned long)x &
    > + return (unsigned long)READ_ONCE(x) & /* Address dependency. */
    > ~(ASSOC_ARRAY_PTR_SUBTYPE_MASK | ASSOC_ARRAY_PTR_TYPE_MASK);
    > }

    Ditto.

    > - ptr = READ_ONCE(node->slots[slot]);
    > + ptr = READ_ONCE(node->slots[slot]); /* Address dependency. */
    > has_meta |= (unsigned long)ptr;
    > if (ptr && assoc_array_ptr_is_leaf(ptr)) {
    > - /* We need a barrier between the read of the pointer
    > - * and dereferencing the pointer - but only if we are
    > - * actually going to dereference it.
    > - */
    > - smp_read_barrier_depends();
    > -

    For example, you can see the READ_ONCE() here; that is already done.

    Can you also not just delete the comment, but rephrase it to indicate that a
    barrier is necessary and it's done by READ_ONCE(), please?

    David

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