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    SubjectRe: [PATCH 2/2] rculist.h: use the rcu API
    On Wed, May 14, 2008 at 11:26:18PM +0200, Franck Bui-Huu wrote:
    > This patch makes almost all list mutation primitives use
    > rcu_assign_pointer().
    >
    > The main point of this being readability improvement.

    Which is not an improvement at all.

    > --- a/include/linux/rculist.h
    > +++ b/include/linux/rculist.h
    > @@ -17,9 +18,8 @@ static inline void __list_add_rcu(struct list_head *new,
    > {
    > new->next = next;
    > new->prev = prev;
    > - smp_wmb();
    > + rcu_assign_pointer(prev->next, new);
    > next->prev = new;
    > - prev->next = new;
    > }

    Nice chunk to demonstrate.

    Before one could write this like:

    smp_wmb(); smp_wmb();
    next->prev = new; or prev->next = new;
    prev->next = new; next->prev = new;

    And both examples aren't buggy.

    After, you can't write:

    next->prev = new;
    rcu_assign_pointer(prev->next, new);

    Guess why?

    This barrier is related not only to next assignment, but to the whole
    group of assignments.

    > @@ -108,9 +108,8 @@ static inline void list_replace_rcu(struct list_head
    > *old,
    > {
    > new->next = old->next;
    > new->prev = old->prev;
    > - smp_wmb();
    > + rcu_assign_pointer(new->prev->next, new);
    > new->next->prev = new;
    > - new->prev->next = new;
    > old->prev = LIST_POISON2;
    > }



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