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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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