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Message-ID: <20080515055011.GA12809@martell.zuzino.mipt.ru>
Date: Thu, 15 May 2008 09:50:11 +0400
From: Alexey Dobriyan <adobriyan@...il.com>
To: Franck Bui-Huu <vagabon.xyz@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
paulmck@...ux.vnet.ibm.com, Josh Triplett <josh@...nel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [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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