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From: Alexey Dobriyan <adobriyan@gmail.com>
To: Franck Bui-Huu <vagabon.xyz@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	paulmck@linux.vnet.ibm.com, Josh Triplett <josh@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/2] rculist.h: use the rcu API
Date: Thu, 15 May 2008 09:50:11 +0400	[thread overview]
Message-ID: <20080515055011.GA12809@martell.zuzino.mipt.ru> (raw)
In-Reply-To: <482B58FA.30000@gmail.com>

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;
> }


  reply	other threads:[~2008-05-15  4:54 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-05-14 21:24 [PATCH 1/2] Split list.h and move rcu-protected lists into rculist.h Franck Bui-Huu
2008-05-14 21:26 ` [PATCH 2/2] rculist.h: use the rcu API Franck Bui-Huu
2008-05-15  5:50   ` Alexey Dobriyan [this message]
2008-05-15 14:48     ` Paul E. McKenney
     [not found] <47BAC1E6.2050901@gmail.com>
2008-02-19 11:51 ` Franck Bui-Huu
  -- strict thread matches above, loose matches on Subject: below --
2008-01-17 20:46 [PATCH 0/2] Make RCU lists use the RCU API Franck Bui-Huu
2008-01-17 20:48 ` [PATCH 2/2] rculist.h: use the rcu API Franck Bui-Huu
2008-01-18  5:55   ` Paul E. McKenney

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