linux-kernel - Re: [RFC PATCH 05/21] list: Comment missing WRITE_ONCE() in __list

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Message-ID: <20200330231456.GA19865@paulmck-ThinkPad-P72>
Date:   Mon, 30 Mar 2020 16:14:56 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Will Deacon <will@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Maddie Stone <maddiestone@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, kernel-team@...roid.com,
        kernel-hardening@...ts.openwall.com
Subject: Re: [RFC PATCH 05/21] list: Comment missing WRITE_ONCE() in
 __list_del()

On Tue, Mar 24, 2020 at 03:36:27PM +0000, Will Deacon wrote:
> Although __list_del() is called from the RCU list implementation, it
> omits WRITE_ONCE() when updating the 'prev' pointer for the 'next' node.
> This is reasonable because concurrent RCU readers only access the 'next'
> pointers.
> 
> Although it's obvious after reading the code, add a comment.
> 
> Cc: Paul E. McKenney <paulmck@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Will Deacon <will@...nel.org>

OK, I will take the easy one.  ;-)

Reviewed-by: Paul E. McKenney <paulmck@...nel.org>

> ---
>  include/linux/list.h | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/include/linux/list.h b/include/linux/list.h
> index 4d9f5f9ed1a8..ec1f780d1449 100644
> --- a/include/linux/list.h
> +++ b/include/linux/list.h
> @@ -109,6 +109,7 @@ static inline void list_add_tail(struct list_head *new, struct list_head *head)
>   */
>  static inline void __list_del(struct list_head * prev, struct list_head * next)
>  {
> +	/* RCU readers don't read the 'prev' pointer */
>  	next->prev = prev;
>  	WRITE_ONCE(prev->next, next);
>  }
> -- 
> 2.20.1
>