linux-kernel - Re: [RFC PATCH 12/21] list: Poison ->next pointer for non-RCU deletion of 'hlist_nulls

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Message-ID: <20200330233229.GG19865@paulmck-ThinkPad-P72>
Date:   Mon, 30 Mar 2020 16:32:29 -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 12/21] list: Poison ->next pointer for non-RCU
 deletion of 'hlist_nulls_node'

On Tue, Mar 24, 2020 at 03:36:34PM +0000, Will Deacon wrote:
> hlist_nulls_del() is used for non-RCU deletion of an 'hlist_nulls_node'
> and so we can safely poison the ->next pointer without having to worry
> about concurrent readers, just like we do for other non-RCU list
> deletion primitives
> 
> Cc: Paul E. McKenney <paulmck@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Will Deacon <will@...nel.org>

Agreed, any code having difficulty with this change should use instead
hlist_nulls_del_rcu().

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

> ---
>  include/linux/list_nulls.h | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/include/linux/list_nulls.h b/include/linux/list_nulls.h
> index fd154ceb5b0d..48f33ae16381 100644
> --- a/include/linux/list_nulls.h
> +++ b/include/linux/list_nulls.h
> @@ -99,6 +99,7 @@ static inline void __hlist_nulls_del(struct hlist_nulls_node *n)
>  static inline void hlist_nulls_del(struct hlist_nulls_node *n)
>  {
>  	__hlist_nulls_del(n);
> +	n->next = LIST_POISON1;
>  	n->pprev = LIST_POISON2;
>  }
>  
> -- 
> 2.20.1
>