linux-kernel - Re: [RFC PATCH 06/21] list: Remove superfluous WRITE_ONCE() from hlist

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Message-ID: <20200330232114.GC19865@paulmck-ThinkPad-P72>
Date:   Mon, 30 Mar 2020 16:21:14 -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 06/21] list: Remove superfluous WRITE_ONCE() from
 hlist_nulls implementation

On Tue, Mar 24, 2020 at 03:36:28PM +0000, Will Deacon wrote:
> Commit 860c8802ace1 ("rcu: Use WRITE_ONCE() for assignments to ->pprev
> for hlist_nulls") added WRITE_ONCE() invocations to hlist_nulls_add_head()
> and hlist_nulls_del().
> 
> Since these functions should not ordinarily run concurrently with other
> list accessors, restore the plain C assignments so that KCSAN can yell
> if a data race occurs.
> 
> Cc: Eric Dumazet <edumazet@...gle.com>
> Cc: Paul E. McKenney <paulmck@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Will Deacon <will@...nel.org>

And this means that the lockless uses of hlist_nulls_empty() need
attention, correct?

							Thanx, Paul

> ---
>  include/linux/list_nulls.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/list_nulls.h b/include/linux/list_nulls.h
> index fa51a801bf32..fd154ceb5b0d 100644
> --- a/include/linux/list_nulls.h
> +++ b/include/linux/list_nulls.h
> @@ -80,10 +80,10 @@ static inline void hlist_nulls_add_head(struct hlist_nulls_node *n,
>  	struct hlist_nulls_node *first = h->first;
>  
>  	n->next = first;
> -	WRITE_ONCE(n->pprev, &h->first);
> +	n->pprev = &h->first;
>  	h->first = n;
>  	if (!is_a_nulls(first))
> -		WRITE_ONCE(first->pprev, &n->next);
> +		first->pprev = &n->next;
>  }
>  
>  static inline void __hlist_nulls_del(struct hlist_nulls_node *n)
> @@ -99,7 +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);
> -	WRITE_ONCE(n->pprev, LIST_POISON2);
> +	n->pprev = LIST_POISON2;
>  }
>  
>  /**
> -- 
> 2.20.1
>