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Date:   Mon, 30 Mar 2020 16:07:52 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Will Deacon <will@kernel.org>
Cc:     linux-kernel@vger.kernel.org, Eric Dumazet <edumazet@google.com>,
        Jann Horn <jannh@google.com>,
        Kees Cook <keescook@chromium.org>,
        Maddie Stone <maddiestone@google.com>,
        Marco Elver <elver@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>, kernel-team@android.com,
        kernel-hardening@lists.openwall.com
Subject: Re: [RFC PATCH 02/21] list: Remove hlist_nulls_unhashed_lockless()
Message-ID: <20200330230752.GY19865@paulmck-ThinkPad-P72>
Reply-To: paulmck@kernel.org
References: <20200324153643.15527-1-will@kernel.org>
 <20200324153643.15527-3-will@kernel.org>
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On Tue, Mar 24, 2020 at 03:36:24PM +0000, Will Deacon wrote:
> Commit 02b99b38f3d9 ("rcu: Add a hlist_nulls_unhashed_lockless() function")
> introduced the (as yet unused) hlist_nulls_unhashed_lockless() function
> to avoid KCSAN reports when an RCU reader checks the 'hashed' status
> of an 'hlist_nulls' concurrently undergoing modification.
> 
> Remove the unused function and add a READ_ONCE() to hlist_nulls_unhashed(),
> just like we do already for hlist_nulls_empty().
> 
> Cc: Paul E. McKenney <paulmck@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Will Deacon <will@kernel.org>

Well, I guess that the added docbook comment might be seen as worthwhile.

Acked-by: Paul E. McKenney <paulmck@kernel.org>

> ---
>  include/linux/list_nulls.h | 14 --------------
>  1 file changed, 14 deletions(-)
> 
> diff --git a/include/linux/list_nulls.h b/include/linux/list_nulls.h
> index fa6e8471bd22..3a9ff01e9a11 100644
> --- a/include/linux/list_nulls.h
> +++ b/include/linux/list_nulls.h
> @@ -65,20 +65,6 @@ static inline unsigned long get_nulls_value(const struct hlist_nulls_node *ptr)
>   * but hlist_nulls_del() does not.
>   */
>  static inline int hlist_nulls_unhashed(const struct hlist_nulls_node *h)
> -{
> -	return !h->pprev;
> -}
> -
> -/**
> - * hlist_nulls_unhashed_lockless - Has node been removed and reinitialized?
> - * @h: Node to be checked
> - *
> - * Not that not all removal functions will leave a node in unhashed state.
> - * For example, hlist_del_init_rcu() leaves the node in unhashed state,
> - * but hlist_nulls_del() does not.  Unlike hlist_nulls_unhashed(), this
> - * function may be used locklessly.
> - */
> -static inline int hlist_nulls_unhashed_lockless(const struct hlist_nulls_node *h)
>  {
>  	return !READ_ONCE(h->pprev);
>  }
> -- 
> 2.20.1
>