linux-kernel - Re: [PATCH] rculist.h: docu: fix wrong function name

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Date:   Wed, 20 Sep 2023 03:53:08 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Philipp Stanner <pstanner@...hat.com>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Zqiang <qiang.zhang1211@...il.com>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rculist.h: docu: fix wrong function name

On Tue, Sep 19, 2023 at 09:47:55PM +0200, Philipp Stanner wrote:
> The header contains a comment that details why the functions
> list_empty_rcu() and list_first_entry_rcu() don't exist. It explains
> that they don't exist because standard list_empty() can be used just as
> well, but one can not expect sane results from a subsequent, quote,
> "list_first_entry_rcu()".
> 
> This function (obviously) does not exist. What the comment's author
> actually meant was the standard list-function list_first_entry().
> 
> Change the function name in that comment from list_first_entry_rcu() to
> list_first_entry().
> 
> Additionally, add the parenthesis to list_first_or_null_rcu to be congruent
> with that entire comment's style.
> 
> Signed-off-by: Philipp Stanner <pstanner@...hat.com>
> ---
> Hi!
> I hope this helps.
> I wasn't 100.000000% sure if that's correct, but I thought asking is for
> free 8-)
> 
> Regards,
> P.

Thank you for sending this!  Please see below.

							Thanx, Paul

> ---
>  include/linux/rculist.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/rculist.h b/include/linux/rculist.h
> index d29740be4833..4837d8892691 100644
> --- a/include/linux/rculist.h
> +++ b/include/linux/rculist.h
> @@ -331,9 +331,9 @@ static inline void list_splice_tail_init_rcu(struct list_head *list,
>   * rcu_dereference() is not needed), which means that list_empty() can be
>   * used anywhere you would want to use list_empty_rcu().  Just don't
>   * expect anything useful to happen if you do a subsequent lockless
> - * call to list_first_entry_rcu()!!!
> + * call to list_first_entry()!!!

You are quite correct that the original is incorrect, given that it does
not exist, but a better change would be to list_entry_rcu().  The reason
being that list_first_entry() does not have READ_ONCE(), allowing the
compiler to play all sorts of games (see https://lwn.net/Articles/793253
for some examples).

>   *
> - * See list_first_or_null_rcu for an alternative.
> + * See list_first_or_null_rcu() for an alternative.

Good catch!

Please do feel free to send an update.

							Thanx, Paul

>   */
>  
>  /**
> -- 
> 2.41.0
>