linux-kernel - Re: [PATCH v4 2/2] mm/slab: break up RCU readers on SLAB_TYPESAFE_BY

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Message-ID: <d3c609f6-4d0e-4953-8873-697ee3af13cc@paulmck-laptop>
Date:   Mon, 24 Apr 2023 11:34:42 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     SeongJae Park <sj@...nel.org>
Cc:     akpm@...ux-foundation.org, vbabka@...e.cz, willy@...radead.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/2] mm/slab: break up RCU readers on
 SLAB_TYPESAFE_BY_RCU example code

On Mon, Apr 17, 2023 at 07:04:50PM +0000, SeongJae Park wrote:
> The SLAB_TYPESAFE_BY_RCU example code snippet uses a single RCU
> read-side critical section for retries.
> 'Documentation/RCU/rculist_nulls.rst' has similar example code snippet,
> and commit da82af04352b ("doc: Update and wordsmith rculist_nulls.rst")
> broke it up.  Apply the change to SLAB_TYPESAFE_BY_RCU example code
> snippet, too.
> 
> Signed-off-by: SeongJae Park <sj@...nel.org>

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

> ---
>  include/linux/slab.h | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index b18e56c6f06c..6acf1b7c6551 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -53,16 +53,18 @@
>   * stays valid, the trick to using this is relying on an independent
>   * object validation pass. Something like:
>   *
> + * begin:
>   *  rcu_read_lock();
> - * again:
>   *  obj = lockless_lookup(key);
>   *  if (obj) {
>   *    if (!try_get_ref(obj)) // might fail for free objects
> - *      goto again;
> + *      rcu_read_unlock();
> + *      goto begin;
>   *
>   *    if (obj->key != key) { // not the object we expected
>   *      put_ref(obj);
> - *      goto again;
> + *      rcu_read_unlock();
> + *      goto begin;
>   *    }
>   *  }
>   *  rcu_read_unlock();
> -- 
> 2.25.1
>