linux-kernel - Re: [PATCH 4/4] Docs/RCU/rculist_nulls: Drop unnecessary '

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Message-ID: <CAEXW_YQFqW2QcAuHZEhc_GaUaB-=QOS0WgUOizd=FYwtFQ8vag@mail.gmail.com>
Date:   Fri, 19 May 2023 14:52:50 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     SeongJae Park <sj@...nel.org>
Cc:     paulmck@...nel.org, corbet@....net, rcu@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] Docs/RCU/rculist_nulls: Drop unnecessary '_release'
 in insert function

On Thu, May 18, 2023 at 6:40 PM SeongJae Park <sj@...nel.org> wrote:
>
> The document says we can avoid extra smp_rmb() in lockless_lookup() and
> extra _release() in insert function when hlist_nulls is used.  However,
> the example code snippet for the insert function is still using the
> extra _release().  Drop it.
>
> Signed-off-by: SeongJae Park <sj@...nel.org>
> ---
>  Documentation/RCU/rculist_nulls.rst | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/Documentation/RCU/rculist_nulls.rst b/Documentation/RCU/rculist_nulls.rst
> index 5cd6f3f8810f..463270273d89 100644
> --- a/Documentation/RCU/rculist_nulls.rst
> +++ b/Documentation/RCU/rculist_nulls.rst
> @@ -191,7 +191,7 @@ scan the list again without harm.
>    obj = kmem_cache_alloc(cachep);
>    lock_chain(); // typically a spin_lock()
>    obj->key = key;
> -  atomic_set_release(&obj->refcnt, 1); // key before refcnt
> +  atomic_set(&obj->refcnt, 1);
>    /*
>     * insert obj in RCU way (readers might be traversing chain)
>     */

If write to ->refcnt of 1 is reordered with setting of ->key, what
prevents the 'lookup algorithm' from doing a key match (obj->key ==
key) before the refcount has been initialized?

Are we sure the reordering mentioned in the document is the same as
the reordering prevented by the atomic_set_release()?

For the other 3 patches, feel free to add:
Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>

thanks,

 - Joel