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References: <20230518224008.2468-1-sj@kernel.org> <20230518224008.2468-5-sj@kernel.org>
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From:   Joel Fernandes <joel@joelfernandes.org>
Date:   Fri, 19 May 2023 14:52:50 -0400
Message-ID: <CAEXW_YQFqW2QcAuHZEhc_GaUaB-=QOS0WgUOizd=FYwtFQ8vag@mail.gmail.com>
Subject: Re: [PATCH 4/4] Docs/RCU/rculist_nulls: Drop unnecessary '_release'
 in insert function
To:     SeongJae Park <sj@kernel.org>
Cc:     paulmck@kernel.org, corbet@lwn.net, rcu@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org
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On Thu, May 18, 2023 at 6:40=E2=80=AFPM SeongJae Park <sj@kernel.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@kernel.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/rcul=
ist_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 =3D kmem_cache_alloc(cachep);
>    lock_chain(); // typically a spin_lock()
>    obj->key =3D 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 =3D=3D
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@joelfernandes.org>

thanks,

 - Joel