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From:   SeongJae Park <sj@kernel.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     SeongJae Park <sj@kernel.org>, paulmck@kernel.org, corbet@lwn.net,
        rcu@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 4/4] Docs/RCU/rculist_nulls: Drop unnecessary '_release' in insert function
Date:   Fri,  9 Jun 2023 19:12:06 +0000
Message-Id: <20230609191206.30465-1-sj@kernel.org>
In-Reply-To: <CAEXW_YQFqW2QcAuHZEhc_GaUaB-=QOS0WgUOizd=FYwtFQ8vag@mail.gmail.com>
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On Fri, 19 May 2023 14:52:50 -0400 Joel Fernandes <joel@joelfernandes.org> wrote:

> On Thu, May 18, 2023 at 6:40 PM 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/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()?

Paul, may I ask your opinion?


Thanks,
SJ

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