linux-kernel - Re: Question about the barrier() in hlist_nulls_for_each_entry

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Message-ID: <CANn89iLqU=huOuCt2kXmrXf68TUU-N90aQnMykkYcZ+Arx9-aA@mail.gmail.com>
Date:   Thu, 20 Jul 2023 23:11:34 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Alan Huang <mmpgouride@...il.com>
Cc:     linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        rcu@...r.kernel.org, "Paul E. McKenney" <paulmck@...nel.org>,
        roman.gushchin@...ux.dev
Subject: Re: Question about the barrier() in hlist_nulls_for_each_entry_rcu()

On Thu, Jul 20, 2023 at 10:00 PM Alan Huang <mmpgouride@...il.com> wrote:
>
>
> > 2023年7月21日 03:22，Eric Dumazet <edumazet@...gle.com> 写道：
> >
> > On Thu, Jul 20, 2023 at 8:54 PM Alan Huang <mmpgouride@...il.com> wrote:
> >>
> >> Hi,
> >>
> >> I noticed a commit c87a124a5d5e(“net: force a reload of first item in hlist_nulls_for_each_entry_rcu”)
> >> and a related discussion [1].
> >>
> >> After reading the whole discussion, it seems like that ptr->field was cached by gcc even with the deprecated
> >> ACCESS_ONCE(), so my question is:
> >>
> >>        Is that a compiler bug? If so, has this bug been fixed today, ten years later?
> >>
> >>        What about READ_ONCE(ptr->field)?
> >
> > Make sure sparse is happy.
>
> It caused a problem without barrier(), and the deprecated ACCESS_ONCE() didn’t help:
>
>         https://lore.kernel.org/all/519D19DA.50400@yandex-team.ru/
>
> So, my real question is: With READ_ONCE(ptr->field), are there still some unusual cases where gcc
> decides not to reload ptr->field?

I can not really answer without seeing an actual patch...

Why are you asking ? Are you tracking compiler bug fixes ?

>
> >
> > Do you have a patch for review ?
>
> Possibly next month. :)
>
> >
> >
> >>
> >>
> >> [1] https://lore.kernel.org/all/1369699930.3301.494.camel@edumazet-glaptop/
> >>
> >> Thanks,
> >> Alan
>