[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <43d29007-3c59-4497-a1e5-26f182a7f4c5@paulmck-laptop>
Date: Mon, 31 Jul 2023 13:09:53 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Alan Huang <mmpgouride@...il.com>
Cc: Joel Fernandes <joel@...lfernandes.org>,
Eric Dumazet <edumazet@...gle.com>,
linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
rcu@...r.kernel.org, roman.gushchin@...ux.dev
Subject: Re: Question about the barrier() in hlist_nulls_for_each_entry_rcu()
On Fri, Jul 21, 2023 at 10:27:04PM +0800, Alan Huang wrote:
>
> > 2023年7月21日 20:54,Joel Fernandes <joel@...lfernandes.org> 写道:
> >
> >
> >
> >> On Jul 20, 2023, at 4:00 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 am a bit doubtful there will be strong (any?) interest in replacing the barrier() with READ_ONCE() without any tangible reason, regardless of whether a gcc issue was fixed.
> >
> > But hey, if you want to float the idea…
>
> We already had the READ_ONCE() in rcu_deference_raw().
>
> The barrier() here makes me think we need write code like below:
>
> READ_ONCE(head->first);
> barrier();
> READ_ONCE(head->first);
>
> With READ_ONCE (or the deprecated ACCESS_ONCE),
> I don’t think a compiler should cache the value of head->first.
Apologies for the late reply!
If both are READ_ONCE(), you should not need the barrier(). Unless there
is some other code not shown in your example that requires it, that is.
Thanx, Paul
> > Thanks,
> >
> > - Joel
> >
> >>
> >>>
> >>> Do you have a patch for review ?
> >>
> >> Possibly next month. :)
> >>
> >>>
> >>>
> >>>>
> >>>>
> >>>> [1] https://lore.kernel.org/all/1369699930.3301.494.camel@edumazet-glaptop/
> >>>>
> >>>> Thanks,
> >>>> Alan
> >>
>
Powered by blists - more mailing lists