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

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Message-ID: <bf86abde-6a90-4ea0-a298-abe5b367f4f9@paulmck-laptop>
Date:   Thu, 3 Aug 2023 06:53:54 -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 Thu, Aug 03, 2023 at 09:40:11PM +0800, Alan Huang wrote:
> 
> > 2023年8月1日 上午4:09，Paul E. McKenney <paulmck@...nel.org> 写道：
> > 
> > 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.
> 
> And unless the compiler has a bug. :) 
> 
> So, the barrier() in hlist_nulls_for_each_entry_rcu() is a workaround for a compiler bug.

Fair enough!!!  ;-)


							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
>