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

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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
> >> 
>