Re: Question about the barrier() in hlist_nulls_for_each_entry_rcu()

From: Paul E. McKenney
Date: Thu Aug 03 2023 - 09:54:01 EST


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@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxx> 写道:
> >>>
> >>>
> >>>
> >>>> On Jul 20, 2023, at 4:00 PM, Alan Huang <mmpgouride@xxxxxxxxx> wrote:
> >>>>
> >>>> 
> >>>>> 2023年7月21日 03:22,Eric Dumazet <edumazet@xxxxxxxxxx> 写道:
> >>>>>
> >>>>>> On Thu, Jul 20, 2023 at 8:54 PM Alan Huang <mmpgouride@xxxxxxxxx> 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@xxxxxxxxxxxxxx/
> >>>>
> >>>> 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
>