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 >
