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. 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 > >> >
