> 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. > > 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
