> 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. > > 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 >>
