> 2023年7月21日 05:11，Eric Dumazet <edumazet@xxxxxxxxxx> 写道： > > On Thu, Jul 20, 2023 at 10:00 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 can not really answer without seeing an actual patch... The content of the potential patch: diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h index 89186c499dd4..bcd39670f359 100644 --- a/include/linux/rculist_nulls.h +++ b/include/linux/rculist_nulls.h @@ -158,15 +158,9 @@ static inline void hlist_nulls_add_fake(struct hlist_nulls_node *n) * @pos: the &struct hlist_nulls_node to use as a loop cursor. * @head: the head of the list. * @member: the name of the hlist_nulls_node within the struct. - * - * The barrier() is needed to make sure compiler doesn't cache first element [1], - * as this loop can be restarted [2] - * [1] Documentation/memory-barriers.txt around line 1533 - * [2] Documentation/RCU/rculist_nulls.rst around line 146 */ #define hlist_nulls_for_each_entry_rcu(tpos, pos, head, member) \ - for (({barrier();}), \ - pos = rcu_dereference_raw(hlist_nulls_first_rcu(head)); \ + for (pos = rcu_dereference_raw(hlist_nulls_first_rcu(head)); \ (!is_a_nulls(pos)) && \ ({ tpos = hlist_nulls_entry(pos, typeof(*tpos), member); 1; }); \ pos = rcu_dereference_raw(hlist_nulls_next_rcu(pos))) @@ -180,8 +174,7 @@ static inline void hlist_nulls_add_fake(struct hlist_nulls_node *n) * @member: the name of the hlist_nulls_node within the struct. */ #define hlist_nulls_for_each_entry_safe(tpos, pos, head, member) \ - for (({barrier();}), \ - pos = rcu_dereference_raw(hlist_nulls_first_rcu(head)); \ + for (pos = rcu_dereference_raw(hlist_nulls_first_rcu(head)); \ (!is_a_nulls(pos)) && \ ({ tpos = hlist_nulls_entry(pos, typeof(*tpos), member); \ pos = rcu_dereference_raw(hlist_nulls_next_rcu(pos)); 1; });) > > Why are you asking ? Are you tracking compiler bug fixes ? The barrier() here makes me confused. If we really need that, do we need: READ_ONCE(head->first); barrier(); READ_ONCE(head->first); ? > >> >>> >>> Do you have a patch for review ? >> >> Possibly next month. :) >> >>> >>> >>>> >>>> >>>> [1] https://lore.kernel.org/all/1369699930.3301.494.camel@edumazet-glaptop/ >>>> >>>> Thanks, >>>> Alan
