On Wed, 2013-05-29 at 14:09 +0400, Roman Gushchin wrote: > Unfortunately, using barrier() can also prevent gcc from making some > (acceptable) code optimizations, because barrier() has a global effect, > and everything we want to reload is the (head->first) pointer. > So, to be absolutely precise, we have to introduce and use > the ACCESS_FIELD_ONCE() macro.
Yep, ACCESS_ONCE() seems to not work for a field, at least the ACCESS_ONCE() used in __rcu_dereference_check() > > In any case, it doesn't look like a big problem. > In my mind, the best solution is to use the ACCESS_FIELD_ONCE() macro, > but using barrier() is also an acceptable solution. > True, these lookup functions are usually structured the same around the hlist_nulls_for_each_entry_rcu() loop. A barrier() right before the loop seems to be a benefit, the size of assembly code is reduced by 48 bytes. And its one of the documented way to handle this kind of problems (Documentation/atomic_ops.txt line 114) I guess we should amend this documentation, eventually. Thanks, please add you "Signed-off-by" if you agree with the patch. [PATCH] net: force a reload of first item in hlist_nulls_for_each_entry_rcu Roman Gushchin discovered that udp4_lib_lookup2() was not reloading first item in the rcu protected list, in case the loop was restarted. This produced soft lockups as in https://lkml.org/lkml/2013/4/16/37 rcu_dereference(X)/ACCESS_ONCE(X) seem to not work as intended if X is ptr->field : In some cases, gcc caches the value or ptr->field in a register. Use a barrier() to disallow such caching, as documented in Documentation/atomic_ops.txt line 114 Thanks a lot to Roman for providing analysis and numerous patches. Diagnosed-by: Roman Gushchin <kl...@yandex-team.ru> Signed-off-by: Eric Dumazet <eduma...@google.com> Reported-by: Boris Zhmurov <zhmu...@yandex-team.ru> --- include/linux/rculist_nulls.h | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h index 2ae1371..c7557fa 100644 --- a/include/linux/rculist_nulls.h +++ b/include/linux/rculist_nulls.h @@ -105,9 +105,14 @@ static inline void hlist_nulls_add_head_rcu(struct hlist_nulls_node *n, * @head: the head for your 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/atomic_ops.txt around line 114 + * [2] Documentation/RCU/rculist_nulls.txt around line 146 */ #define hlist_nulls_for_each_entry_rcu(tpos, pos, head, member) \ - for (pos = rcu_dereference_raw(hlist_nulls_first_rcu(head)); \ + for (({barrier();}), \ + 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))) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
