From 9fb13f7e3023941beb41536f399c065acdb4a1a6 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Wed, 29 May 2013 09:06:27 +0000 Subject: [PATCH 1/1] net: force a reload of first item in hlist_nulls_for_each_entry_rcu [ Upstream commit c87a124a5d5e8cf8e21c4363c3372bcaf53ea190 ] 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 Signed-off-by: Eric Dumazet Reported-by: Boris Zhmurov Signed-off-by: Roman Gushchin Acked-by: Paul E. McKenney Signed-off-by: David S. Miller Signed-off-by: Ben Hutchings --- 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 2ae13714828b..1c33dd7da4a7 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))) -- 2.39.2