Messages in this thread | | | Date | Tue, 21 May 2013 05:58:56 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] rcu: fix a race in hlist_nulls_for_each_entry_rcu macro |
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On Tue, May 21, 2013 at 04:46:54PM +0400, Roman Gushchin wrote: > On 21.05.2013 16:09, Paul E. McKenney wrote: > >On Tue, May 21, 2013 at 01:05:48PM +0400, Roman Gushchin wrote: > >>Hi, all! > >> > >>This is a fix for a problem described here: > >>https://lkml.org/lkml/2013/4/16/371 . > >>--- > >> > >>Some network functions (udp4_lib_lookup2(), for instance) use the > >>hlist_nulls_for_each_entry_rcu macro in a way that assumes restarting > >>of a loop. In this case, it is strictly necessary to reread the head->first > >>value from the memory before each scan. > >>Without additional hints, gcc caches this value in a register. In this case, > >>if a cached node is moved to another chain during the scan, we can loop > >>forever getting wrong nulls values and restarting the loop uninterruptedly. > >> > >>Signed-off-by: Roman Gushchin <klamm@yandex-team.ru> > >>Reported-by: Boris Zhmurov <zhmurov@yandex-team.ru> > >>--- > >> include/linux/rculist_nulls.h | 5 +++-- > >> 1 file changed, 3 insertions(+), 2 deletions(-) > >> > >>diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h > >>index 2ae1371..efd51bf 100644 > >>--- a/include/linux/rculist_nulls.h > >>+++ b/include/linux/rculist_nulls.h > >>@@ -37,8 +37,9 @@ static inline void hlist_nulls_del_init_rcu(struct > >>hlist_nulls_node *n) > >> } > >> } > >> > >>-#define hlist_nulls_first_rcu(head) \ > >>- (*((struct hlist_nulls_node __rcu __force **)&(head)->first)) > >>+#define hlist_nulls_first_rcu(head) \ > >>+ (*((struct hlist_nulls_node __rcu __force **) \ > >>+ &((volatile typeof(*head) *)head)->first)) > > > >Why not use ACCESS_ONCE() or (better) rcu_dereference_raw() here? > > It will be nice, but will require to keep the old variant too (for > using in hlist_nulls_add_head_rcu() as in rcu_assign_pointer() > argument). Do you think, it's better?
Both ACCESS_ONCE() and rcu_dereference_raw() can be used by updaters as well as readers, so yes, I do think that it is better. Better to keep the encapsulation rather than having to search for lots of volatile casts should this idiom ever need to change.
Thanx, Paul
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