Messages in this thread | | | Date | Tue, 21 May 2013 16:46:54 +0400 | From | Roman Gushchin <> | Subject | Re: [PATCH] rcu: fix a race in hlist_nulls_for_each_entry_rcu macro |
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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?
Regards, Roman
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