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 <kl...@yandex-team.ru>
> Reported-by: Boris Zhmurov <zhmu...@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?
Thanx, Paul
> #define hlist_nulls_next_rcu(node) \
> (*((struct hlist_nulls_node __rcu __force **)&(node)->next))
> --
> 1.8.1.2
>
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