Re: [PATCH v2] rcu: fix a race in hlist_nulls_for_each_entry_rcu macro

[Roman Gushchin <klamm@yandex-team.ru>]

On 29.05.2013 23:06, Eric Dumazet wrote:
> On Wed, 2013-05-29 at 14:09 +0400, Roman Gushchin wrote:
>
> 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.
>

Signed-off-by: Roman Gushchin <klamm@yandex-team.ru>

Many thanks to you, Paul E. McKenney and David Laight for your
patches, help and participation in this discussion.

>
> [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 <klamm@yandex-team.ru>
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Reported-by: Boris Zhmurov <zhmurov@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)))
>
>