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

From: "Paul E. McKenney" <[email-masked]>
Date: Tue, 21 May 2013 05:09:07 -0700

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 <[email-masked]>
> Reported-by: Boris Zhmurov <[email-masked]>
> ---
>  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
>