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

[Eric Dumazet <eric.dumazet@gmail.com>]

On Tue, 2013-05-21 at 22:12 +0400, Roman Gushchin wrote:
> > If other rcu accessors have the same problem, a more complete patch is
> > needed.
> 
> [PATCH] rcu: fix a race in rcu lists traverse macros
> 
> Some network functions (udp4_lib_lookup2(), for instance) use the
> rcu lists traverse macros (hlist_nulls_for_each_entry_rcu, for instance)
> 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/compiler.h      | 6 ++++++
>   include/linux/rculist.h       | 6 ++++--
>   include/linux/rculist_nulls.h | 3 ++-
>   3 files changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 92669cd..0e05d7c 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -351,6 +351,12 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
>    */
>   #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
> 
> +/*
> + * Prevent the compiler from optimizing accesses to structure member.
> + */
> +#define GET_STRUCT_FIELD_VOLATILE(struct_ptr, field)		\
> +	(((volatile typeof(*struct_ptr) *)struct_ptr)->field)
> +
>   /* Ignore/forbid kprobes attach on very low level functions marked by this attribute: */
>   #ifdef CONFIG_KPROBES
>   # define __kprobes	__attribute__((__section__(".kprobes.text")))
> diff --git a/include/linux/rculist.h b/include/linux/rculist.h
> index 8089e35..6c3eea7 100644
> --- a/include/linux/rculist.h
> +++ b/include/linux/rculist.h
> @@ -282,7 +282,8 @@ static inline void list_splice_init_rcu(struct list_head *list,
>    * as long as the traversal is guarded by rcu_read_lock().
>    */
>   #define list_for_each_entry_rcu(pos, head, member) \
> -	for (pos = list_entry_rcu((head)->next, typeof(*pos), member); \
> +	for (pos = list_entry_rcu(GET_STRUCT_FIELD_VOLATILE(head, next), \
> +				  typeof(*pos), member); \
>   		&pos->member != (head); \
>   		pos = list_entry_rcu(pos->member.next, typeof(*pos), member))
> 
> @@ -348,7 +349,8 @@ static inline void hlist_replace_rcu(struct hlist_node *old,
>   /*
>    * return the first or the next element in an RCU protected hlist
>    */
> -#define hlist_first_rcu(head)	(*((struct hlist_node __rcu **)(&(head)->first)))
> +#define hlist_first_rcu(head)	(*((struct hlist_node __rcu **)		\
> +				   (&GET_STRUCT_FIELD_VOLATILE(head, first))))
>   #define hlist_next_rcu(node)	(*((struct hlist_node __rcu **)(&(node)->next)))
>   #define hlist_pprev_rcu(node)	(*((struct hlist_node __rcu **)((node)->pprev)))
> 
> diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h
> index 2ae1371..b04fd0a 100644
> --- a/include/linux/rculist_nulls.h
> +++ b/include/linux/rculist_nulls.h
> @@ -38,7 +38,8 @@ 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))
> +	(*((struct hlist_nulls_node __rcu __force **)	\
> +	   &(GET_STRUCT_FIELD_VOLATILE(head, first))))
> 
>   #define hlist_nulls_next_rcu(node) \
>   	(*((struct hlist_nulls_node __rcu __force **)&(node)->next))

Please use ACCESS_ONCE(), which is the standard way to deal with this,
and remove the rcu_dereference_raw() in 
hlist_nulls_for_each_entry_rcu()

something like : (for the nulls part only)

diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h
index 2ae1371..1d485d3 100644
--- a/include/linux/rculist_nulls.h
+++ b/include/linux/rculist_nulls.h
@@ -40,6 +40,9 @@ 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_once(head) \
+	ACCESS_ONCE(*((struct hlist_nulls_node __rcu __force **)&(head)->first))
+
 #define hlist_nulls_next_rcu(node) \
 	(*((struct hlist_nulls_node __rcu __force **)&(node)->next))
 
@@ -107,7 +110,7 @@ static inline void hlist_nulls_add_head_rcu(struct hlist_nulls_node *n,
  *
  */
 #define hlist_nulls_for_each_entry_rcu(tpos, pos, head, member)                        \
-	for (pos = rcu_dereference_raw(hlist_nulls_first_rcu(head));            \
+	for (pos = hlist_nulls_first_rcu_once(head);                            \
 		(!is_a_nulls(pos)) &&                                           \
 		({ tpos = hlist_nulls_entry(pos, typeof(*tpos), member); 1; }); \
                pos = rcu_dereference_raw(hlist_nulls_next_rcu(pos)))