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 <kl...@yandex-team.ru>
> Reported-by: Boris Zhmurov <zhmu...@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)))
--
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