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

From: Roman Gushchin
Date: Tue May 21 2013 - 08:53:12 EST

On 21.05.2013 16:09, Paul E. McKenney wrote:
On Tue, May 21, 2013 at 01:05:48PM +0400, Roman Gushchin wrote:
Hi, all!

This is a fix for a problem described here: .

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 <klamm@xxxxxxxxxxxxxx>
Reported-by: Boris Zhmurov <zhmurov@xxxxxxxxxxxxxx>
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?

It will be nice, but will require to keep the old variant too (for using in hlist_nulls_add_head_rcu() as in rcu_assign_pointer() argument). Do you think, it's better?


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