On 21.05.2013 16:09, Paul E. McKenney wrote:

On Tue, May 21, 2013 at 01:05:48PM +0400, Roman Gushchin wrote:

quoted

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 <redacted>
Reported-by: Boris Zhmurov <redacted>
---
  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?

Regards,
Roman