[PATCH] rcu: fix a race in hlist_nulls_for_each_entry_rcu macro

[Roman Gushchin]

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 <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))

 #define hlist_nulls_next_rcu(node) \
 	(*((struct hlist_nulls_node __rcu __force **)&(node)->next))
--
1.8.1.2
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