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