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

From: Roman Gushchin
Date: Tue May 21 2013 - 11:51:22 EST

On 21.05.2013 19:16, Eric Dumazet wrote:
On Tue, 2013-05-21 at 18:47 +0400, Roman Gushchin wrote:
On 21.05.2013 17:44, Eric Dumazet wrote:
On Tue, 2013-05-21 at 05:09 -0700, Paul E. McKenney wrote:

-#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?

More exactly we have :

#define list_entry_rcu(ptr, type, member) \
({typeof (*ptr) __rcu *__ptr = (typeof (*ptr) __rcu __force *)ptr; \
container_of((typeof(ptr))rcu_dereference_raw(__ptr), type, member); \

#define list_for_each_entry_rcu(pos, head, member) \
for (pos = list_entry_rcu((head)->next, typeof(*pos), member); \
&pos->member != (head); \
pos = list_entry_rcu(pos->, typeof(*pos), member))
<< and >>

#define hlist_nulls_for_each_entry_rcu(tpos, pos, head, member) \
for (pos = rcu_dereference_raw(hlist_nulls_first_rcu(head)); \
(!is_a_nulls(pos)) && \
({ tpos = hlist_nulls_entry(pos, typeof(*tpos), member); 1; }); \
pos = rcu_dereference_raw(hlist_nulls_next_rcu(pos)))

We need to change hlist_nulls_for_each_entry_rcu() to use same construct,
so that the rcu_dereference_raw() is performed at the right place.


This code has the same mistake: it is rcu_dereference_raw(head->first),
so there is nothing that prevents gcc to store the (head->first) value
in a register.

Please read again what I wrote, you misundertood.

hlist_nulls_for_each_entry_rcu() should use same construct than
list_for_each_entry_rcu(), and not use rcu_dereference_raw()

Is that clear, or do you want me to send the patch ?

If you think, that it will solve the problem, please, send a patch. I think, you are wrong here.
If you think only that it will look better, I agree.


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