Messages in this thread | | | Subject | Re: [PATCH] rcu: fix a race in hlist_nulls_for_each_entry_rcu macro | From | Eric Dumazet <> | Date | Tue, 21 May 2013 08:16:21 -0700 |
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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->member.next, 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. > > No. > > 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 ?
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