netdev - Re: [PATCH] rcu: fix a race in hlist_nulls_for_each_entry

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Date:	Tue, 21 May 2013 08:16:21 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Roman Gushchin <klamm@...dex-team.ru>
Cc:	paulmck@...ux.vnet.ibm.com, Dipankar Sarma <dipankar@...ibm.com>,
	zhmurov@...dex-team.ru, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>
Subject: Re: [PATCH] rcu: fix a race in hlist_nulls_for_each_entry_rcu macro

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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