From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH] rcu: fix a race in hlist_nulls_for_each_entry_rcu macro Date: Tue, 21 May 2013 05:09:07 -0700 Message-ID: <20130521120906.GD3578@linux.vnet.ibm.com> References: <519B38EC.90401@yandex-team.ru> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Dipankar Sarma , zhmurov@yandex-team.ru, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, "David S. Miller" , Eric Dumazet , Alexey Kuznetsov , James Morris , Hideaki YOSHIFUJI , Patrick McHardy To: Roman Gushchin Return-path: Content-Disposition: inline In-Reply-To: <519B38EC.90401@yandex-team.ru> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org 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 > Reported-by: Boris Zhmurov > --- > 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? Thanx, Paul > #define hlist_nulls_next_rcu(node) \ > (*((struct hlist_nulls_node __rcu __force **)&(node)->next)) > -- > 1.8.1.2 >