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

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Date:	Tue, 21 May 2013 16:46:54 +0400
From:	Roman Gushchin <klamm@...dex-team.ru>
To:	paulmck@...ux.vnet.ibm.com
CC:	Dipankar Sarma <dipankar@...ibm.com>, zhmurov@...dex-team.ru,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	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 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 <klamm@...dex-team.ru>
>> Reported-by: Boris Zhmurov <zhmurov@...dex-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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