netdev - Re: [PATCH net-next v2 1/6] bonding: simplify and use RCU protection for 3ad xmit path

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Date:	Wed, 04 Sep 2013 22:53:20 +0800
From:	Ding Tianhong <dthxman@...il.com>
To:	Veaceslav Falico <vfalico@...hat.com>
CC:	Ding Tianhong <dingtianhong@...wei.com>,
	Jay Vosburgh <fubar@...ibm.com>,
	Andy Gospodarek <andy@...yhouse.net>,
	"David S. Miller" <davem@...emloft.net>,
	Nikolay Aleksandrov <nikolay@...hat.com>,
	Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next v2 1/6] bonding: simplify and use RCU protection
 for 3ad xmit path

于 2013/9/4 18:18, Veaceslav Falico 写道:
> On Wed, Sep 04, 2013 at 05:43:45PM +0800, Ding Tianhong wrote:
> ...snip...
>> +/**
>> + * IMPORTANT: bond_first/last_slave_rcu can return NULL in case of 
>> an empty list
>> + * Caller must hold rcu_read_lock
>> + */
>> +#define bond_first_slave_rcu(bond) \
>> + list_first_or_null_rcu(&(bond)->slave_list, struct slave, list)
>> +#define bond_last_slave_rcu(bond) \
>> + (list_empty(&(bond)->slave_list) ? NULL : \
>> + bond_to_slave_rcu((bond)->slave_list.prev))
>
> Here, bond_last_slave_rcu() is racy. The list can be non-empty when
> list_empty() is verified, however afterwards it might become empty, when
> you call bond_to_slave_rcu(), and thus you'll get
> bond_to_slave(bond->slave_list) in the result, which is not a slave.
>
> Take a look at list_first_or_null_rcu() for a reference. The main idea is
> that it first gets the ->next pointer, with RCU protection, and then
> verifies if it's the list head or not, and if not - it gets the container
> already. This way the ->next pointer won't get away.
>
> These kind of bugs are really rare, but are *EXTREMELY* hard to debug.
Thanks for your response and opinions, but I think your miss something,
the slave_list will not changed in the rcu_read_lock, so ,the bugs will not
happen.

>
>> +
>> #define bond_is_first_slave(bond, pos) ((pos)->list.prev == 
>> &(bond)->slave_list)
>> #define bond_is_last_slave(bond, pos) ((pos)->list.next == 
>> &(bond)->slave_list)
>>
>> @@ -93,6 +106,15 @@
>> (bond_is_first_slave(bond, pos) ? bond_last_slave(bond) : \
>> bond_to_slave((pos)->list.prev))
>>
>> +/* Since bond_first/last_slave_rcu can return NULL, these can return 
>> NULL too */
>> +#define bond_next_slave_rcu(bond, pos) \
>> + (bond_is_last_slave(bond, pos) ? bond_first_slave_rcu(bond) : \
>> + bond_to_slave_rcu((pos)->list.next))
>> +
>> +#define bond_prev_slave_rcu(bond, pos) \
>> + (bond_is_first_slave(bond, pos) ? bond_last_slave_rcu(bond) : \
>> + bond_to_slave_rcu((pos)->list.prev))
>> +
>
> These two are also racy. bond_is_last/first_slave() is not rcu-ified, and
> thus you can't rely on it without proper locking. Same ideas apply as per
> bond_first_slave_rcu().
> -- 
refer to the above answer.

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