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

From: Ding Tianhong <[email-masked]>
Date: Thu, 5 Sep 2013 10:06:03 +0800

On 2013/9/5 0:25, David Miller wrote:
> From: Veaceslav Falico <[email-masked]>
> Date: Wed, 4 Sep 2013 12:18:24 +0200
> 
>> 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.
> 
> I agree with this analysis.
> 
> Ding, "rcu_read_lock()" doesn't "lock" anything.  It's just a memory
> barrier.
> 
> All the list can still change on you asynchronously to your accesses.
> 
> That's why list_first_or_null_rcu() is so carefully arranged.
> Therefore, you must make similar accomodations.
> 
> 
> 

yes, after a long time thinking, I found the problem and know how to do next, repair and resend it later.

> .