From: Ding Tianhong <dingtianhong@huawei•com>
To: David Miller <davem@davemloft•net>
Cc: <vfalico@redhat•com>, <fubar@us•ibm.com>, <andy@greyhouse•net>,
<nikolay@redhat•com>, <netdev@vger•kernel.org>
Subject: Re: [PATCH net-next v2 1/6] bonding: simplify and use RCU protection for 3ad xmit path
Date: Thu, 5 Sep 2013 10:06:03 +0800 [thread overview]
Message-ID: <5227E70B.5040407@huawei.com> (raw)
In-Reply-To: <20130904.122512.1633906087065495330.davem@davemloft.net>
On 2013/9/5 0:25, David Miller wrote:
> From: Veaceslav Falico <vfalico@redhat•com>
> 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.
> .
>
prev parent reply other threads:[~2013-09-05 2:06 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-09-04 9:43 [PATCH net-next v2 1/6] bonding: simplify and use RCU protection for 3ad xmit path Ding Tianhong
2013-09-04 10:18 ` Veaceslav Falico
2013-09-04 14:53 ` Ding Tianhong
2013-09-04 16:25 ` David Miller
2013-09-05 2:06 ` Ding Tianhong [this message]
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