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 12:25:12 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	vfalico@...hat.com
Cc:	dingtianhong@...wei.com, fubar@...ibm.com, andy@...yhouse.net,
	nikolay@...hat.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next v2 1/6] bonding: simplify and use RCU
 protection for 3ad xmit path

From: Veaceslav Falico <vfalico@...hat.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.


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