netdev - Re: [PATCH net] bonding: fix div by zero while enslaving and transmitting

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Date:	Fri, 12 Sep 2014 07:45:52 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Nikolay Aleksandrov <nikolay@...hat.com>
Cc:	netdev@...r.kernel.org, Jiri Pirko <jiri@...nulli.us>,
	Andy Gospodarek <andy@...yhouse.net>,
	Jay Vosburgh <j.vosburgh@...il.com>,
	Veaceslav Falico <vfalico@...il.com>
Subject: Re: [PATCH net] bonding: fix div by zero while enslaving and
 transmitting

On Fri, 2014-09-12 at 15:33 +0200, Nikolay Aleksandrov wrote:

> One more thing, netdev_master_upper_dev_link_private() which is called
> after the increment uses list_add_rcu() (i.e. rcu_assign_pointer) to insert
> the slave, so there's a barrier there to ensure this is visible before the
> slave is linked.

You missed my point.

You fixed the writer side, without adding barriers on the read side.

Without looking at the code, just reading your patch, I spot a problem.

Following code is fundamentally broken :

rcu_read_lock();

if (bond->list) {
    x = y % bond->slave;  // bug : cpu could fetch bond->slave before  bond->list

rcu_read_unlock();


Because it needs a read barrier, since the writer does after your patch:

	update bond->slave
	smp_wmb();
	update bond->list

I repeat : adding few rcu_read_lock() / rcu_read_unlock() /
rcu_assign_pointer() calls is not enough to guarantee code is not
broken.



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