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

[Nikolay Aleksandrov <nikolay@redhat.com>]

On 09/12/2014 04:45 PM, Eric Dumazet wrote:
> 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();
> 
> 
Ah yes, you're absolutely correct.
Then get the value of slave_cnt in a local variable using ACCESS_ONCE() + a
check for != 0 would be a better solution than adding a read barrier, what
do you think ?
Since it's not a problem to see slave_cnt > 0 when there're no slaves, but
the other way around is problematic.

> 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.
> 
Yes, I see now.

Thanks,
 Nik
> 
> 
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