netdev - Re: [PATCH net] bonding: fix packets_per

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Message-ID: <20131205124259.GE23210@redhat.com>
Date:	Thu, 5 Dec 2013 13:42:59 +0100
From:	Veaceslav Falico <vfalico@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Nikolay Aleksandrov <nikolay@...hat.com>, netdev@...r.kernel.org,
	Andy Gospodarek <andy@...yhouse.net>,
	Jay Vosburgh <fubar@...ibm.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH net] bonding: fix packets_per_slave showing

On Thu, Dec 05, 2013 at 04:33:55AM -0800, Eric Dumazet wrote:
>On Thu, 2013-12-05 at 12:08 +0100, Veaceslav Falico wrote:
>> On Thu, Dec 05, 2013 at 11:36:58AM +0100, Nikolay Aleksandrov wrote:
>> >There's an issue when showing the value of packets_per_slave due to
>> >using signed integer. The value may be < 0 and thus not put through
>> >reciprocal_value() before showing. This patch makes it use unsigned
>> >integer when showing it.
>>
>> I was already checking my basic algebra knowledge here,
>> reciprocal_value(reciprocal_value(0..USHRT_MAX)) can become negative?!? :)
>>
>> If anyone's also wondering...
>>
>> packets_per_slave is reciprocal_value(0..USHRT_MAX), and thus can indeed be
>> negative, and then the code
>>
>> 	if (packets_per_slave > 1)
>> 		packets_per_slave = reciprocal_value(packets_per_slave);
>>
>> would fail to recognise that it's a reciprocal_divide() value, and not a
>> standard 0/1 option (in bond_rr_gen_slave_id() we verify it via a switch,
>> so we're safe there) - and thus output nonsense.
>>
>> Acked-by: Veaceslav Falico <vfalico@...hat.com>
>
>This code is very confusing.
>
>Please rename bond->params.packets_per_slave
>to bond->params.reciprocal_packets_per_slave
>
>To make clear that its the reciprocal value.
>
>Also the module parameter is named packets_per_slave, it would be nice
>if same name was not reused as local variable in bond_rr_gen_slave_id()

Agreed, but I think it'd be rather net-next material. Here we have a clear
bug...

>
>bond_check_params() reads the sys value several times.
>
>This is racy with /sys access.
>
>You should use ACCESS_ONCE() to make sure nothing bad happens.

Hrm? bond_check_params() isn't involved in sysfs at all :-/, it's called
only via bonding_init(). And bond->params.packets_per_slave isn't read
there at all, only assigned.

Or, given the naming confusions, I'm again missing something?
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