Re: Fw: [Bug 199995] New: Ramdomly sent TCP Reset from Kernel with bonding mode "brodcast"

[Eric Dumazet <eric.dumazet@gmail.com>]

On 06/08/2018 02:04 PM, Michal Kubecek wrote:
> On Fri, Jun 08, 2018 at 09:59:54AM -0700, Stephen Hemminger wrote:
>>
>> https://bugzilla.kernel.org/show_bug.cgi?id=199995
>>
>>             Bug ID: 199995
>>            Summary: Ramdomly sent TCP Reset from Kernel with bonding mode
>>                     "brodcast"
>>
>> after a dist upgrade from Ubuntu 17.10 (Kernel 4.13.x) to Ubuntu 18.04 (Kernel
>> 4.15.0) I suffer from ramdomly generated TCP RST packets sent (presumably) by
>> the Kernel 
>> on a bonding device that uses bonding mode "brodcast" with 2 physical NICs.
>>
>> With tcpdump/whireshark I can see that the kernel randomly sends TCP-RST
>> packets after the SYN/ACK/ACK packet is received (see attached PCAP).
>> This only happens if the kernel receives the initial SYN packet on both
>> physical NICs (and therefore seeing it twice), before the connection is
>> established by sending SYN/ACK.
>> It's not happening in 100% of all cases and only, if the system can use two or
>> more CPU cores/threads. With only one CPU available to the system, this
>> behaviour is not reproducable.
> 
> I have seen similar report earlier from one of our customers running
> SLE12 SP2 (kernel 4.4). The problem is that if duplicated SYN packet is
> received on both slaves, these two copies can be processed by the
> lockless listener simultaneously on different CPUs and each can reply by
> SYNACK with different sequence number which results in a reset.
> 
> I tried to think of a way to prevent this race without losing the
> performance gain of lockless listener but couldn't come with anything.
> Eventually, I managed to persuade the customer that this setup (where
> each packet is received twice under normal circumstances) is not what
> broadcast mode was designed for (based on the description in
> Documentation/networking/bonding.txt).
> 
> However, the lockless listener was introduced in 4.4 so it's not clear
> why reporter started encountering this after an upgrade from 4.13 to
> 4.15.

Yes, I do not buy this at all.

If two identical SYN are received by two cpus, we should create one SYN_RECV and send
two SYNACK.

But it is a bit hard to test this :/

I will take a look, thanks.