Re: [PATCH v2] net/ixgbe: add proper memory barriers for some Rx functions

[zhoumin <zhoumin@loongson.cn>]

Hi Morten,

On Thur, May 4, 2023 at 9:21PM, Morten Brørup wrote:
>> From: zhoumin [mailto:zhoumin@loongson.cn]
>> Sent: Thursday, 4 May 2023 15.17
>>
>> Hi Konstantin,
>>
>> Thanks for your  comments.
>>
>> On 2023/5/1 下午9:29, Konstantin Ananyev wrote:
>>>> Segmentation fault has been observed while running the
>>>> ixgbe_recv_pkts_lro() function to receive packets on the Loongson 3C5000
>>>> processor which has 64 cores and 4 NUMA nodes.
>>>>
>>>>  From the ixgbe_recv_pkts_lro() function, we found that as long as the
>>>> first
>>>> packet has the EOP bit set, and the length of this packet is less
>>>> than or
>>>> equal to rxq->crc_len, the segmentation fault will definitely happen
>>>> even
>>>> though on the other platforms, such as X86.
>>>>
>>>> Because when processd the first packet the first_seg->next will be
>>>> NULL, if
>>>> at the same time this packet has the EOP bit set and its length is less
>>>> than or equal to rxq->crc_len, the following loop will be excecuted:
>>>>
>>>>      for (lp = first_seg; lp->next != rxm; lp = lp->next)
>>>>          ;
>>>>
>>>> We know that the first_seg->next will be NULL under this condition.
>>>> So the
>>>> expression of lp->next->next will cause the segmentation fault.
>>>>
>>>> Normally, the length of the first packet with EOP bit set will be
>>>> greater
>>>> than rxq->crc_len. However, the out-of-order execution of CPU may
>>>> make the
>>>> read ordering of the status and the rest of the descriptor fields in
>>>> this
>>>> function not be correct. The related codes are as following:
>>>>
>>>>          rxdp = &rx_ring[rx_id];
>>>>   #1     staterr = rte_le_to_cpu_32(rxdp->wb.upper.status_error);
>>>>
>>>>          if (!(staterr & IXGBE_RXDADV_STAT_DD))
>>>>              break;
>>>>
>>>>   #2     rxd = *rxdp;
>>>>
>>>> The sentence #2 may be executed before sentence #1. This action is
>>>> likely
>>>> to make the ready packet zero length. If the packet is the first
>>>> packet and
>>>> has the EOP bit set, the above segmentation fault will happen.
>>>>
>>>> So, we should add rte_rmb() to ensure the read ordering be correct.
>>>> We also
>>>> did the same thing in the ixgbe_recv_pkts() function to make the rxd
>>>> data
>>>> be valid even thougth we did not find segmentation fault in this
>>>> function.
>>>>
>>>> Signed-off-by: Min Zhou <zhoumin@loongson.cn>
>>>> ---
>>>> v2:
>>>> - Make the calling of rte_rmb() for all platforms
>>>> ---
>>>>   drivers/net/ixgbe/ixgbe_rxtx.c | 3 +++
>>>>   1 file changed, 3 insertions(+)
>>>>
>>>> diff --git a/drivers/net/ixgbe/ixgbe_rxtx.c
>>>> b/drivers/net/ixgbe/ixgbe_rxtx.c
>>>> index c9d6ca9efe..302a5ab7ff 100644
>>>> --- a/drivers/net/ixgbe/ixgbe_rxtx.c
>>>> +++ b/drivers/net/ixgbe/ixgbe_rxtx.c
>>>> @@ -1823,6 +1823,8 @@ ixgbe_recv_pkts(void *rx_queue, struct rte_mbuf
>>>> **rx_pkts,
>>>>           staterr = rxdp->wb.upper.status_error;
>>>>           if (!(staterr & rte_cpu_to_le_32(IXGBE_RXDADV_STAT_DD)))
>>>>               break;
>>>> +
>>>> +        rte_rmb();
>>>>           rxd = *rxdp;
>>>
>>>
>>> Indeed, looks like a problem to me on systems with relaxed MO.
>>> Strange that it was never hit on arm or ppc - cc-ing ARM/PPC maintainers.
>> The LoongArch architecture uses the Weak Consistency model which can
>> cause the problem, especially in scenario with many cores, such as
>> Loongson 3C5000 with four NUMA node, which has 64 cores. I cannot
>> reproduce it on Loongson 3C5000 with one NUMA node, which just has 16 cores.
>>> About a fix - looks right, but a bit excessive to me -
>>> as I understand all we need here is to prevent re-ordering by CPU itself.
>> Yes, thanks for cc-ing.
>>> So rte_smp_rmb() seems enough here.
>>> Or might be just:
>>> staterr = __atomic_load_n(&rxdp->wb.upper.status_error,
>>> __ATOMIC_ACQUIRE);
>>>
>> Does __atomic_load_n() work on Windows if we use it to solve this problem ?
> Yes, __atomic_load_n() works on Windows too.
>
Thank you, Morten. I got it.

I will compare those barriers and choose a proper one for this problem.


Best regards,

Min