netdev - Re: [PATCH] net/mlx4_en: ensure rx_desc updating reaches HW before prod db updating

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Date:   Tue, 23 Jan 2018 11:25:55 +0800
From:   "jianchao.wang" <jianchao.w.wang@...cle.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Tariq Toukan <tariqt@...lanox.com>, junxiao.bi@...cle.com,
        netdev@...r.kernel.org, linux-rdma@...r.kernel.org,
        linux-kernel@...r.kernel.org, Saeed Mahameed <saeedm@...lanox.com>
Subject: Re: [PATCH] net/mlx4_en: ensure rx_desc updating reaches HW before
 prod db updating

Hi Jason

Thanks for your kindly response.

On 01/22/2018 11:47 PM, Jason Gunthorpe wrote:
>>> Yeah, mlx4 NICs in Google fleet receive trillions of packets per
>>> second, and we never noticed an issue.
>>>
>>> Although we are using a slightly different driver, using order-0 pages
>>> and fast pages recycling.
>>>
>>>
>> The driver we use will will set the page reference count to (size of pages)/stride, the
>> pages will be freed by networking stack when the reference become zero, and the order-3
>> pages maybe allocated soon, this give NIC device a chance to corrupt the pages which have
>>  been allocated by others, such as slab.
> But it looks like the wmb() is placed when stuffing new rx descriptors
> into the device - how can it prevent corruption of pages where
> ownership was transfered from device to the host? That sounds more like a
> rmb() is missing someplace to me...
> 
The device may see the prod_db updating before rx_desc updating.
Then it will get stale rx_descs.
These stale rx_descs may contain pages that has been freed to host.
 

> (Granted the missing wmb() is a bug, but it may not be fully solving this
>  issue??)the wmb() here fix one of the customer's test case.

Thanks
Jianchao