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SubjectRe: [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.


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