netdev - Re: [PATCH v3 2/2] net: io_uring/zcrx: call netdev_queue_get_dma

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Message-ID: <1f708620-303f-4466-b248-3490a8e9e424@davidwei.uk>
Date: Mon, 3 Nov 2025 10:29:31 -0800
From: David Wei <dw@...idwei.uk>
To: Pavel Begunkov <asml.silence@...il.com>, io-uring@...r.kernel.org,
 netdev@...r.kernel.org
Cc: Jens Axboe <axboe@...nel.dk>, "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>
Subject: Re: [PATCH v3 2/2] net: io_uring/zcrx: call
 netdev_queue_get_dma_dev() under instance lock

On 2025-11-03 10:21, Pavel Begunkov wrote:
> On 11/3/25 17:47, David Wei wrote:
>> On 2025-11-03 05:51, Pavel Begunkov wrote:
>>> On 11/1/25 02:24, David Wei wrote:
>>>> netdev ops must be called under instance lock or rtnl_lock, but
>>>> io_register_zcrx_ifq() isn't doing this for netdev_queue_get_dma_dev().
>>>> Fix this by taking the instance lock using netdev_get_by_index_lock().
>>>>
>>>> Extended the instance lock section to include attaching a memory
>>>> provider. Could not move io_zcrx_create_area() outside, since the dmabuf
>>>> codepath IORING_ZCRX_AREA_DMABUF requires ifq->dev.
>>>
>>> It's probably fine for now, but this nested waiting feels
>>> uncomfortable considering that it could be waiting for other
>>> devices to finish IO via dmabuf fences.
>>>
>>
>> Only the dmabuf path requires ifq->dev in io_zcrx_create_area(); I could
>> split this into two and then unlock netdev instance lock between holding
>> a ref and calling net_mp_open_rxq().
>>
>> So the new ordering would be:
>>
>>    1. io_zcrx_create_area() for !IORING_ZCRX_AREA_DMABUF
>>    2. netdev_get_by_index_lock(), hold netdev ref, unlock netdev
>>    3. io_zcrx_create_area() for IORING_ZCRX_AREA_DMABUF
>>    4. net_mp_open_rxq()
> 
> To avoid dragging it on, can you do it as a follow up please? And
> it's better to avoid splitting on IORING_ZCRX_AREA_DMABUF, either it
> works for both or it doesn't at all.
> 

Of course, follow ups are always my preference.