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

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Message-ID: <7805c473-448a-430c-a53b-a42e8d2c24bf@gmail.com>
Date: Mon, 3 Nov 2025 18:21:26 +0000
From: Pavel Begunkov <asml.silence@...il.com>
To: David Wei <dw@...idwei.uk>, 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 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.

-- 
Pavel Begunkov