netdev - Re: [PATCH v2 3/5] io_uring/zcrx: share an ifq between rings

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Message-ID: <fdf328c5-c933-4b9c-a683-c910c6fc16a4@davidwei.uk>
Date: Sun, 26 Oct 2025 08:06:55 -0700
From: David Wei <dw@...idwei.uk>
To: Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org,
 netdev@...r.kernel.org
Cc: Pavel Begunkov <asml.silence@...il.com>
Subject: Re: [PATCH v2 3/5] io_uring/zcrx: share an ifq between rings

On 2025-10-26 06:16, Jens Axboe wrote:
> On 10/25/25 10:12 PM, David Wei wrote:
>> Sorry I missed this during the splitting. Will include in v3.
>>
>>>
>>>> +    ifq->proxy = src_ifq;
>>>
>>> For this, since the ifq is shared and reference counted, why don't they
>>> just point at the same memory here? Would avoid having this ->proxy
>>> thing and just skipping to that in other spots where the actual
>>> io_zcrx_ifq is required?
>>>
>>
>> I wanted a way to separate src and dst rings, while also decrementing
>> refcounts once and only once. I used separate ifq objects to do this,
>> but having learnt about xarray marks, I think I can use that instead.
> 
> I'm confused, why do you even need that? You already have
> ifq->proxy which is just a "link" to the shared queue, why aren't both
> rings just using the same ifq structure? You already increment the
> refcount when you add proxy, why can't the new ring just store the same
> ifq?
> 

The main reason is I want only-once semantics for decrementing the
refcounts. I used a separate ifq to do this, marking it with -1
afterwards, but I learnt about xarray marks which lets me do the same
thing.

Hopefully v3 will make it clearer what I mean. It's looking really
clean.