netdev - Re: [PATCH net-next v11 10/21] net: add helpers for setting a memory provider on an rx queue

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Message-ID: <20250117180856.42514e9b@kernel.org>
Date: Fri, 17 Jan 2025 18:08:56 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Pavel Begunkov <asml.silence@...il.com>
Cc: Mina Almasry <almasrymina@...gle.com>, David Wei <dw@...idwei.uk>,
 io-uring@...r.kernel.org, netdev@...r.kernel.org, Jens Axboe
 <axboe@...nel.dk>, Paolo Abeni <pabeni@...hat.com>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jesper Dangaard
 Brouer <hawk@...nel.org>, David Ahern <dsahern@...nel.org>, Stanislav
 Fomichev <stfomichev@...il.com>, Joe Damato <jdamato@...tly.com>, Pedro
 Tammela <pctammela@...atatu.com>
Subject: Re: [PATCH net-next v11 10/21] net: add helpers for setting a
 memory provider on an rx queue

On Fri, 17 Jan 2025 23:20:58 +0000 Pavel Begunkov wrote:
> > True, so not twice, but the race is there. It's not correct to call
> > ops of a device which has already been unregistered.  
> 
> Ok, from what you're saying it's regardless of the netdev still
> having refs lingering. In this case it was better a version ago
> where io_uring was just taking the rtnl lock, which protects
> against concurrent unregistration while io_uring is checking
> netdev.

Yes, v9 didn't have this race, it just didn't release the netdev ref
correctly. Plus we plan to lift the rtnl_lock requirement on this API 
in 6.14, so the locking details best live under net/

The change I suggested to earlier should be fine.

 - If uninstall path wins it will clear and put the netdev under the
   spin lock and the close path will do nothing.

 - If the close path grabs the netdev pointer the uninstall path will 
   do nothing in io_uring, just clear the pointers in net/ side. Then 
   the close path will grab the lock in net_mp_open_rxq() see the netdev
   as unregistered, return early, put the ref.

Did I miss something?

> Does your patch below covers that?

My patch below is semi-related. It prevents us from calling 
the start/stop but we shouldn't call the alloc/free either, 
after we enter unregister. IOW this patch fixes devmem but
io_uring needs more. I think the "entry point" such as
net_mp_close_rxq() is appropriate for netdev state validation,
rather then the bowels of netdev_rx_queue_restart().

> Or does it have to be resolved in this set? I assume you're going to
> queue it as a fix.

Yes, just hoping to hear from Mina first.