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

[Jakub Kicinski <kuba@kernel.org>]

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.