Re: [PATCH net-next v5 3/4] virtio-net: Map NAPIs to queues

[Joe Damato <jdamato@fastly.com>]

On Mon, Mar 03, 2025 at 04:03:55PM -0800, Jakub Kicinski wrote:
> On Mon, 3 Mar 2025 13:33:10 -0500 Joe Damato wrote:
> > 
> > How about we don't use the API at all from refill_work?
> > 
> > Patch 4 adds consistent NAPI config state and refill_work isn't a
> > queue resize maybe we don't need to call the netif_queue_set_napi at
> > all since the NAPI IDs are persisted in the NAPI config state and
> > refill_work shouldn't change that?
> > 
> > In which case, we could go back to what refill_work was doing
> > before and avoid the problem entirely.
> > 
> > What do you think ?
> 
> Should work, I think. Tho, I suspect someone will want to add queue API
> support to virtio sooner or later, and they will run into the same
> problem with the netdev instance lock, as all of ndo_close() will then
> be covered with netdev->lock.
> 
> More thorough and idiomatic way to solve the problem would be to cancel
> the work non-sync in ndo_close, add cancel with _sync after netdev is
> unregistered (in virtnet_remove()) when the lock is no longer held, then
> wrap the entire work with a relevant lock and check if netif_running()
> to return early in case of a race.
> 
> Middle ground would be to do what you suggested above and just leave 
> a well worded comment somewhere that will show up in diffs adding queue
> API support?

Seems like Jason agrees that leaving refill_work unmodified will
work [1].

I think leaving a comment is a good idea and am happy to do so. Not
sure where would be a good spot for it.

Two spots that come to mind are:
 - in virtnet_probe where all the other netdev ops are plumbed
   through, or
 - above virtnet_disable_queue_pair which I assume a future queue
   API implementor would need to call for ndo_queue_stop

I get the feeling you have a much better suggestion in mind though
:)

[1]: https://lore.kernel.org/netdev/CACGkMEvWuRjBbc3PvOUpDFkjcby5QNLw5hA_FpNSPyWjkEXD_Q@mail.gmail.com/