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

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Message-ID: <Z8cXh43GJq2lolxE@LQ3V64L9R2>
Date: Tue, 4 Mar 2025 10:08:55 -0500
From: Joe Damato <jdamato@...tly.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: netdev@...r.kernel.org, mkarsten@...terloo.ca,
	gerhard@...leder-embedded.com, jasowang@...hat.com,
	xuanzhuo@...ux.alibaba.com, mst@...hat.com, leiyang@...hat.com,
	Eugenio Pérez <eperezma@...hat.com>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	"open list:VIRTIO CORE AND NET DRIVERS" <virtualization@...ts.linux.dev>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next v5 3/4] virtio-net: Map NAPIs to queues

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:
> > > > @@ -2880,6 +2880,13 @@ static void refill_work(struct work_struct *work)
> > > >         bool still_empty;
> > > >         int i;
> > > > 
> > > > +       spin_lock(&vi->refill_lock);
> > > > +       if (!vi->refill_enabled) {
> > > > +               spin_unlock(&vi->refill_lock);
> > > > +               return;
> > > > +       }
> > > > +       spin_unlock(&vi->refill_lock);
> > > > +
> > > >         for (i = 0; i < vi->curr_queue_pairs; i++) {
> > > >                 struct receive_queue *rq = &vi->rq[i];
> > > >  
> > > 
> > > Err, I suppose this also doesn't work because:
> > > 
> > > CPU0                       CPU1
> > > rtnl_lock                  (before CPU0 calls disable_delayed_refill) 
> > >   virtnet_close            refill_work
> > >                              rtnl_lock()
> > >   cancel_sync <= deadlock
> > > 
> > > Need to give this a bit more thought.  
> > 
> > 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.

Thanks for the guidance. I am happy to make an attempt at
implementing this in a future, separate series that follows this
one (probably after netdev conf in a few weeks :).

> 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?

Jason, Michael, et. al.:  what do you think ? I don't want to spin
up a v6 if you are opposed to proceeding this way. Please let me
know.