netdev - Re: [PATCH net-next 3/3] virtio

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Message-ID: <Z4WSfER6O6n3hxXh@LQ3V64L9R2>
Date: Mon, 13 Jan 2025 14:23:56 -0800
From: Joe Damato <jdamato@...tly.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Jason Wang <jasowang@...hat.com>, netdev@...r.kernel.org,
	mkarsten@...terloo.ca, "Michael S. Tsirkin" <mst@...hat.com>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.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 3/3] virtio_net: Map NAPIs to queues

On Mon, Jan 13, 2025 at 02:04:46PM -0800, Jakub Kicinski wrote:
> On Mon, 13 Jan 2025 09:30:20 -0800 Joe Damato wrote:
> > > >  static void virtnet_napi_enable_lock(struct virtqueue *vq,
> > > > -                                    struct napi_struct *napi)
> > > > +                                    struct napi_struct *napi,
> > > > +                                    bool need_rtnl)
> > > >  {
> > > > +       struct virtnet_info *vi = vq->vdev->priv;
> > > > +       int q = vq2rxq(vq);
> > > > +
> > > >         virtnet_napi_do_enable(vq, napi);
> > > > +
> > > > +       if (q < vi->curr_queue_pairs) {
> > > > +               if (need_rtnl)
> > > > +                       rtnl_lock();  
> > > 
> > > Can we tweak the caller to call rtnl_lock() instead to avoid this trick?  
> > 
> > The major problem is that if the caller calls rtnl_lock() before
> > calling virtnet_napi_enable_lock, then virtnet_napi_do_enable (and
> > thus napi_enable) happen under the lock.
> > 
> > Jakub mentioned in a recent change [1] that napi_enable may soon
> > need to sleep.
> > 
> > Given the above constraints, the only way to avoid the "need_rtnl"
> > would be to refactor the code much more, placing calls (or wrappers)
> > to netif_queue_set_napi in many locations.
> > 
> > IMHO: This implementation seemed cleaner than putting calls to
> > netif_queue_set_napi throughout the driver.
> > 
> > Please let me know how you'd like to proceed on this.
> > 
> > [1]: https://lore.kernel.org/netdev/20250111024742.3680902-1-kuba@kernel.org/
> 
> I'm going to make netif_queue_set_napi() take netdev->lock, and remove
> the rtnl_lock requirement ~this week. If we need conditional locking
> perhaps we're better off waiting?

That seems reasonable to me and I can wait.

Please CC me on that series so I can take a look and I'll adjust the
v2 of this series to avoid the locking once your series is merged.

Thanks!