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

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Message-ID: <Z8Xc0muOV8jtHBkX@LQ3V64L9R2>
Date: Mon, 3 Mar 2025 11:46:10 -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 Fri, Feb 28, 2025 at 06:27:59PM -0800, Jakub Kicinski wrote:
> On Thu, 27 Feb 2025 18:50:13 +0000 Joe Damato wrote:
> > @@ -2870,9 +2883,15 @@ static void refill_work(struct work_struct *work)
> >  	for (i = 0; i < vi->curr_queue_pairs; i++) {
> >  		struct receive_queue *rq = &vi->rq[i];
> >  
> > +		rtnl_lock();
> >  		virtnet_napi_disable(rq);
> > +		rtnl_unlock();
> > +
> >  		still_empty = !try_fill_recv(vi, rq, GFP_KERNEL);
> > +
> > +		rtnl_lock();
> >  		virtnet_napi_enable(rq);
> > +		rtnl_unlock();
> 
> Looks to me like refill_work is cancelled _sync while holding rtnl_lock
> from the close path. I think this could deadlock?

Good catch, thank you!

It looks like this is also the case in the failure path on
virtnet_open.

Jason: do you have any suggestions?

It looks like in both open and close disable_delayed_refill is
called first, before the cancel_delayed_work_sync.

Would something like this solve the problem?

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 76dcd65ec0f2..457115300f05 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -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];