netdev - Re: [PATCH V2] virtio-net: fix the race between refill work and close

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Date:   Thu, 30 Jun 2022 14:07:50 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     mst <mst@...hat.com>, davem <davem@...emloft.net>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        netdev <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2] virtio-net: fix the race between refill work and close

On Thu, Jun 30, 2022 at 10:51 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Thu, 30 Jun 2022 10:08:04 +0800 Jason Wang wrote:
> > +static void enable_refill_work(struct virtnet_info *vi)
> > +{
> > +     spin_lock(&vi->refill_lock);
> > +     vi->refill_work_enabled = true;
> > +     spin_unlock(&vi->refill_lock);
> > +}
> > +
> > +static void disable_refill_work(struct virtnet_info *vi)
> > +{
> > +     spin_lock(&vi->refill_lock);
> > +     vi->refill_work_enabled = false;
> > +     spin_unlock(&vi->refill_lock);
> > +}
> > +
> >  static void virtqueue_napi_schedule(struct napi_struct *napi,
> >                                   struct virtqueue *vq)
> >  {
> > @@ -1527,8 +1547,12 @@ static int virtnet_receive(struct receive_queue *rq, int budget,
> >       }
> >
> >       if (rq->vq->num_free > min((unsigned int)budget, virtqueue_get_vring_size(rq->vq)) / 2) {
> > -             if (!try_fill_recv(vi, rq, GFP_ATOMIC))
> > -                     schedule_delayed_work(&vi->refill, 0);
> > +             if (!try_fill_recv(vi, rq, GFP_ATOMIC)) {
> > +                     spin_lock(&vi->refill_lock);
> > +                     if (vi->refill_work_enabled)
> > +                             schedule_delayed_work(&vi->refill, 0);
> > +                     spin_unlock(&vi->refill_lock);
>
> Are you sure you can use the basic spin_lock() flavor in all cases?
> Isn't the disable/enable called from a different context than this
> thing here?

This function will only be called in bh so it's safe.

>
> The entire delayed work construct seems a little risky because the work
> may go to sleep after disabling napi, causing large latency spikes.

Yes, but it only happens on OOM.

> I guess you must have a good reason no to simply reschedule the NAPI
> and keep retrying with GFP_ATOMIC...

Less pressure on the memory allocator on OOM probably, but it looks
like an independent issue that might be optimized in the future.

>
> Please add the target tree name to the subject.

Ok

Thanks

>