linux-kernel - Re: [PATCH 6/7] vdpa/mlx5: Avoid using reslock in event

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Date:   Wed, 23 Nov 2022 12:00:42 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Eli Cohen <elic@...dia.com>
Cc:     "mst@...hat.com" <mst@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "si-wei.liu@...cle.com" <si-wei.liu@...cle.com>,
        "eperezma@...hat.com" <eperezma@...hat.com>,
        "lulu@...hat.com" <lulu@...hat.com>
Subject: Re: [PATCH 6/7] vdpa/mlx5: Avoid using reslock in event_handler

On Mon, Nov 14, 2022 at 4:58 PM Eli Cohen <elic@...dia.com> wrote:
>
> > From: Jason Wang <jasowang@...hat.com>
> > Sent: Monday, 14 November 2022 9:53
> > To: Eli Cohen <elic@...dia.com>
> > Cc: mst@...hat.com; linux-kernel@...r.kernel.org; virtualization@...ts.linux-
> > foundation.org; si-wei.liu@...cle.com; eperezma@...hat.com;
> > lulu@...hat.com
> > Subject: Re: [PATCH 6/7] vdpa/mlx5: Avoid using reslock in event_handler
> >
> > On Sun, Nov 13, 2022 at 9:45 PM Eli Cohen <elic@...dia.com> wrote:
> > >
> > > event_handler runs under atomic context and may not acquire reslock. We
> > > can still guarantee that the handler won't be called after suspend by
> > > clearing nb_registered, unregistering the handler and flushing the
> > > workqueue.
> > >
> > > Signed-off-by: Eli Cohen <elic@...dia.com>
> > > ---
> > >  drivers/vdpa/mlx5/net/mlx5_vnet.c | 14 +++-----------
> > >  1 file changed, 3 insertions(+), 11 deletions(-)
> > >
> > > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > index 6e6490c85be2..bebfba530247 100644
> > > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > @@ -2872,8 +2872,8 @@ static int mlx5_vdpa_suspend(struct vdpa_device
> > *vdev)
> > >         int i;
> > >
> > >         down_write(&ndev->reslock);
> > > -       mlx5_notifier_unregister(mvdev->mdev, &ndev->nb);
> > >         ndev->nb_registered = false;
> > > +       mlx5_notifier_unregister(mvdev->mdev, &ndev->nb);
> >
> > I wonder why this can help anything.
> I think you were concerned that async events will come when the device was suspended. Since we can't take reslock, I think this guarantees that we won't get any events after suspension.
>

Ok, I see.

> > And if it does, we have simliar
> > logic in mlx5_vdpa_dev_del() do we need to fix that as well?
> >
> We have the same construct there only that I set nb_registered = false after unregistering the notifier. So I probably need to move it before mlx5_notifier_unregister().

Right.

Thanks

>
> > Thanks
> >
> > >         flush_workqueue(ndev->mvdev.wq);
> > >         for (i = 0; i < ndev->cur_num_vqs; i++) {
> > >                 mvq = &ndev->vqs[i];
> > > @@ -3051,7 +3051,7 @@ static void update_carrier(struct work_struct
> > *work)
> > >         else
> > >                 ndev->config.status &= cpu_to_mlx5vdpa16(mvdev,
> > ~VIRTIO_NET_S_LINK_UP);
> > >
> > > -       if (ndev->config_cb.callback)
> > > +       if (ndev->nb_registered && ndev->config_cb.callback)
> > >                 ndev->config_cb.callback(ndev->config_cb.private);
> > >
> > >         kfree(wqent);
> > > @@ -3068,21 +3068,13 @@ static int event_handler(struct notifier_block
> > *nb, unsigned long event, void *p
> > >                 switch (eqe->sub_type) {
> > >                 case MLX5_PORT_CHANGE_SUBTYPE_DOWN:
> > >                 case MLX5_PORT_CHANGE_SUBTYPE_ACTIVE:
> > > -                       down_read(&ndev->reslock);
> > > -                       if (!ndev->nb_registered) {
> > > -                               up_read(&ndev->reslock);
> > > -                               return NOTIFY_DONE;
> > > -                       }
> > >                         wqent = kzalloc(sizeof(*wqent), GFP_ATOMIC);
> > > -                       if (!wqent) {
> > > -                               up_read(&ndev->reslock);
> > > +                       if (!wqent)
> > >                                 return NOTIFY_DONE;
> > > -                       }
> > >
> > >                         wqent->mvdev = &ndev->mvdev;
> > >                         INIT_WORK(&wqent->work, update_carrier);
> > >                         queue_work(ndev->mvdev.wq, &wqent->work);
> > > -                       up_read(&ndev->reslock);
> > >                         ret = NOTIFY_OK;
> > >                         break;
> > >                 default:
> > > --
> > > 2.38.1
> > >
>