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

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Message-ID: <DM8PR12MB54008F8D33409AFCA5878AAAAB059@DM8PR12MB5400.namprd12.prod.outlook.com>
Date:   Mon, 14 Nov 2022 08:58:19 +0000
From:   Eli Cohen <elic@...dia.com>
To:     Jason Wang <jasowang@...hat.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

> 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.

> 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().

> 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
> >