Re: [PATCH v2] vduse: Fix race condition between resetting and irq injecting

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Oct 13, 2021 at 08:30:40PM +0800, Yongji Xie wrote:
> On Wed, Oct 13, 2021 at 7:10 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Wed, Sep 29, 2021 at 04:30:50PM +0800, Xie Yongji wrote:
> > > The interrupt might be triggered after a reset since there is
> > > no synchronization between resetting and irq injecting.
> >
> > In fact, irq_lock is already used to synchronize with
> > irqs. Why isn't taking and releasing it enough?
> >
> 
> For example:
> 
> CPU 0
>                   CPU1
> ---------
>                       --------
> vduse_dev_ioctl()
>   check DRIVER_OK
> 
>                         vduse_dev_reset()
> 
>                           flush_work(&vq->inject);
>     queue_work(vduse_irq_wq, &vq->inject);
> 
>                         virtio_vdpa_probe()
> 
>                           virtio_vdpa_find_vqs()
>    vduse_vq_irq_inject()
>      vq->cb.callback(vq->cb.private);
> 
>                         set DRIVER_OK
> 
> In the above case, the irq callback is still triggered before DRIVER_OK is set.
> 
> But now I found it seems to be better to just check DRIVER_OK again in
> vduse_vq_irq_inject().

And then pesumably make sure each time we set status
it's done under the irq lock?

> > > And it
> > > might break something if the interrupt is delayed until a new
> > > round of device initialization.
> > >
> > > Fixes: c8a6153b6c59 ("vduse: Introduce VDUSE - vDPA Device in Userspace")
> > > Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
> > > ---
> > >  drivers/vdpa/vdpa_user/vduse_dev.c | 37 +++++++++++++++++++++++++------------
> > >  1 file changed, 25 insertions(+), 12 deletions(-)
> > >
> > > diff --git a/drivers/vdpa/vdpa_user/vduse_dev.c b/drivers/vdpa/vdpa_user/vduse_dev.c
> > > index cefb301b2ee4..841667a896dd 100644
> > > --- a/drivers/vdpa/vdpa_user/vduse_dev.c
> > > +++ b/drivers/vdpa/vdpa_user/vduse_dev.c
> > > @@ -80,6 +80,7 @@ struct vduse_dev {
> > >       struct vdpa_callback config_cb;
> > >       struct work_struct inject;
> > >       spinlock_t irq_lock;
> > > +     struct rw_semaphore rwsem;
> > >       int minor;
> > >       bool broken;
> > >       bool connected;
> >
> > What does this lock protect? Use a more descriptive name pls,
> > and maybe add a comment.
> >
> 
> This lock is used to ensure there is no more inflight irq kwork after reset.
> 
> >
> > > @@ -410,6 +411,8 @@ static void vduse_dev_reset(struct vduse_dev *dev)
> > >       if (domain->bounce_map)
> > >               vduse_domain_reset_bounce_map(domain);
> > >
> > > +     down_write(&dev->rwsem);
> > > +
> > >       dev->status = 0;
> > >       dev->driver_features = 0;
> > >       dev->generation++;
> > > @@ -443,6 +446,8 @@ static void vduse_dev_reset(struct vduse_dev *dev)
> > >               flush_work(&vq->inject);
> > >               flush_work(&vq->kick);
> > >       }
> > > +
> > > +     up_write(&dev->rwsem);
> > >  }
> > >
> > >  static int vduse_vdpa_set_vq_address(struct vdpa_device *vdpa, u16 idx,
> > > @@ -885,6 +890,23 @@ static void vduse_vq_irq_inject(struct work_struct *work)
> > >       spin_unlock_irq(&vq->irq_lock);
> > >  }
> > >
> > > +static int vduse_dev_queue_irq_work(struct vduse_dev *dev,
> > > +                                 struct work_struct *irq_work)
> > > +{
> > > +     int ret = -EINVAL;
> > > +
> > > +     down_read(&dev->rwsem);
> > > +     if (!(dev->status & VIRTIO_CONFIG_S_DRIVER_OK))
> > > +             goto unlock;
> > > +
> > > +     ret = 0;
> > > +     queue_work(vduse_irq_wq, irq_work);
> > > +unlock:
> > > +     up_read(&dev->rwsem);
> > > +
> > > +     return ret;
> > > +}
> > > +
> > >  static long vduse_dev_ioctl(struct file *file, unsigned int cmd,
> > >                           unsigned long arg)
> > >  {
> >
> >
> > so that's a lot of overhead for an irq.
> > Normally the way to address races like this is to add
> > flushing to the reset path, not locking to irq path.
> >
> 
> Yes, we already call flush_work() in the reset path.
> 
> Thanks,
> Yongji

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