Re: [PATCH 2/2] virtio_pmem: set device ready in probe()

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

On Wed, Jun 22, 2022 at 03:24:15PM +0800, Jason Wang wrote:
> On Wed, Jun 22, 2022 at 2:29 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Tue, Jun 21, 2022 at 03:38:35PM -0700, Dan Williams wrote:
> > > Jason Wang wrote:
> > > > The NVDIMM region could be available before the virtio_device_ready()
> > > > that is called by virtio_dev_probe(). This means the driver tries to
> > > > use device before DRIVER_OK which violates the spec, fixing this by
> > > > set device ready before the nvdimm_pmem_region_create().
> > >
> > > Can you clarify the failure path. What race is virtio_device_ready()
> > > losing?
> > >
> > > >
> > > > Note that this means the virtio_pmem_host_ack() could be triggered
> > > > before the creation of the nd region, this is safe since the
> > > > virtio_pmem_host_ack() since pmem_lock has been initialized and we
> > > > check if we've added any buffer before trying to proceed.
> > >
> > > I got a little bit lost with the usage of "we" here. Can you clarify
> > > which function / context is making which guarantee?
> > >
> > > >
> > > > Fixes 6e84200c0a29 ("virtio-pmem: Add virtio pmem driver")
> > > > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > > > ---
> > > >  drivers/nvdimm/virtio_pmem.c | 12 ++++++++++++
> > > >  1 file changed, 12 insertions(+)
> > > >
> > > > diff --git a/drivers/nvdimm/virtio_pmem.c b/drivers/nvdimm/virtio_pmem.c
> > > > index 48f8327d0431..173f2f5adaea 100644
> > > > --- a/drivers/nvdimm/virtio_pmem.c
> > > > +++ b/drivers/nvdimm/virtio_pmem.c
> > > > @@ -84,6 +84,17 @@ static int virtio_pmem_probe(struct virtio_device *vdev)
> > > >     ndr_desc.provider_data = vdev;
> > > >     set_bit(ND_REGION_PAGEMAP, &ndr_desc.flags);
> > > >     set_bit(ND_REGION_ASYNC, &ndr_desc.flags);
> > > > +   /*
> > > > +    * The NVDIMM region could be available before the
> > > > +    * virtio_device_ready() that is called by
> > > > +    * virtio_dev_probe(), so we set device ready here.
> > > > +    *
> > > > +    * The callback - virtio_pmem_host_ack() is safe to be called
> > > > +    * before the nvdimm_pmem_region_create() since the pmem_lock
> > > > +    * has been initialized and legality of a used buffer is
> > > > +    * validated before moving forward.
> > >
> > > This comment feels like changelog material. Just document why
> > > virtio_device_ready() must be called before device_add() of the
> > > nd_region.
> >
> > Agree here. More specifically if you are documenting why is it
> > safe to invoke each callback then that belongs to the callback itself.
> 
> Ok, so I will move it to the callback and leave a simple comment like
> 
> " See comment in virtio_pmem_host_ack(), it is safe to be called
> before nvdimm_pmem_region_create()"
> 
> Thanks

No, just document why virtio_device_ready() must be called before device_add()

I don't think the idea of working around these issues by adding code
to  virtio_device_ready worked so far, not at all sure this approach
is here to stay.


> >
> > > > +    */
> > > > +   virtio_device_ready(vdev);
> > > >     nd_region = nvdimm_pmem_region_create(vpmem->nvdimm_bus, &ndr_desc);
> > > >     if (!nd_region) {
> > > >             dev_err(&vdev->dev, "failed to create nvdimm region\n");
> > > > @@ -92,6 +103,7 @@ static int virtio_pmem_probe(struct virtio_device *vdev)
> > > >     }
> > > >     return 0;
> > > >  out_nd:
> > > > +   virtio_reset_device(vdev);
> > > >     nvdimm_bus_unregister(vpmem->nvdimm_bus);
> > > >  out_vq:
> > > >     vdev->config->del_vqs(vdev);
> > > > --
> > > > 2.25.1
> > > >
> >