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

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

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.

> > +	 */
> > +	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
> >