Re: [PATCH v2 02/13] vfio/mdev: Allow the mdev_parent_ops to specify the device driver to bind

[Leon Romanovsky <leon@kernel.org>]

On Mon, Apr 26, 2021 at 05:00:04PM -0300, Jason Gunthorpe wrote:
> This allows a mdev driver to opt out of using vfio_mdev.c, instead the
> driver will provide a 'struct mdev_driver' and register directly with the
> driver core.
> 
> Much of mdev_parent_ops becomes unused in this mode:
> - create()/remove() are done via the mdev_driver probe()/remove()
> - mdev_attr_groups becomes mdev_driver driver.dev_groups
> - Wrapper function callbacks are replaced with the same ones from
>   struct vfio_device_ops
> 
> Following patches convert all the drivers.
> 
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
>  drivers/vfio/mdev/mdev_core.c   | 64 ++++++++++++++++++++++++++++-----
>  drivers/vfio/mdev/mdev_driver.c | 17 ++++++++-
>  include/linux/mdev.h            |  3 ++
>  3 files changed, 75 insertions(+), 9 deletions(-)

<...>

> +/*
> + * mdev drivers can refuse to bind during probe(), in this case we want to fail
> + * the creation of the mdev all the way back to sysfs. This is a weird model
> + * that doesn't fit in the driver core well, nor does it seem to appear any
> + * place else in the kernel, so use a simple hack.
> + */
> +static int mdev_bind_driver(struct mdev_device *mdev)
> +{
> +	struct mdev_driver *drv = mdev->type->parent->ops->device_driver;
> +	int ret;
> +
> +	if (!drv)
> +		drv = &vfio_mdev_driver;
> +
> +	while (1) {
> +		device_lock(&mdev->dev);
> +		if (mdev->dev.driver == &drv->driver) {
> +			ret = 0;
> +			goto out_unlock;
> +		}
> +		if (mdev->probe_err) {
> +			ret = mdev->probe_err;
> +			goto out_unlock;
> +		}
> +		device_unlock(&mdev->dev);
> +		ret = device_attach(&mdev->dev);

The sequence above looks sketchy:
1. lock
2. check for driver
3. unlock
4. device_attach - it takes internally same lock as in step 1.

Why don't you rely on internal to device_attach() driver check?

Thanks