Re: [PATCH 6/6] rust: driver: drop device private data post unbind

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Jan 07, 2026 at 11:35:05AM +0100, Danilo Krummrich wrote:
> @@ -548,6 +548,10 @@ static DEVICE_ATTR_RW(state_synced);
>  static void device_unbind_cleanup(struct device *dev)
>  {
>  	devres_release_all(dev);
> +#ifdef CONFIG_RUST

Nit, let's not put #ifdef in .c files, the overhead of an empty pointer
for all drivers is not a big deal.

> +	if (dev->driver->p_cb.post_unbind)
> +		dev->driver->p_cb.post_unbind(dev);
> +#endif
>  	arch_teardown_dma_ops(dev);
>  	kfree(dev->dma_range_map);
>  	dev->dma_range_map = NULL;
> diff --git a/include/linux/device/driver.h b/include/linux/device/driver.h
> index cd8e0f0a634b..51a9ebdd8a2d 100644
> --- a/include/linux/device/driver.h
> +++ b/include/linux/device/driver.h
> @@ -85,6 +85,8 @@ enum probe_type {
>   *		uevent.
>   * @p:		Driver core's private data, no one other than the driver
>   *		core can touch this.
> + * @p_cb:	Callbacks private to the driver core; no one other than the
> + *		driver core is allowed to touch this.
>   *
>   * The device driver-model tracks all of the drivers known to the system.
>   * The main reason for this tracking is to enable the driver core to match
> @@ -119,6 +121,15 @@ struct device_driver {
>  	void (*coredump) (struct device *dev);
>  
>  	struct driver_private *p;
> +#ifdef CONFIG_RUST

Again, no #ifdef.

> +	struct {
> +		/*
> +		 * Called after remove() and after all devres entries have been
> +		 * processed.
> +		 */
> +		void (*post_unbind)(struct device *dev);

post_unbind_rust_only()?

> -impl<T: RegistrationOps> Registration<T> {
> +impl<T: RegistrationOps + 'static> Registration<T> {
> +    extern "C" fn post_unbind_callback(dev: *mut bindings::device) {
> +        // SAFETY: The driver core only ever calls the post unbind callback with a valid pointer to
> +        // a `struct device`.
> +        //
> +        // INVARIANT: `dev` is valid for the duration of the `post_unbind_callback()`.
> +        let dev = unsafe { &*dev.cast::<device::Device<device::CoreInternal>>() };
> +
> +        // `remove()` and all devres callbacks have been completed at this point, hence drop the
> +        // driver's device private data.
> +        //
> +        // SAFETY: By the safety requirements of the `Driver` trait, `T::DriverData` is the
> +        // driver's device private data.
> +        drop(unsafe { dev.drvdata_obtain::<T::DriverData>() });

I don't mind this, but why don't we also do this for all C drivers?
Just null out the pointer at this point in time so that no one can touch
it, just like you are doing here (in a way.)

thanks,

greg k-h