Re: [RFC PATCH v2 12/22] sound: usb: card: Introduce USB SND platform op callbacks

[Pierre-Louis Bossart]

> +int snd_usb_register_platform_ops(struct snd_usb_platform_ops *ops)
> +{
> +	if (platform_ops)
> +		return -EEXIST;
> +
> +	platform_ops = ops;
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(snd_usb_register_platform_ops);
> +
> +int snd_usb_unregister_platform_ops(void)
> +{
> +	platform_ops = NULL;
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(snd_usb_unregister_platform_ops);

I find this super-racy.

If the this function is called just before ...

>  
>  /*
>   * disconnect streams
> @@ -910,6 +928,10 @@ static int usb_audio_probe(struct usb_interface *intf,
>  	usb_set_intfdata(intf, chip);
>  	atomic_dec(&chip->active);
>  	mutex_unlock(&register_mutex);
> +
> +	if (platform_ops->connect_cb)
> +		platform_ops->connect_cb(intf, chip);
> +

... this, then you have a risk of using a dandling pointer.

You also didn't test that the platform_ops != NULL, so there's a risk of
dereferencing a NULL pointer.

Not so good, eh?

It's a classic (I've had the same sort of issues with SoundWire), when
you export ops from one driver than can be removed, then additional
protection is needed when using those callbacks.