Conceptual bug on SoundWire probe/remove?

[Pierre-Louis Bossart]

Hi,
I could use feedback/guidance on a possible conceptual bug in the 
SoundWire probe and bus handling.

When we probe a driver, the code does this:

static int sdw_drv_probe(struct device *dev)
{
	struct sdw_slave *slave = dev_to_sdw_dev(dev);
	struct sdw_driver *drv = drv_to_sdw_driver(dev->driver);
	const struct sdw_device_id *id;
	const char *name;
	int ret;

	/*
	 * fw description is mandatory to bind
	 */
	if (!dev->fwnode)
		return -ENODEV;

	if (!IS_ENABLED(CONFIG_ACPI) && !dev->of_node)
		return -ENODEV;

	id = sdw_get_device_id(slave, drv);
	if (!id)
		return -ENODEV;

	slave->ops = drv->ops;

The last line is the problematic one. If at some point, the user does an 
rmmod and unbinds the SoundWire codec driver, the .remove will be called 
and the 'drv' will no longer be valid, but we will still have a 
reference to drv->ops and use that pointer in the bus code, e.g.

		/* Update the Slave driver */
		if (slave_notify && slave->ops &&
		    slave->ops->interrupt_callback) {
			slave_intr.sdca_cascade = sdca_cascade;
			slave_intr.control_port = clear;
			memcpy(slave_intr.port, &port_status,
			       sizeof(slave_intr.port));

			slave->ops->interrupt_callback(slave,
&slave_intr);
		}

I noodled with a potential fix in
https://github.com/thesofproject/linux/pull/3534/commits/82d64fb0fd39b532263f060a8ec86e47e9ab305b

where I force-reset this slave->ops pointer, but it is likely to be very 
racy.

We probably need to avoid such references, or have a clean mechanism to 
unbind, e.g. with all commands and interrupts stopped while the codec 
driver .remove routine is handled.

Initial error reports at https://github.com/thesofproject/linux/issues/3531

Suggestions and comments welcome, thanks!
-Pierre