Re: [PATCH] spi: Fix spi device unregister flow

From: Saravana Kannan
Date: Mon May 03 2021 - 13:22:40 EST

On Mon, May 3, 2021 at 3:07 AM Lukas Wunner <lukas@xxxxxxxxx> wrote:
> On Mon, Apr 26, 2021 at 04:56:38PM -0700, Saravana Kannan wrote:
> > When an SPI device is unregistered, the spi->controller->cleanup() is
> > called in the device's release callback. That's wrong for a couple of
> > reasons:
> >
> > 1. spi_dev_put() can be called before spi_add_device() is called. And
> > it's spi_add_device() that calls spi_setup(). This will cause clean()
> > to get called without the spi device ever being setup.
> Well, yes, but it's not a big problem in practice so far:
> I've checked all drivers and there are only four which are affected
> by this: spi-mpc512x-psc.c spi-pic32.c spi-s3c64xx.c spi-st-ssc4.c
> They all fiddle with the chipselect GPIO in their ->cleanup hook
> and the GPIO may not have been requested yet because that happens
> during ->setup.
> All the other drivers merely invoke kzalloc() on ->setup and kfree()
> on ->cleanup. The order doesn't matter in this case because
> kfree(NULL) is a no-op.

That's making a lot of assumptions about drivers not doing certain
things in the future or making assumptions about the hardware (chip
select or whatever other configuration that might happen). Totally
unnecessary and error prone.

> > 2. There's no guarantee that the controller's driver would be present by
> > the time the spi device's release function gets called.
> How so? spi_devices are instantiated on ->probe of the controller
> via spi_register_controller() and destroyed on ->remove via
> spi_unregister_controller(). I don't see how the controller driver
> could ever be unavailable, so this point seems moot.

Just because put_device() is called on a struct device doesn't mean
it's getting destroyed immediately. The refcount needs to reach zero
for ->cleanup() to be called eventually. And there's no guarantee that
by the time the ref count hits zero that your controller driver is
still around. So, it's not a moot point.

> > Fix these issues by simply moving the cleanup from the device release
> > callback to the actual spi_unregister_device() function.
> Unfortunately the fix is wrong, it introduces a new problem:
> > @@ -713,6 +717,8 @@ void spi_unregister_device(struct spi_device *spi)
> > if (!spi)
> > return;
> >
> > + spi_cleanup(spi);
> > +
> > if (spi->dev.of_node) {
> > of_node_clear_flag(spi->dev.of_node, OF_POPULATED);
> > of_node_put(spi->dev.of_node);
> Now you're running ->cleanup before the SPI slave's driver is unbound.

By "slave" device, you mean struct spi_device, right?

Sorry if I'm mistaken about my understanding of the SPI framework.
Please explain how that's happening here. The main place
spi_unregister_device() is getting called from is
spi_controller_unregister(). If the controller's child/slave
spi_device's aren't unbound by then, you've got bigger problems even
without my patch?

> That's bad, the driver may need to access the physical device on unbound,
> e.g. to quiesce interrupts. That may not work now because the
> slave's controller_state is gone.
> NAK, this needs to be reverted.

Please help me understand how this is broken. It's not clear to me.