Re: Deadlock in spi_add_device() -- device core problem

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, Oct 07, 2021 at 06:05:24PM +0200, Uwe Kleine-König wrote:
> Hello,
> 
> On an ARM board with an spi-mux I observe a deadlock during boot. This
> happens because spi_add_device() (in drivers/spi/spi.c) for the spi-mux
> device calls device_add() while holding &spi_add_lock. The spi-mux
> driver's probe routine than creates another controller and for the
> devices on that bus spi_add_device() is called again, trying to grab
> &spi_add_lock again.
> 
> The easy workaround would be to replace &spi_add_lock with a
> per-controller lock, but I have the expectation that it should be
> possible to not hold a lock while calling device_add().

No, busses should not be adding new devices to the same bus from within
a probe function.

Drivers for a bus bind to the bus, they should not be creating new
devices for that same bus, as that does not seem correct.

> The difficulty (and the reason that this lock exists) is that it should
> be prevented that more than one device is added for a any chipselect.
> My first guess was, this check isn't necessary, because the devices are
> called $busname.$chipselect and a second device on the same bus with the
> same chipselect would fail when device_add() tries to create the
> duplicate name. However for devices instanciated by ACPI the naming is
> different (see spi_dev_set_name(), and commit b6fb8d3a1f15). Also there
> is a comment "We need to make sure there's no other device with this
> chipselect **BEFORE** we call setup(), else we'll trash its
> configuration." A problem there might be that spi_setup() might toggle
> the chipselect line (which is a problem if the chipselect polarity
> isn't the same for the two devices, or the earlier device is currently
> busy). Anything else?
> 
> There is also a check:
> 
> 	if (IS_ENABLED(CONFIG_SPI_DYNAMIC) &&
> 	    !device_is_registered(&ctlr->dev)) {
> 		return -NODEV;
> 	}
> 
> which catches the race that the controller (which is also the device's
> parent) is about to go while we try to add a new device to it. Is this a
> real problem? (The spi_unregister_controller() function unregisters all
> children which might race with adding a new child without locking. This
> check was implemented by Lukas Wunner (on Cc:) in commit ddf75be47ca7.)
> Doesn't all children of a given device are unregistered automatically by
> the device core somehow when said device is unregistered? (I checked,
> but didn't find anything.)
> 
> Does this all sound about right? Greg, do you have a recommendation how
> to properly fix this problem?

Don't allow it :)

Since when do spi devices find new spi devices on their same bus?  This
feels very wrong...

thanks,

greg k-h