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

[Lukas Wunner <lukas@xxxxxxxxx>]

On Thu, Oct 07, 2021 at 06:05:24PM +0200, Uwe Kleine-König wrote:
> 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,

Either that or register the controller from spi-mux asynchronously
by way of a work item.


> 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.)

No, the device core makes no such guarantee.

Thanks,

Lukas