Re: [PATCH 2/2] cxl/core: Hold grandparent port lock while dport adding

[Gregory Price <gourry@gourry.net>]

On Sun, Feb 01, 2026 at 05:30:02PM +0800, Li Ming wrote:
> When CXL subsystem adds a cxl port to a hierarchy, there is a small
> window where the new port becomes visible before it is bound to a
> driver. This happens because device_add() adds a device to bus device
> list before bus_probe_device() binds it to a driver.
> So if two cxl memdevs are trying to add a dport to a same port via
> devm_cxl_enumerate_ports(), the second cxl memdev may observe the port
> and attempt to add a dport, but fails because the port has not yet been
> attached to cxl port driver.
> the sequence is like:
> 
> CPU 0					CPU 1
> devm_cxl_enumerate_ports()
>   # port not found, add it
>   add_port_attach_ep()
>     # hold the parent port lock
>     # to add the new port
>     devm_cxl_create_port()
>       device_add()
> 	# Add dev to bus devs list
> 	bus_add_device()
> 					devm_cxl_enumerate_ports()
> 					# found the port
> 					find_cxl_port_by_uport()
> 					# hold port lock to add a dport
> 					device_lock(the port)
> 					find_or_add_dport()
> 					  cxl_port_add_dport()
> 					    return -ENXIO because port->dev.driver is NULL
> 					device_unlock(the port)
> 	bus_probe_device()
> 	  # hold the port lock
> 	  # for attaching
> 	  device_lock(the port)
> 	    attaching the new port
> 	  device_unlock(the port)
> 
> To fix this race, require that dport addition holds the parent port lock
> of the target port. The CXL subsystem already requires holding the
> parent port lock while attaching a new port. Therefore, successfully
> acquiring the parent port lock ganrantees that port attaching has
> completed.
> 

With just a a cursory look, I'm immediately concerned that you're fixing
a race condition with a lock inversion.

Can you guarantee the following is not happening

  Thread A         Thread B
  ----------------------------
  lock(parent)     lock(port)
  lock(port)       lock(parent)

~Gregory