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

[Li Ming]

From:  <dan.j.williams@xxxxxxxxx>
To: "Li Ming"<ming.li@xxxxxxxxxxxx>, <dave@xxxxxxxxxxxx>, <jonathan.cameron@xxxxxxxxxx>, <dave.jiang@xxxxxxxxx>, <alison.schofield@xxxxxxxxx>, <vishal.l.verma@xxxxxxxxx>, <ira.weiny@xxxxxxxxx>, <dan.j.williams@xxxxxxxxx>
Cc: <linux-cxl@xxxxxxxxxxxxxxx>, <linux-kernel@xxxxxxxxxxxxxxx>, "Li Ming"<ming.li@xxxxxxxxxxxx>
Date: Tue, 03 Feb 2026 08:07:13 +0800
Subject: Re: [PATCH 2/2] cxl/core: Hold grandparent port lock while dport adding

 > 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.
 > 
 > Are you seeing this case fail permanently? The expectation is that the
 > one that loses the race iterates up the topology and retries.
 > 
 > So yes, you can lose this race once, but not twice is the expectation.
 > 
Hi Dan,

My understanding is that would not trigger enumeration retry, because enumerating ports flow retries the enumeration only when find_or_add_dport() returns a -EAGAIN. but the port's driver checking in cxl_port_add_dport() returns a -ENXIO, so it makes devm_cxl_enumerate_ports() failure directly.

Ming