The vmd driver creates a "domain" symlink in sysfs for each VMD bridge.
Previously this symlink was created after pci_bus_add_devices() added
devices below the VMD bridge and emitted udev events to announce them to
userspace.

This led to a race between userspace consumers of the udev events and the
kernel creation of the symlink.  One such consumer is mdadm, which
assembles block devices into a RAID array, and for devices below a VMD
bridge, mdadm depends on the "domain" symlink.

If mdadm loses the race, it may be unable to assemble a RAID array, which
may cause a boot failure or other issues, with complaints like this:

  (udev-worker)[2149]: nvme1n1: '/sbin/mdadm -I /dev/nvme1n1'(err) 'mdadm: Unable to get real path for '/sys/bus/pci/drivers/vmd/0000:c7:00.5/domain/device''
  (udev-worker)[2149]: nvme1n1: '/sbin/mdadm -I /dev/nvme1n1'(err) 'mdadm: /dev/nvme1n1 is not attached to Intel(R) RAID controller.'
  (udev-worker)[2149]: nvme1n1: '/sbin/mdadm -I /dev/nvme1n1'(err) 'mdadm: No OROM/EFI properties for /dev/nvme1n1'
  (udev-worker)[2149]: nvme1n1: '/sbin/mdadm -I /dev/nvme1n1'(err) 'mdadm: no RAID superblock on /dev/nvme1n1.'
  (udev-worker)[2149]: nvme1n1: Process '/sbin/mdadm -I /dev/nvme1n1' failed with exit code 1.

This symptom prevents the OS from booting successfully.

After a NVMe disk is probed/added by the nvme driver, udevd invokes mdadm
to detect if there is a mdraid associated with this NVMe disk, and mdadm
determines if a NVMe device is connected to a particular VMD domain by
checking the "domain" symlink. For example:

  Thread A                   Thread B             Thread mdadm
  vmd_enable_domain
    pci_bus_add_devices
      __driver_probe_device
       ...
       work_on_cpu
         schedule_work_on
         : wakeup Thread B
                             nvme_probe
                             : wakeup scan_work
                               to scan nvme disk
                               and add nvme disk
                               then wakeup udevd
                                                  : udevd executes
                                                    mdadm command
         flush_work                               main
         : wait for nvme_probe done                ...
      __driver_probe_device                        find_driver_devices
      : probe next nvme device                     : 1) Detect domain symlink
      ...                                            2) Find domain symlink
      ...                                               from vmd sysfs
      ...                                            3) Domain symlink not
      ...                                               created yet; failed
    sysfs_create_link
    : create domain symlink

Create the VMD "domain" symlink before invoking pci_bus_add_devices() to
avoid this race.
Suggested-by: Adrian Huang <ahuang12@lenovo.com>
Link: https://lore.kernel.org/linux-pci/20240605124844.24293-1-sjiwei@163.comSigned-off-by: Jiwei Sun <sunjw10@lenovo.com>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
[bhelgaas: commit log]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Nirmal Patel <nirmal.patel@linux.intel.com>