PCI: Warn on missing cfg_access_lock during secondary bus reset

The recent adventure with adding lockdep tracking for cfg_access_lock,
while it yielded many false positives [1], did catch a true positive in the
pci_reset_bus() path [2].

So, while lockdep is difficult to deploy, open coding a check that
cfg_access_lock is held during the reset is feasible.

While this does not offer a full backtrace, it should be sufficient to
implicate the caller of pci_bridge_secondary_bus_reset() as a path that
needs investigation.

Link: https://lore.kernel.org/r/171711746953.1628941.4692125082286867825.stgit@dwillia2-xfh.jf.intel.com
Link: https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_134186v1/shard-dg2-1/igt@[email protected] [1]
Link: http://lore.kernel.org/r/[email protected] [2]
Signed-off-by: Dan Williams <[email protected]>
Signed-off-by: Bjorn Helgaas <[email protected]>
Tested-by: Hans de Goede <[email protected]>
Tested-by: Kalle Valo <[email protected]>
Reviewed-by: Dave Jiang <[email protected]>

drivers/pci/pci.c

@@ -4884,6 +4884,9 @@ void __weak pcibios_reset_secondary_bus(struct pci_dev *dev)
 int pci_bridge_secondary_bus_reset(struct pci_dev *dev)
 {
 	lock_map_assert_held(&dev->cfg_access_lock);
+	if (!dev->block_cfg_access)
+		pci_warn_once(dev, "unlocked secondary bus reset via: %pS\n",
+			      __builtin_return_address(0));
 	pcibios_reset_secondary_bus(dev);
 
 	return pci_bridge_wait_for_secondary_bus(dev, "bus reset");