On Thu, Feb 23, 2023 at 01:53:45PM -0600, Bjorn Helgaas wrote: > Hmm. Good question. Off the top of my head, I can't explain the > difference between pci_rescan_remove_lock and pci_bus_sem, so I'm > confused, too. I added Lukas in case he has a ready explanation. pci_bus_sem is a global lock which protects the "devices" list of all pci_bus structs. We do have a bunch of places left where the "devices" list is accessed without holding pci_bus_sem, though I've tried to slowly eliminate them. pci_rescan_remove_lock is a global "big kernel lock" which serializes any device addition and removal. pci_rescan_remove_lock is known to be far too course-grained and thus deadlock-prone, particularly if hotplug ports are nested (as is the case with Thunderbolt). It needs to be split up into several smaller locks which protect e.g. allocation of resources of a bus (bus numbers or MMIO / IO space) and whatever else needs to be protected. It's just that nobody has gotten around to identify what exactly needs to be protected, adding the new locks and removing pci_rescan_remove_lock. Thanks, Lukas
