PCI: pciehp: Avoid unnecessary device replacement check

In the Linux kernel, the following vulnerability has been resolved:

PCI: pciehp: Avoid unnecessary device replacement check

Hot-removal of nested PCI hotplug ports suffers from a long-standing race

condition which can lead to a deadlock: A parent hotplug port acquires

pci_lock_rescan_remove(), then waits for pciehp to unbind from a child

hotplug port. Meanwhile that child hotplug port tries to acquire

pci_lock_rescan_remove() as well in order to remove its own children.

The deadlock only occurs if the parent acquires pci_lock_rescan_remove()

first, not if the child happens to acquire it first.

Several workarounds to avoid the issue have been proposed and discarded

over the years, e.g.:

https://lore.kernel.org/r/4c882e25194ba8282b78fe963fec8faae7cf23eb.1529173804.git.lukas@wunner.de/

A proper fix is being worked on, but needs more time as it is nontrivial

and necessarily intrusive.

Recent commit 9d573d19547b ("PCI: pciehp: Detect device replacement during

system sleep") provokes more frequent occurrence of the deadlock when

removing more than one Thunderbolt device during system sleep. The commit

sought to detect device replacement, but also triggered on device removal.

Differentiating reliably between replacement and removal is impossible

because pci_get_dsn() returns 0 both if the device was removed, as well as

if it was replaced with one lacking a Device Serial Number.

Avoid the more frequent occurrence of the deadlock by checking whether the

hotplug port itself was hot-removed. If so, there's no sense in checking

whether its child device was replaced.

This works because the ->resume_noirq() callback is invoked in top-down

order for the entire hierarchy: A parent hotplug port detecting device

replacement (or removal) marks all children as removed using

pci_dev_set_disconnected() and a child hotplug port can then reliably

detect being removed.