Urgent ACPI Revert For Linux 6.18 To Deal With Some Hardware Crashing

The Linux 6.18 kernel is anticipated for release this coming Sunday while this week a last-minute crisis was averted following reports of a kernel crash from recent ACPI code changes.

Borislav Petkov of AMD reported on Monday with the latest development code he was hitting a null pointer dereference within the ACPI code and in turn a crash at boot. This was noticed on an old AMD Phenom II era system with MSI MS-7599 motherboard.

This was tracked down to a commit made back during the Linux 6.18 merge window to optimize the ACPI idle driver registration:

"Currently, the ACPI idle driver is registered from within a CPU hotplug callback. Although this didn't cause any functional issues, this is questionable and confusing. And it is better to register the cpuidle driver when all of the CPUs have been brought up.

So add a new function to initialize acpi_idle_driver based on the power management information of an available CPU and register cpuidle driver in acpi_processor_driver_init()."

While trying to address "questionable and confusing" code, for at least some hardware this ends up causing the kernel crash and thus is being reverted ahead of Linux 6.18 final.

Linux power management subsystem maintainer Rafael Wysocki today sent out the urgent ACPI pull to revert this problematic code:

"This reverts a commit that attempted to make the code in the ACPI processor driver more straightforward, but it turned out to cause the kernel to crash on at least one system, along with some further cleanups on top of it."

The entire scope isn't known beyond the old AMD system where it was discovered to happen, but at least this ACPI revert is now set to happen just ahead of what should be the Linux 6.18 stable release on Sunday.