This is a note to let you know that I've just added the patch titled arch/x86: Handle non enumerated CPU after physical hotplug to the 4.8-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: arch-x86-handle-non-enumerated-cpu-after-physical-hotplug.patch and it can be found in the queue-4.8 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 2a51fe083eba7f99cbda72f5ef90cdf2f4df882c Mon Sep 17 00:00:00 2001 From: Prarit Bhargava <prarit@xxxxxxxxxx> Date: Mon, 3 Oct 2016 13:07:12 -0400 Subject: arch/x86: Handle non enumerated CPU after physical hotplug From: Prarit Bhargava <prarit@xxxxxxxxxx> commit 2a51fe083eba7f99cbda72f5ef90cdf2f4df882c upstream. When a CPU is physically added to a system then the MADT table is not updated. If subsequently a kdump kernel is started on that physically added CPU then the ACPI enumeration fails to provide the information for this CPU which is now the boot CPU of the kdump kernel. As a consequence, generic_processor_info() is not invoked for that CPU so the number of enumerated processors is 0 and none of the initializations, including the logical package id management, are performed. We have code which relies on the correctness of the logical package map and other information which is initialized via generic_processor_info(). Executing such code will result in undefined behaviour or kernel crashes. This problem applies only to the kdump kernel because a normal kexec will switch to the original boot CPU, which is enumerated in MADT, before jumping into the kexec kernel. The boot code already has a check for num_processors equal 0 in prefill_possible_map(). We can use that check as an indicator that the enumeration of the boot CPU did not happen and invoke generic_processor_info() for it. That initializes the relevant data for the boot CPU and therefore prevents subsequent failure. [ tglx: Refined the code and rewrote the changelog ] Signed-off-by: Prarit Bhargava <prarit@xxxxxxxxxx> Fixes: 1f12e32f4cd5 ("x86/topology: Create logical package id") Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Len Brown <len.brown@xxxxxxxxx> Cc: Borislav Petkov <bp@xxxxxxx> Cc: Andi Kleen <ak@xxxxxxxxxxxxxxx> Cc: Jiri Olsa <jolsa@xxxxxxxxxx> Cc: Juergen Gross <jgross@xxxxxxxx> Cc: dyoung@xxxxxxxxxx Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx> Cc: kexec@xxxxxxxxxxxxxxxxxxx Link: http://lkml.kernel.org/r/1475514432-27682-1-git-send-email-prarit@xxxxxxxxxx Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- arch/x86/kernel/smpboot.c | 18 +++++++++++++++--- 1 file changed, 15 insertions(+), 3 deletions(-) --- a/arch/x86/kernel/smpboot.c +++ b/arch/x86/kernel/smpboot.c @@ -1406,9 +1406,21 @@ __init void prefill_possible_map(void) { int i, possible; - /* no processor from mptable or madt */ - if (!num_processors) - num_processors = 1; + /* No boot processor was found in mptable or ACPI MADT */ + if (!num_processors) { + int apicid = boot_cpu_physical_apicid; + int cpu = hard_smp_processor_id(); + + pr_warn("Boot CPU (id %d) not listed by BIOS\n", cpu); + + /* Make sure boot cpu is enumerated */ + if (apic->cpu_present_to_apicid(0) == BAD_APICID && + apic->apic_id_valid(apicid)) + generic_processor_info(apicid, boot_cpu_apic_version); + + if (!num_processors) + num_processors = 1; + } i = setup_max_cpus ?: 1; if (setup_possible_cpus == -1) { Patches currently in stable-queue which might be from prarit@xxxxxxxxxx are queue-4.8/arch-x86-handle-non-enumerated-cpu-after-physical-hotplug.patch -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html