linux-stable.git/Documentation/arch/arm64/cpu-hotplug.rst, branch linux-rolling-stable Linux kernel stable tree cpu/hotplug: Fix NULL kobject warning in cpuhp_smt_enable() 2026-07-24T14:21:20+00:00 Jinjie Ruan ruanjinjie@huawei.com 2026-06-10T07:52:02+00:00 901a489d89ee9c854624c8444090e38e70aed234 commit f9a82544c7174851f5c7524622f5966dcafd3a47 upstream. On arm64, when booting with `maxcpus` greater than the number of present CPUs (e.g., QEMU -smp cpus=4,maxcpus=8), some CPUs are marked as 'present' but have not yet been registered via register_cpu(). Consequently, the per-cpu device objects for these CPUs are not yet initialized. In cpuhp_smt_enable(), the code iterates over all present CPUs. Calling _cpu_up() for these unregistered CPUs eventually leads to sysfs_create_group() being called with a NULL kobject (or a kobject without a directory), triggering the following warning in fs/sysfs/group.c: WARNING: fs/sysfs/group.c:137 at internal_create_group+0x41c/0x4bc, CPU#2: sh/181 [...] Call trace: internal_create_group+0x41c/0x4bc (P) sysfs_create_group+0x18/0x24 topology_add_dev+0x1c/0x28 cpuhp_invoke_callback+0x104/0x20c __cpuhp_invoke_callback_range+0x94/0x11c _cpu_up+0x200/0x37c When booting with ACPI, arm64 smp_prepare_cpus() currently sets all enumerated CPUs as "present" regardless of their status in the MADT. This causes issues with SMT hotplug control. For instance, with QEMU's "-smp 4,maxcpus=8" configuration, the MADT GICC entries are populated as follows: 1. The first four CPUs: `Enabled` set but `Online Capable` not set. 2. The remaining four CPUs: `Online Capable` set but `Enabled` not set to support potential hot-plugging. Fix this by: 1. When booting with ACPI, checking the ACPI_MADT_ENABLED flag in the GICC entry before calling set_cpu_present() during SMP initialization. 2. Properly managing the present mask in acpi_map_cpu() and acpi_unmap_cpu() to support actual CPU hotplug events, This aligns with other architectures like x86 and LoongArch. 3. Update the arm64 CPU hotplug documentation to no longer state that all online-capable vCPUs are marked as present by the kernel at boot time. This ensures that only physically available or explicitly enabled CPUs are in the present mask, keeping the SMT control logic consistent with the actual hardware state. Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Jonathan Cameron <jic23@kernel.org> Cc: James Morse <james.morse@arm.com> Cc: Yicong Yang <yangyicong@hisilicon.com> Cc: stable@vger.kernel.org Link: https://uefi.org/specs/ACPI/6.5/05_ACPI_Software_Programming_Model.html#gic-cpu-interface-gicc-structure Fixes: eed4583bcf9a ("arm64: Kconfig: Enable HOTPLUG_SMT") Reviewed-by: Catalin Marinas <catalin.marinas@arm.com> Suggested-by: Catalin Marinas <catalin.marinas@arm.com> Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com> Signed-off-by: Will Deacon <will@kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> 
commit f9a82544c7174851f5c7524622f5966dcafd3a47 upstream.

On arm64, when booting with `maxcpus` greater than the number of present
CPUs (e.g., QEMU -smp cpus=4,maxcpus=8), some CPUs are marked as 'present'
but have not yet been registered via register_cpu(). Consequently,
the per-cpu device objects for these CPUs are not yet initialized.

In cpuhp_smt_enable(), the code iterates over all present CPUs. Calling
_cpu_up() for these unregistered CPUs eventually leads to
sysfs_create_group() being called with a NULL kobject (or a kobject
without a directory), triggering the following warning in
fs/sysfs/group.c:

  WARNING: fs/sysfs/group.c:137 at internal_create_group+0x41c/0x4bc, CPU#2: sh/181
  [...]
  Call trace:
    internal_create_group+0x41c/0x4bc (P)
    sysfs_create_group+0x18/0x24
    topology_add_dev+0x1c/0x28
    cpuhp_invoke_callback+0x104/0x20c
    __cpuhp_invoke_callback_range+0x94/0x11c
    _cpu_up+0x200/0x37c

When booting with ACPI, arm64 smp_prepare_cpus() currently sets all
enumerated CPUs as "present" regardless of their status in the MADT. This
causes issues with SMT hotplug control. For instance, with QEMU's
"-smp 4,maxcpus=8" configuration, the MADT GICC entries are populated as
follows:

1. The first four CPUs: `Enabled` set but `Online Capable` not set.

2. The remaining four CPUs: `Online Capable` set but `Enabled` not set
   to support potential hot-plugging.

Fix this by:

1. When booting with ACPI, checking the ACPI_MADT_ENABLED flag in the GICC
   entry before calling set_cpu_present() during SMP initialization.

2. Properly managing the present mask in acpi_map_cpu() and
   acpi_unmap_cpu() to support actual CPU hotplug events, This aligns with
   other architectures like x86 and LoongArch.

3. Update the arm64 CPU hotplug documentation to no longer state that all
   online-capable vCPUs are marked as present by the kernel at boot time.

This ensures that only physically available or explicitly enabled CPUs
are in the present mask, keeping the SMT control logic consistent with
the actual hardware state.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: James Morse <james.morse@arm.com>
Cc: Yicong Yang <yangyicong@hisilicon.com>
Cc: stable@vger.kernel.org
Link: https://uefi.org/specs/ACPI/6.5/05_ACPI_Software_Programming_Model.html#gic-cpu-interface-gicc-structure
Fixes: eed4583bcf9a ("arm64: Kconfig: Enable HOTPLUG_SMT")
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Documentation: Fix spelling mistakes 2024-09-05T20:35:45+00:00 Amit Vadhavana av2082000@gmail.com 2024-08-17T07:27:24+00:00 4538480b27a94522ff6432b2d728fafc74f61829 Correct spelling mistakes in the documentation to improve readability. Signed-off-by: Amit Vadhavana <av2082000@gmail.com> Reviewed-by: Bjorn Helgaas <bhelgaas@google.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net> Link: https://lore.kernel.org/r/20240817072724.6861-1-av2082000@gmail.com 
Correct spelling mistakes in the documentation to improve readability.

Signed-off-by: Amit Vadhavana <av2082000@gmail.com>
Reviewed-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240817072724.6861-1-av2082000@gmail.com
arm64: document virtual CPU hotplug's expectations 2024-06-28T17:38:32+00:00 James Morse james.morse@arm.com 2024-05-29T13:34:45+00:00 828ce929d1c3aff0e9a330fea40d29980e17e256 Add a description of physical and virtual CPU hotplug, explain the differences and elaborate on what is required in ACPI for a working virtual hotplug system. Signed-off-by: James Morse <james.morse@arm.com> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com> Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk> Tested-by: Miguel Luis <miguel.luis@oracle.com> Reviewed-by: Gavin Shan <gshan@redhat.com> Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com> Link: https://lore.kernel.org/r/20240529133446.28446-19-Jonathan.Cameron@huawei.com Signed-off-by: Catalin Marinas <catalin.marinas@arm.com> 
Add a description of physical and virtual CPU hotplug, explain the
differences and elaborate on what is required in ACPI for a working
virtual hotplug system.

Signed-off-by: James Morse <james.morse@arm.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Tested-by: Miguel Luis <miguel.luis@oracle.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20240529133446.28446-19-Jonathan.Cameron@huawei.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>