[PATCH] cpu/hotplug: Fix NULL kobject warning in cpuhp_smt_enable()

[Jinjie Ruan]

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:

	if (WARN_ON(!kobj || (!update && !kobj->sd)))
		return -EINVAL;

Fix this by adding a check for get_cpu_device(cpu). This ensures that
SMT is only enabled for CPUs that have successfully completed their
base device registration in sysfs.

How to reproduce:

	1. echo off > /sys/devices/system/cpu/smt/control
		psci: CPU1 killed (polled 0 ms)
		psci: CPU3 killed (polled 0 ms)

	2. echo 2 > /sys/devices/system/cpu/smt/control

	Detected PIPT I-cache on CPU1
	GICv3: CPU1: found redistributor 1 region 0:0x00000000080c0000
	CPU1: Booted secondary processor 0x0000000001 [0x410fd082]
	Detected PIPT I-cache on CPU3
	GICv3: CPU3: found redistributor 3 region 0:0x0000000008100000
	CPU3: Booted secondary processor 0x0000000003 [0x410fd082]
	------------[ cut here ]------------
	WARNING: fs/sysfs/group.c:137 at internal_create_group+0x41c/0x4bc, CPU#2: sh/181
	Modules linked in:
	CPU: 2 UID: 0 PID: 181 Comm: sh Not tainted 7.0.0-rc1-00010-g8d13386c7624 #142 PREEMPT
	Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
	pstate: 20000005 (nzCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
	pc : internal_create_group+0x41c/0x4bc
	lr : sysfs_create_group+0x18/0x24
	sp : ffff80008078ba40
	x29: ffff80008078ba40 x28: ffff296c980ad000 x27: ffff00007fb94128
	x26: 0000000000000054 x25: ffffd693e845f3f0 x24: 0000000000000001
	x23: 0000000000000001 x22: 0000000000000004 x21: 0000000000000000
	x20: ffffd693e845fc10 x19: 0000000000000004 x18: 00000000ffffffff
	x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000000
	x14: 0000000000000358 x13: 0000000000000007 x12: 0000000000000350
	x11: 0000000000000008 x10: 0000000000000407 x9 : 0000000000000400
	x8 : ffff00007fbf3b60 x7 : 0000000000000000 x6 : ffffd693e845f3f0
	x5 : ffff00007fb94128 x4 : 0000000000000000 x3 : ffff000000f4eac0
	x2 : ffffd693e7095a08 x1 : 0000000000000000 x0 : 0000000000000000
	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
	 cpuhp_smt_enable+0xbc/0x114
	 control_store+0xe8/0x1d4
	 dev_attr_store+0x18/0x2c
	 sysfs_kf_write+0x7c/0x94
	 kernfs_fop_write_iter+0x128/0x1b8
	 vfs_write+0x2b0/0x354
	 ksys_write+0x68/0xfc
	 __arm64_sys_write+0x1c/0x28
	 invoke_syscall+0x48/0x10c
	 el0_svc_common.constprop.0+0x40/0xe8
	 do_el0_svc+0x20/0x2c
	 el0_svc+0x34/0x124
	 el0t_64_sync_handler+0xa0/0xe4
	 el0t_64_sync+0x198/0x19c
	---[ end trace 0000000000000000 ]---

Cc: stable@xxxxxxxxxxxxxxx
Fixes: eed4583bcf9a6 ("arm64: Kconfig: Enable HOTPLUG_SMT")
Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
---
 kernel/cpu.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index bc4f7a9ba64e..df725d92ad4f 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2706,6 +2706,13 @@ int cpuhp_smt_enable(void)
 	cpu_maps_update_begin();
 	cpu_smt_control = CPU_SMT_ENABLED;
 	for_each_present_cpu(cpu) {
+		/*
+		 * Skip CPUs that have not been registered in sysfs yet.
+		 * This avoids triggering NULL kobject warnings for maxcpus.
+		 */
+		if (!get_cpu_device(cpu))
+			continue;
+
 		/* Skip online CPUs and CPUs on offline nodes */
 		if (cpu_online(cpu) || !node_online(cpu_to_node(cpu)))
 			continue;
-- 
2.34.1