Re: [tip:sched/core] [sched/smp] 06ddd17521: BUG:using_smp_processor_id()in_preemptible

["Chen, Yu C" <yu.c.chen@intel.com>]

On 7/10/2025 10:58 AM, kernel test robot wrote:
> 
> 
> Hello,
> 
> kernel test robot noticed "BUG:using_smp_processor_id()in_preemptible" on:
> 
> commit: 06ddd17521bf11a3e7f59dafdf5c148f29467d2c ("sched/smp: Always define is_percpu_thread() and scheduler_ipi()")
> https://git.kernel.org/cgit/linux/kernel/git/tip/tip.git sched/core
> 
> [test failed on linux-next/master 835244aba90de290b4b0b1fa92b6734f3ee7b3d9]
> 
> in testcase: boot
> 
> config: x86_64-randconfig-123-20250702
> compiler: gcc-12
> test machine: qemu-system-x86_64 -enable-kvm -cpu SandyBridge -smp 2 -m 16G
> 
> (please refer to attached dmesg/kmsg for entire log/backtrace)
> 
> 
> +----------------------------------------------------------+------------+------------+
> |                                                          | cac5cefbad | 06ddd17521 |
> +----------------------------------------------------------+------------+------------+
> | boot_successes                                           | 20         | 0          |
> | boot_failures                                            | 0          | 18         |
> | BUG:using_smp_processor_id()in_preemptible               | 0          | 18         |
> | BUG:using__this_cpu_write()in_preemptible[#]code:swapper | 0          | 18         |
> | BUG:using__this_cpu_read()in_preemptible                 | 0          | 18         |
> +----------------------------------------------------------+------------+------------+
> 
> 
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <oliver.sang@intel.com>
> | Closes: https://lore.kernel.org/oe-lkp/202507100448.6b88d6f1-lkp@intel.com
> 
> 
> [   25.232998][    T1] BUG: using smp_processor_id() in preemptible [00000000] code: swapper/0/1
> [ 25.233645][ T1] caller is __kvm_is_vmx_supported (arch/x86/include/asm/cpuid/api.h:74 arch/x86/include/asm/cpuid/api.h:113 arch/x86/kvm/vmx/vmx.c:2789)
> [   25.234128][    T1] CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.16.0-rc1-00023-g06ddd17521bf #1 PREEMPT(lazy)
> [   25.234931][    T1] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
> [   25.235357][    T1] Call Trace:
> [   25.235357][    T1]  <TASK>
> [ 25.235357][ T1] dump_stack_lvl (lib/dump_stack.c:123 (discriminator 1))
> [ 25.235357][ T1] check_preemption_disabled (arch/x86/include/asm/preempt.h:85 lib/smp_processor_id.c:53)
> [ 25.235357][ T1] __kvm_is_vmx_supported (arch/x86/include/asm/cpuid/api.h:74 arch/x86/include/asm/cpuid/api.h:113 arch/x86/kvm/vmx/vmx.c:2789)
> [ 25.235357][ T1] ? __cpuid (arch/x86/include/asm/paravirt.h:179)
> [ 25.235357][ T1] ? __sanitizer_cov_trace_pc (kernel/kcov.c:217)
> [ 25.235357][ T1] ? ftrace_likely_update (arch/x86/include/asm/smap.h:53 kernel/trace/trace_branch.c:223)
> [ 25.235357][ T1] ? __sanitizer_cov_trace_pc (kernel/kcov.c:217)
> [ 25.235357][ T1] ? ftrace_likely_update (arch/x86/include/asm/smap.h:53 kernel/trace/trace_branch.c:223)
> [ 25.235357][ T1] ? __sanitizer_cov_trace_pc (kernel/kcov.c:217)
> [ 25.235357][ T1] ? ftrace_likely_update (arch/x86/include/asm/smap.h:53 kernel/trace/trace_branch.c:223)
> [ 25.235357][ T1] vmx_init (arch/x86/kvm/vmx/vmx.c:2808 arch/x86/kvm/vmx/vmx.c:8653)
> [ 25.235357][ T1] vt_init (arch/x86/kvm/vmx/main.c:1072)
> [ 25.235357][ T1] ? pi_init_cpu (arch/x86/kvm/vmx/main.c:1067)
> [ 25.235357][ T1] do_one_initcall (init/main.c:1273)
> [ 25.235357][ T1] ? trace_initcall_level (init/main.c:1264)
> [ 25.235357][ T1] ? __sanitizer_cov_trace_pc (kernel/kcov.c:217)
> [ 25.235357][ T1] ? ftrace_likely_update (arch/x86/include/asm/smap.h:53 kernel/trace/trace_branch.c:223)
> [ 25.235357][ T1] ? __sanitizer_cov_trace_pc (kernel/kcov.c:217)
> [ 25.235357][ T1] ? ftrace_likely_update (arch/x86/include/asm/smap.h:53 kernel/trace/trace_branch.c:223)
> [ 25.235357][ T1] do_initcalls (init/main.c:1334 init/main.c:1351)
> [ 25.235357][ T1] kernel_init_freeable (init/main.c:1587)
> [ 25.235357][ T1] ? rest_init (init/main.c:1465)
> [ 25.235357][ T1] kernel_init (init/main.c:1475)
> [ 25.235357][ T1] ? rest_init (init/main.c:1465)
> [ 25.235357][ T1] ret_from_fork (arch/x86/kernel/process.c:154)
> [ 25.235357][ T1] ? rest_init (init/main.c:1465)
> [ 25.235357][ T1] ret_from_fork_asm (arch/x86/entry/entry_64.S:255)
> [   25.235357][    T1]  </TASK>
> 

I took a glance at the warning, before this patch,
is_percpu_thread() always return true when CONFIG_SMP is not set.
After this patch,
is_percpu_thread() checks the current task's CPU affinity.
So debug_smp_processor_id() -> check_preemption_disabled() ->
is_percpu_thread() might not always return true anymore, which caused
the warning.

Actually the issue is in __kvm_is_vmx_supported(), should
we use something like this below:

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 5c5766467a61..9cba7b061892 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -2787,19 +2787,22 @@ static int setup_vmcs_config(struct vmcs_config 
*vmcs_conf,

  static bool __kvm_is_vmx_supported(void)
  {
-       int cpu = smp_processor_id();
+       int cpu = get_cpu();

         if (!(cpuid_ecx(1) & feature_bit(VMX))) {
                 pr_err("VMX not supported by CPU %d\n", cpu);
+               put_cpu();
                 return false;
         }

         if (!this_cpu_has(X86_FEATURE_MSR_IA32_FEAT_CTL) ||
             !this_cpu_has(X86_FEATURE_VMX)) {
                 pr_err("VMX not enabled (by BIOS) in MSR_IA32_FEAT_CTL 
on CPU %d\n", cpu);
+               put_cpu();
                 return false;
         }

+       put_cpu();
         return true;
  }


thanks,
Chenyu