linux-kernel - [PATCH] smp: Suppress false DEBUG_PREEMPT warning in smp_call_on

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Message-Id: <20251024070714.4047816-1-namcao@linutronix.de>
Date: Fri, 24 Oct 2025 09:07:14 +0200
From: Nam Cao <namcao@...utronix.de>
To: Ingo Molnar <mingo@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Mel Gorman <mgorman@...e.de>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Shrikanth Hegde <sshegde@...ux.ibm.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Valentin Schneider <vschneid@...hat.com>,
	linux-kernel@...r.kernel.org,
	Nam Cao <namcao@...utronix.de>
Subject: [PATCH] smp: Suppress false DEBUG_PREEMPT warning in smp_call_on_cpu()

While booting UP (uniprocessor) kernel with CONFIG_DEBUG_PREEMPT=y, the
following warning is observed:

BUG: using smp_processor_id() in preemptible [00000000] code: swapper/0/1
caller is debug_smp_processor_id+0x1c/0x28
CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.18.0-rc2-00012-g002733e953a7 #111 PREEMPT_RT
Hardware name: MangoPi MQ Pro (DT)
Call Trace:
    dump_backtrace
    show_stack
    dump_stack_lvl
    dump_stack
    check_preemption_disabled.isra.0
    debug_smp_processor_id
    check_unaligned_access
    smp_call_on_cpu
    check_unaligned_access_all_cpus
    do_one_initcall
    kernel_init_freeable
    kernel_init
    ret_from_fork_kernel
    ret_from_fork_kernel_asm

This is a false warning. The UP-variant of smp_call_on_cpu() simply calls
the callback, and thus debug_smp_processor_id() thinks the context is
unsafe for smp_processor_id(), which is obviously false because this is UP
kernel.

This appears after commit 06ddd17521bf ("sched/smp: Always define
is_percpu_thread() and scheduler_ipi()"). Before this commit,
is_percpu_thread() always returns true on UP kernel and thus
debug_smp_processor_id() always sees a per-cpu thread and never warns. But
now is_percpu_thread() returns false for this case.

Suppress this warning with a migrate_disable()+migrate_enable() pair.

Fixes: 06ddd17521bf ("sched/smp: Always define is_percpu_thread() and scheduler_ipi()")
Signed-off-by: Nam Cao <namcao@...utronix.de>
---
grep shows me that the followings also have the same false warning:

    arch/arm64/kernel/watchdog_hld.c
    drivers/hwmon/dell-smm-hwmon.c
    drivers/platform/x86/dell/dcdbas.c
    kernel/watchdog.c
---
 kernel/up.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/up.c b/kernel/up.c
index df50828cc2f0..37c9f0d39b36 100644
--- a/kernel/up.c
+++ b/kernel/up.c
@@ -64,7 +64,14 @@ int smp_call_on_cpu(unsigned int cpu, int (*func)(void *), void *par, bool phys)
 
 	if (phys)
 		hypervisor_pin_vcpu(0);
+
+	/* suppress warnings from debug_smp_processor_id() */
+	migrate_disable();
+
 	ret = func(par);
+
+	migrate_enable();
+
 	if (phys)
 		hypervisor_pin_vcpu(-1);
 
-- 
2.39.5