linux-kernel - Re: [PATCH linux-next] powerpc: use raw_smp_processor_id in arch_touch_nmi

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Date:   Thu, 14 Jul 2022 11:31:00 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     Zhouyi Zhou <zhouzhouyi@...il.com>, mpe@...erman.id.au,
        benh@...nel.crashing.org, paulus@...ba.org, npiggin@...il.com,
        ldufour@...ux.ibm.com, pmladek@...e.com, Julia.Lawall@...ia.fr,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        lance@...osl.org, paulmck@...nel.org, rcu@...r.kernel.org
Cc:     Zhouyi Zhou <zhouzhouyi@...il.com>
Subject: Re: [PATCH linux-next] powerpc: use raw_smp_processor_id in
 arch_touch_nmi_watchdog

On 2022-07-14, Zhouyi Zhou <zhouzhouyi@...il.com> wrote:
> use raw_smp_processor_id() in arch_touch_nmi_watchdog
> because when called from watchdog, the cpu is preemptible.

I would expect the correct solution is to make it a non-migration
section. Something like the below (untested) patch.

John Ogness

diff --git a/arch/powerpc/kernel/watchdog.c b/arch/powerpc/kernel/watchdog.c
index bfc27496fe7e..9d34aa809241 100644
--- a/arch/powerpc/kernel/watchdog.c
+++ b/arch/powerpc/kernel/watchdog.c
@@ -450,17 +450,23 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 void arch_touch_nmi_watchdog(void)
 {
 	unsigned long ticks = tb_ticks_per_usec * wd_timer_period_ms * 1000;
-	int cpu = smp_processor_id();
+	int cpu;
 	u64 tb;
 
-	if (!cpumask_test_cpu(cpu, &watchdog_cpumask))
+	cpu = get_cpu();
+
+	if (!cpumask_test_cpu(cpu, &watchdog_cpumask)) {
+		goto out;
 		return;
+	}
 
 	tb = get_tb();
 	if (tb - per_cpu(wd_timer_tb, cpu) >= ticks) {
 		per_cpu(wd_timer_tb, cpu) = tb;
 		wd_smp_clear_cpu_pending(cpu);
 	}
+out:
+	put_cpu();
 }
 EXPORT_SYMBOL(arch_touch_nmi_watchdog);