Re: [PATCH v2] kcsan: Don't enter an nbcon emergency section for verbose reports

[John Ogness <john.ogness@linutronix.de>]

On 2026-11-04, Breno Leitao <leitao@debian.org> wrote:
>> Wouldn't the below diff be the proper fix?
>> 
>> John
>> 
>> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
>> index c3dc84a7cef26..71f1b4b8f06d5 100644
>> --- a/kernel/locking/lockdep.c
>> +++ b/kernel/locking/lockdep.c
>> @@ -4245,8 +4245,6 @@ void print_irqtrace_events(struct task_struct *curr)
>>  {
>>  	const struct irqtrace_events *trace = &curr->irqtrace;
>>  
>> -	nbcon_cpu_emergency_enter();
>> -
>>  	printk("irq event stamp: %u\n", trace->irq_events);
>>  	printk("hardirqs last  enabled at (%u): [<%px>] %pS\n",
>>  		trace->hardirq_enable_event, (void *)trace->hardirq_enable_ip,
>> @@ -4260,8 +4258,6 @@ void print_irqtrace_events(struct task_struct *curr)
>>  	printk("softirqs last disabled at (%u): [<%px>] %pS\n",
>>  		trace->softirq_disable_event, (void *)trace->softirq_disable_ip,
>>  		(void *)trace->softirq_disable_ip);
>> -
>> -	nbcon_cpu_emergency_exit();
>>  }
>>  
>>  static int HARDIRQ_verbose(struct lock_class *class)
>> @@ -5797,8 +5793,11 @@ static noinstr void check_flags(unsigned long flags)
>>  	}
>>  #endif
>>  
>> -	if (!debug_locks)
>> +	if (!debug_locks) {
>> +		nbcon_cpu_emergency_enter();
>>  		print_irqtrace_events(current);
>> +		nbcon_cpu_emergency_exit();
>> +	}
>>  
>>  	instrumentation_end();
>>  #endif
>
> Yes, but it was unclear to me what were the invocations of
> print_irqtrace_events() that needed the emergency turned on.
>
> how did you decided that only check_flags() needed it?

The emergency enter/exit's are intended for lockdep splats. AFAICT
check_flags() was the only call to print_irqtrace_events() that was not
already in emergency state for lockdep splats.

KCSAN is not the only non-lockdep caller of print_irqtrace_events(). So
my suggested fix applies to the other non-lockdep users as well. Commit
59cd94ef8009 ("lockdep: Mark emergency sections in lockdep splats") was
only intended for lockdep splats.

John