Re: two locations: was: Re: [PATCH printk v1 03/13] printk: use percpu flag instead of cpu_online()

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

On 2022-02-16, Petr Mladek <pmladek@suse.com> wrote:
>> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
>> index d1b773823d63..b346e60e9e51 100644
>> --- a/kernel/printk/printk.c
>> +++ b/kernel/printk/printk.c
>> @@ -2577,11 +2577,11 @@ static int have_callable_console(void)
>>   *
>>   * Console drivers may assume that per-cpu resources have been allocated. So
>>   * unless they're explicitly marked as being able to cope (CON_ANYTIME) don't
>> - * call them until this CPU is officially up.
>> + * call them until per-cpu resources have been allocated.
>>   */
>>  static inline int can_use_console(void)
>>  {
>> -	return cpu_online(raw_smp_processor_id()) || have_callable_console();
>> +	return (printk_percpu_data_ready() || have_callable_console());
>>  }
>
> cpu_online(raw_smp_processor_id()) check is used also in
> call_console_drivers(). The same logic should be used in both
> locations.
>
> I found this when reviewing 6th patch that replaced both checks
> with a single one.
>
> Note that I am still not sure if this change is correct at all.
> It will allow to always call the console during CPU hotplug
> and I am not sure if it is safe. IMHO, it might cause problems when
> a console driver uses, for example, CPU-bound workqueues.

You are correct. We must take hotplug into account for !CON_ANYTIME
consoles. There may be some hotplug callbacks that make memory
unavailable for the console.

However, I will add the use of printk_percpu_data_ready() in the
check. !CON_ANYTIME consoles also should not be called until the per-cpu
areas are ready. For example, it would be bad if a console queued
irq_work before per-cpu areas are setup (cpu_online() is true during
this time).

One of my main concerns was that raw_smp_processor_id() was used for the
check. It is conceptually wrong to exclude certain consoles based on a
current CPU when migration is still enabled. I understand that the use
of can_use_console() is an optimization to avoid doing extra work where
there are no consoles available. But the task could be preemptible there
and _conceptually_, could get moved to another CPU before its write()
callback is called. The cpu_online() check belongs in code where
preemption is disabled.

If the context is preemptible, I do not think it will ever see
!cpu_online(). So I think if the cpu_online() check is limited to
unlocking when console_trylock() was used, it will be correct.

In the current implementation of printk(), it would be odd to do this
conditional check (perhaps by passing @do_cond_resched to
can_use_console()). But my series does significant refactoring and
actually does need to distinguish between console_lock() and
console_trylock() due to the kthreads and supporting the handover. So it
should work well that the cpu_online() check for !CON_ANYTIME is only
performed when !preemptible.

Regardless, my v2 will keep cpu_online() checks since they are necessary
for hotplug support.

John