linux-kernel - Re: [PATCH printk v1 03/13] printk: use percpu flag instead of cpu

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Date:   Fri, 11 Feb 2022 17:05:58 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH printk v1 03/13] printk: use percpu flag instead of
 cpu_online()

On Mon 2022-02-07 20:49:13, John Ogness wrote:
> The CON_ANYTIME console flag is used to label consoles that will
> work correctly before percpu resources are allocated. To check
> the condition, cpu_online(raw_smp_processor_id()) was used.
> However, this is odd because CPUs can go offline at a later point.
> Also, the function is forced to use the raw_ variant because
> migration is not disabled.
> 
> Since commit ab6f762f0f53 ("printk: queue wake_up_klogd irq_work
> only if per-CPU areas are ready") there is a variable to identify
> if percpu resources have been allocated. Use that variable instead
> of cpu_online(raw_smp_processor_id()).
> 
> Signed-off-by: John Ogness <john.ogness@...utronix.de>

I dug more into the history to be more confident about the change.
I am not sure that it is safe.

CON_ANYTIME really seems to be related to per-CPU variables and early
boot, see the commit 76a8ad293912cd2f01eca ("[PATCH] Make printk work
for really early debugging").

I am not 100% sure. I think that the support for per-CPU variables
is enabled by setup_per_cpu_areas(). It is done for_each_possible_cpu()
So that per-CPU variables should always be available after
printk_percpu_data_ready(). So far, so good.

The problem is the commit ac25575203c11145066ea ("[PATCH] CPU hotplug
printk fix"). It suggests that per-CPU data of slab are freed during
hotplug.

There are many other things that are manipulated during cpu hotplug.
And there are the two notifiers "printk:dead" and "printk:online",
see printk_late_init(). Maybe, we should use them to decide whether
the non-trivial consoles are callable during CPU hotplug.

Best Regards,
Petr