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

[Sergey Senozhatsky]

On (22/02/11 17:05), Petr Mladek wrote:
> On Mon 2022-02-07 20:49:13, John Ogness wrote:
[..]
> 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.

Great findings. Looks like we only set __printk_percpu_data_ready to
true and never set it back to false, relying on cpu_online() in such
cases. But here's the thing: we have printk_percpu_data_ready() in
__printk_recursion_counter() and in wake_up_klogd() and in
defer_console_output(), but why we never check __printk_percpu_data_ready
in __down_trylock_console_sem()/__up_console_sem() and more importantly
in console_trylock_spinning() and those do access this_cpu() in printk safe
enter/exit. Am I missing something?