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

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Date:   Mon, 07 Mar 2022 11:12:15 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
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: two locations: was: Re: [PATCH printk v1 03/13] printk: use
 percpu flag instead of cpu_online()

On 2022-03-04, Petr Mladek <pmladek@...e.com> wrote:
>> 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.
>
> This would require calling console_lock()/console_unlock()
> in a hotplug code when cpu_online() already returns false.
> Do I get it correctly?

Yes, but there is also the other side: Are there preemptible code paths
that are calling console_trylock()? And if yes, then why are they not
using console_lock()?

There is only a handfull of console_trylock() users (about 10
sites). Most of them appear to be differentiating from preemptible and
non-preemptible contexts. I will investigate them in detail.

If the raw_smp_processor_id() is changed to smp_processor_id() within
the console_trylock()/console_unlock() call path, then at least the
kernel's lock debugging would identify if there are preemptible contexts
that are using console_trylock().

John