linux-kernel - Re: [PATCH 2/4] printk: disable optimistic spin during panic

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Message-ID: <87ilu5pmtq.fsf@jogness.linutronix.de>
Date:   Thu, 27 Jan 2022 10:15:05 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Stephen Brennan <stephen.s.brennan@...cle.com>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] printk: disable optimistic spin during panic

On 2022-01-27, Sergey Senozhatsky <senozhatsky@...omium.org> wrote:
> Right. So I also thought about placing panic_in_progress() somewhere in
> console_trylock() and make it fail for anything that is not a panic
> CPU.

I think this is a good idea and console_trylock() is the correct place
for that.

> Back in the days we also had this idea of "detaching" non-panic CPUs from
> printk() by overwriting their printk function pointers.

We need to keep in mind that printk() is no longer the problem. The
records are stored locklessly. The problem is the
console_trylock()/console_unlock() within vprintk_emit(). IMHO adding a
panic check in console_trylock() should solve that race sufficiently.

John