linux-kernel - Re: [PATCH printk v5 1/1] printk: extend console

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Date:   Fri, 17 Jun 2022 18:51:48 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        John Ogness <john.ogness@...utronix.de>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "open list:ARM/Amlogic Meson..." <linux-amlogic@...ts.infradead.org>,
        Theodore Ts'o <tytso@....edu>,
        Alexander Potapenko <glider@...gle.com>,
        Marco Elver <elver@...gle.com>, kasan-dev@...glegroups.com
Subject: Re: [PATCH printk v5 1/1] printk: extend console_lock for
 per-console locking

On 2022-06-09 14:27:11 [+0200], Jason A. Donenfeld wrote:
> Hi Dmitry,
> 
> On Thu, Jun 09, 2022 at 02:18:19PM +0200, Dmitry Vyukov wrote:
> > > AFAIK, CONFIG_PROVE_RAW_LOCK_NESTING is useful for teasing out cases
> > > where RT's raw spinlocks will nest wrong with RT's sleeping spinlocks.
> > > But nobody who wants an RT kernel will be using KFENCE. So this seems
> > > like a non-issue? Maybe just add a `depends on !KFENCE` to
> > > PROVE_RAW_LOCK_NESTING?
> > 
> > Don't know if there are other good solutions (of similar simplicity).
> 
> Fortunately, I found one that solves things without needing to
> compromise on anything:
> https://lore.kernel.org/lkml/20220609121709.12939-1-Jason@zx2c4.com/
> 
> > Btw, should this new CONFIG_PROVE_RAW_LOCK_NESTING be generally
> > enabled on testing systems? We don't have it enabled on syzbot.
> 
> Last time I spoke with RT people about this, the goal was eventually to
> *always* enable it when lock proving is enabled, but there are too many
> bugs and cases now to do that, so it's an opt-in. I might be
> misremembering, though, so CC'ing Sebastian in case he wants to chime
> in.

That is basically still the case. If CONFIG_PROVE_RAW_LOCK_NESTING yells
then there will be yelling on PREEMPT_RT, too. We would like to get
things fixed ;)

Without going through this thread, John is looking at printk and printk
triggers a few of those. That is one of reasons why this is not enabled
by default.

> Jason

Sebastian