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

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Message-ID: <YqHgdECTYFNJgdGc@zx2c4.com>
Date:   Thu, 9 Jun 2022 13:58:44 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     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

Hi John,

On Thu, Jun 09, 2022 at 01:25:15PM +0206, John Ogness wrote:
> (Added RANDOM NUMBER DRIVER and KFENCE people.)

Thanks.

> I am guessing you have CONFIG_PROVE_RAW_LOCK_NESTING enabled?
> 
> We are seeing a spinlock (base_crng.lock) taken while holding a
> raw_spinlock (meta->lock).
> 
> kfence_guarded_alloc()
>   raw_spin_trylock_irqsave(&meta->lock, flags)
>     prandom_u32_max()
>       prandom_u32()
>         get_random_u32()
>           get_random_bytes()
>             _get_random_bytes()
>               crng_make_state()
>                 spin_lock_irqsave(&base_crng.lock, flags);
> 
> I expect it is allowed to create kthreads via kthread_run() in
> early_initcalls.

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?

Jason