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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Thu, 9 Jun 2022 14:18:19 +0200
Message-ID: <CACT4Y+ajfVUkqAjAin73ftqAz=HmLX=p=S=HRV1qe-8_y36J+A@mail.gmail.com>
Subject: Re: [PATCH printk v5 1/1] printk: extend console_lock for per-console locking
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     John Ogness <john.ogness@linutronix.de>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Petr Mladek <pmladek@suse.com>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "open list:ARM/Amlogic Meson..." <linux-amlogic@lists.infradead.org>,
        "Theodore Ts'o" <tytso@mit.edu>,
        Alexander Potapenko <glider@google.com>,
        Marco Elver <elver@google.com>, kasan-dev@googlegroups.com
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On Thu, 9 Jun 2022 at 13:59, Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>
> 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?

Don't know if there are other good solutions (of similar simplicity).
But fwiw this is not about the target production environment. Real
production uses of RT kernels will probably not enable LOCKDEP,
PROVE_RAW_LOCK_NESTING and other debugging configs.
This is about detecting as many bugs as possible in testing
environments. And testing environments can well have both LOCKDEP and
KFENCE enabled. Any such limitation will require doubling the number
of tested configurations.

Btw, should this new CONFIG_PROVE_RAW_LOCK_NESTING be generally
enabled on testing systems? We don't have it enabled on syzbot.