Re: [PATCH printk v5 1/1] printk: extend console_lock for per-console locking

From: "Jason A. Donenfeld" <Jason@zx2c4.com>
To: John Ogness <john.ogness@linutronix.de>
Cc: 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
Subject: Re: [PATCH printk v5 1/1] printk: extend console_lock for per-console locking
Date: Thu, 9 Jun 2022 14:18:44 +0200	[thread overview]
Message-ID: <YqHlJDh1MSYJWBnu@zx2c4.com> (raw)
In-Reply-To: <YqHgdECTYFNJgdGc@zx2c4.com>

Hey again,

On Thu, Jun 09, 2022 at 01:58:44PM +0200, Jason A. Donenfeld 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?

On second thought, the fix is trivial:
https://lore.kernel.org/lkml/20220609121709.12939-1-Jason@zx2c4.com/

Jason

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