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Date: Mon, 15 Jan 2024 09:51:41 +0100
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Petr Mladek <pmladek@suse.com>
Cc: John Ogness <john.ogness@linutronix.de>,
	Sergey Senozhatsky <senozhatsky@chromium.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org,
	Francesco Dolcini <francesco@dolcini.it>,
	kernel test robot <oliver.sang@intel.com>
Subject: Re: [PATCH printk v3 02/14] printk: Adjust mapping for 32bit seq
 macros
Message-ID: <20240115085141.MSS4LLsR@linutronix.de>
References: <20231214214201.499426-1-john.ogness@linutronix.de>
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 <ZaEUUZ6PICA54qVH@alley>
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On 2024-01-12 19:14:44 [+0100], Petr Mladek wrote:
>=20
> That said, I am a bit nervous that a bug like this might cause
> workqueue stall and panic() the kernel.


> At least, this is how I read
> https://lore.kernel.org/oe-lkp/202311171611.78d41dbe-oliver.sang@intel.co=
m/

well, workqueue stalls and RCU as well because the CPU spins. That is a
natural consequence because the CPU makes no progress (at boot). The
panic _might_ be due to panic_on_error or so.
There is no scheduler, nothing so one CPU is blocked and the world ends=E2=
=80=A6

> It looks like it caused some loop and refcout overlow or so.
> But I might be wrong.
>=20
> I would like to better understand this and check if we could prevent
> it somehow.

Based on memory: the problem is that the sign extension bug (the fixed
bug) returned the wrong or too low sequence number. So the printk code
tried again to obtain a new sequence number. And got the wrong
again. And this is what looped during boot.=20

I'm not sure if this sort of lockup can happen now after the bug is
fixed. I can issue a NMI backtrace on all CPUs (32) without the sync (so
they all can printk immediately and not one after the other) and it
prints and continues=E2=80=A6

> Best Regards,
> Petr

Sebastian