linux-kernel - panic context: was: Re: [PATCH printk v2 04/11] printk: nbcon: Provide functions to mark atomic write sections

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Date:   Fri, 6 Oct 2023 14:51:16 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: panic context: was: Re: [PATCH printk v2 04/11] printk: nbcon:
 Provide functions to mark atomic write sections

Adding Linus into Cc. I would like to be sure about the flushing
of atomic consoles in panic context.

> During the demo at LPC2022 we had the situation that there was a large
> backlog when a WARN was hit. With current mainline the first line of the
> WARN is put into the ringbuffer and then the entire backlog is flushed
> before storing the rest of the WARN into the ringbuffer. At the time it
> was obvious that we should finish storing the WARN message and then
> start flushing the backlog.

This talks about the "emergency" context (WARN/OOPS/watchdog).
The system might be in big troubles but it would still try to continue.

Do we really want to defer the flush also for panic() context?

I ask because I was not on LPC 2022 in person and I do not remember
all details.

Anyway, the deferred flush works relatively well for the "emergency" context:

   + flushed from nbcon_atomic_exit()
   + printk kthread might emit the messages while they are being added

But it is tricky in panic(), see 8th patch at
https://lore.kernel.org/r/20230919230856.661435-9-john.ogness@linutronix.de

   + nbcon_atomic_exit() is called only in one code path.

   + nbcon_atomic_flush_all() is used in other paths. It looks like
     a "Whack a mole" game to me.

   + messages are never emitted by printk kthread either because
     CPUs are stopped or the kthread is not allowed to get the lock[*]

I see only one positive of the explicit flush. The consoles would
not delay crash_exec() and the crash dump might be closer to
the point where panic() was called.

Otherwise I see only negatives => IMHO, we want to flush atomic
consoles synchronously from printk() in panic().

Does anyone really want explicit flushes in panic()?

[*] Emitting messages is explicitly blocked on non-panic CPUs. It
    increases the change that panic-CPU would be able to take
    the console lock the safe way.

Best Regards,
Petr