linux-kernel - Re: [PATCH printk v4 17/27] printk: nbcon: Use nbcon consoles in console_flush

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Date: Thu, 18 Apr 2024 00:05:29 +0206
From: John Ogness <john.ogness@...utronix.de>
To: Petr Mladek <pmladek@...e.com>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>, Steven Rostedt
 <rostedt@...dmis.org>, Thomas Gleixner <tglx@...utronix.de>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk v4 17/27] printk: nbcon: Use nbcon consoles in
 console_flush_all()

On 2024-04-11, Petr Mladek <pmladek@...e.com> wrote:
> console_flush_all() is called from console_unlock().
> It might be called in atomic context. But the current
> owner might be theoretically scheduled out.

Nice catch. This problem was introduced in this series after you pointed
out that return value of nbcon_legacy_emit_next_record() did not match
the semantics of console_emit_next_record().

> I see 3 possible solutions:
>
>   1. Enforce that nbcon context can be acquired only with preemtion
>      disabled.

Not acceptable.

>   2. Enforce that nbcon context can be acquired only with
>      interrupts. It would prevent deadlock when some future
>      code interrupt flush in NBCON_PRIO_EMERGENCY context.
>      And then a potential nested console_flush_all() won't be
>      able to takeover the interrupted NBCON_PRIO_CONTEXT
>      and there will be no progress.

Not acceptable.

>   3. console_flush_all() should ignore nbcon console when
>      it is not able to get the context, aka no progress.

This was the previous implementation, but as you point out, it is an
issue that console_flush_all() is no longer reliable.

I will continue this topic by responding to your follow-up message.

John