linux-kernel - Re: [RFC PATCH v1 00/25] printk: new implementation

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Message-ID: <87lg2js547.fsf@linutronix.de>
Date:   Wed, 13 Feb 2019 23:20:08 +0100
From:   John Ogness <john.ogness@...utronix.de>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Daniel Wang <wonderfly@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Jiri Slaby <jslaby@...e.com>,
        Peter Feiner <pfeiner@...gle.com>,
        "linux-serial\@vger.kernel.org" <linux-serial@...r.kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC PATCH v1 00/25] printk: new implementation

On 2019-02-13, David Laight <David.Laight@...LAB.COM> wrote:
> ...
>> - A dedicated kernel thread is created for printing to all consoles in
>>   a fully preemptible context.
>> 
>> - A new (optional) console operation "write_atomic" is introduced that
>>   console drivers may implement. This function must be NMI-safe. An
>>   implementation for the 8250 UART driver is provided.
>> 
>> - The concept of "emergency messages" is introduced that allows
>>   important messages (based on a new emergency loglevel threshold) to
>>   be immediately written to any consoles supporting write_atomic,
>>   regardless of the context.
> ...
>
> Does this address my usual 'gripe' that the output is written to the
> console by syslogd and not by the kernel itself?

If I understand it correctly, your usual 'gripe' is aimed at
distributions that are turning off the kernel writing directly to the
console. I don't see how that is a kernel issue.

> When you are trying to find out where the system is completely
> deadlocking you need the 'old fashioned' completely synchronous kernel
> printf().

Emergency messages will give you that. They differ from the current
implementation by changing printk to have the caller print only _their_
message directly without concern for past unseen non-emergency messages
or which context they are in.

John Ogness