linux-kernel - Re: How to handle concurrent access to /dev/ttyprintk ?

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Message-ID: <YHcoNgbDyMbDKSf1@alley>
Date:   Wed, 14 Apr 2021 19:36:54 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Samo Pogačnik <samo_pogacnik@....net>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Jiri Slaby <jirislaby@...nel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        John Ogness <john.ogness@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: How to handle concurrent access to /dev/ttyprintk ?

On Tue 2021-04-13 17:22:46, Samo Pogačnik wrote:
> Dne 13.04.2021 (tor) ob 16:32 +0200 je Petr Mladek napisal(a):
> > On Tue 2021-04-13 13:10:50, Samo Pogačnik wrote:
> > > Dne 13.04.2021 (tor) ob 11:41 +0200 je Petr Mladek napisal(a):
> > > > On Mon 2021-04-12 14:41:27, Samo Pogačnik wrote:
> > > > > Dne 12.04.2021 (pon) ob 19:39 +0900 je Tetsuo Handa napisal(a):
> > > > > > What is the intended usage of /dev/ttyprintk ?
> > > > > > 
> > > > > 
> > > > > The intended use of 'ttyprintk' is to redirect console to /dev/ttyprintk
> > > > > via the TIOCCONS ioctl. After successfull redirection, all console
> > > > > messages get "merged" with kernel messages and as such automatically
> > > > > processed
> > > > > (stored/transferred) by the syslog service for example.
> > > > 
> > > > The same can be achieved by /dev/kmsg that was created by systemd
> > > > developers.
> > > > 
> > > 
> > > 'kmsg' and 'ttyprintk' are different types of drivers and as such rather
> > > complementary than exclusive. The 'ttyprintk' being a tty driver allows 
> > > for a system wide automatic redirection of anything written to the
> > > console.
> > 
> > I might miss something. But how can one setup ttyprintk as the system
> > wide console? I do not see any code that would use ttyprintk
> > in struct console.
> > 
> 
> You can compile this simple code below and call:
> 
> # ./tioccons /dev/ttyprintk

Thanks a lot for the example.

My head is still spinning around it. I have troubles to update
my mental model around /dev/console with this use case.

My original model is that kernel or processes write into
/dev/console so that a human could see and read it
on the device attached to the console.

kernel is calling console->write() callback when showing new messages
on consoles. The write to the redirected /dev/console
makes the messages appear in the kernel log. It feels like
it should create an infinite loop. Except that it doesn't
work this way. Sigh.

Best Regards,
Petr