linux-kernel - Re: [PATCH] pwm: Add missing '\n' in log messages

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Date:   Tue, 14 Apr 2020 12:04:32 -0700
From:   Joe Perches <joe@...ches.com>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Thierry Reding <thierry.reding@...il.com>,
        paul@...pouillou.net, Dan Carpenter <dan.carpenter@...cle.com>
Cc:     u.kleine-koenig@...gutronix.de, linux-pwm@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] pwm: Add missing '\n' in log messages

On Tue, 2020-04-14 at 20:30 +0200, Christophe JAILLET wrote:
> Being able to detect early missing trailing '\n' would help maintainers 
> and patch providers.
> 
> You are the 2nd person (I've added Paul Cercueil in copy of my reply) 
> who reports that he is thinking that it is no more required to add a '\n'.

The printk subsystem will, for every printk, check
if the last printk has a newline termination and if
it doesn't and the current printk does not start with
KERN_CONT will insert a newline.

The negative to this approach is the last printk,
if it does not have a newline, is buffered and not
emitted until another printk occurs.

There is also the (now small) possibility that
multiple concurrent kernel threads or processes
could interleave printks without a terminating
newline and a different process could emit a
printk that starts with KERN_CONT and the emitted
message could be garbled.

See:

commit 4bcc595ccd80decb4245096e3d1258989c50ed41
Author: Linus Torvalds <torvalds@...ux-foundation.org>
Date:   Sat Oct 8 20:32:40 2016 -0700

    printk: reinstate KERN_CONT for printing continuation lines