linux-kernel - Re: [RFC PATCH 1/1] tracefs: add TP_printk_no_nl

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Date:   Mon, 31 Jul 2023 10:31:03 +0206
From:   John Ogness <john.ogness@...utronix.de>
To:     Jim Cromie <jim.cromie@...il.com>, rostedt@...dmis.org,
        linux-kernel@...r.kernel.org
Cc:     Jim Cromie <jim.cromie@...il.com>,
        Vincent Whitchurch <vincent.whitchurch@...s.com>,
        daniel@...ll.ch, pmladek@...e.com, sergey.senozhatsky@...il.com,
        Simon Ser <contact@...rsion.fr>,
        Sean Paul <seanpaul@...omium.org>
Subject: Re: [RFC PATCH 1/1] tracefs: add TP_printk_no_nl - RFC

On 2023-07-30, Jim Cromie <jim.cromie@...il.com> wrote:
> This variant of TP_printk() does *not* add the trailing newline.
>
> https://lore.kernel.org/lkml/20200825153338.17061-1-vincent.whitchurch@axis.com/
>
> taught dyndbg to send pr_debug() msgs to tracefs, via -x/T flag.
>
> It "reused" the include/trace/events/printk.h console event,
> which does the following:
>
> 	TP_fast_assign(
> 		/*
> 		 * Each trace entry is printed in a new line.
> 		 * If the msg finishes with '\n', cut it off
> 		 * to avoid blank lines in the trace.
> 		 */
> 		if ((len > 0) && (text[len-1] == '\n'))
> 			len -= 1;
>
> 		memcpy(__get_str(msg), text, len);
> 		__get_str(msg)[len] = 0;
> 	),
>
> That trim work could be avoided, *iff* all pr_debug() callers are
> known to have no '\n' to strip.  While thats not true for *all*
> callsites, it is 99+% true for DRM.debug callsites, and can be made
> true for some subsets of prdbg/dyndbg callsites.

Note that the trailing '\n' in printk messages has a purpose.  It
finalizes commitment of the message to the ringbuffer so that the
message cannot be extended by any LOG_CONT printk's that may come along
(from any context).

If it is not intended that a message is extended, then that message
should include a trailing '\n'.

Grepping through the kernel, I am surprised how many messages are
missing the trailing newline when it is obvious they are not intended to
be extended. I consider these bugs.

John Ogness