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Date: Wed, 30 Aug 2017 11:47:03 +0900
From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Joe Perches <joe@perches.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Pavel Machek <pavel@ucw.cz>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Petr Mladek <pmladek@suse.com>, Jan Kara <jack@suse.cz>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jiri Slaby <jslaby@suse.com>, Andreas Mohr <andi@lisas.de>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: printk: what is going on with additional newlines?
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On (08/29/17 19:31), Joe Perches wrote:
[..]
> > the idea is not to do printk() on that seq buffer at all, but to
> > log_store(), atomically, seq buffer messages
> > 
> > 	spin_lock(&logbuf_lock)
> > 	while (offset < seq_buffer->len) {
> > 		...
> > 		log_store(seq->buffer + offset);
> > 		...
> > 	}
> > 	spin_unlock(&logbuf_unlock)
> 
> Why?
> 
> What's wrong with a simple printk?
> It'd still do a log_store.

sure, it will. but in separate logbuf entries, and between two
consequent printk calls on the same CPU a lot of stuff can happen:
IRQs->printks, rescheduling->printks, etc. etc. (not to mention
concurrent printks from other CPUs) so what people want to have is
to have a way to make several printks appear next to each other in
the logs (dmesg or serial log). Tetsuo wants this, for instance,
for OOM reports and backtraces. SCIS/ATA people want it as well.

	-ss