linux-kernel - Re: printk: what is going on with additional newlines?

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Date:   Mon, 28 Aug 2017 21:21:09 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Pavel Machek <pavel@....cz>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>, Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jiri Slaby <jslaby@...e.com>, Andreas Mohr <andi@...as.de>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        linux-kernel@...r.kernel.org
Subject: Re: printk: what is going on with additional newlines?

On (08/28/17 19:28), Sergey Senozhatsky wrote:
> On (08/28/17 11:05), Pavel Machek wrote:
> > Hi!
> > 
> > In 4.13-rc, printk("foo"); printk("bar"); seems to produce
> > foo\nbar. That's... quite surprising/unwelcome. What is going on
> > there? Are timestamps responsible?
> 
> well, one thing we know for sure it is not related to this patch set  ;)
> 
> 
> does any of the below patches fix the problem for you?
> 
> basically it sets up the rule -- if we don't have LOG_NEWLINE lflags
> then we enforce LOG_CONT.

[..]

> @@ -1670,7 +1670,9 @@ static size_t log_output(int facility, int level, enum log_flags lflags, const c
>  	 * write from the same process, try to add it to the buffer.
>  	 */
>  	if (cont.len) {
> 		if (cont.owner == current && (lflags & LOG_CONT)) {


on the other hand... I don't think I like that check at all.
so I *probably* want to change it to -- !LOG_NEWLINE messages of the
same loglevel AND from the same task are getting concatenated.
a message with LOG_NEWLINE flushes the cont buffer.

for example:

       printk("foo"); printk("foo"); printk("bar\n");
       printk("buz"); printk("buz"); printk("buz"); pr_info("INFO msg\n");
       printk("buz"); printk("buz"); printk("buz"); pr_err("ERR msg\n");
       printk(KERN_CONT"foo"); printk(KERN_CONT"foo"); printk(KERN_CONT"bar\n");
       printk(KERN_CONT"foo"); printk(KERN_CONT"foo"); printk(KERN_ERR"bar\n");
       printk(KERN_CONT"foo"); printk(KERN_ERR"foo err"); printk(KERN_ERR"bar err\n");


for instance,
	printk(KERN_ERR"foo err"); printk(KERN_ERR"bar err\n");

should produce    "foo errbar err\n".    from the same task and of
the same loglevel, no new line. must be cont messages with a missing
KERN_CONT. right?


so, for the examples I posted above, I think, the output must be

 foofoobar
 buzbuzbuz
 INFO msg
 buzbuzbuz
 ERR msg
 foofoobar
 foofoo
 bar
 foo
 foo errbar err


how about something like this?

---

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index fc47863f629c..675febf84dc8 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1670,10 +1670,9 @@ static size_t log_output(int facility, int level, enum log_flags lflags, const c
 	 * write from the same process, try to add it to the buffer.
 	 */
 	if (cont.len) {
-		if (cont.owner == current && (lflags & LOG_CONT)) {
+		if (cont.owner == current && cont.level == level)
 			if (cont_add(facility, level, lflags, text, text_len))
 				return text_len;
-		}
 		/* Otherwise, make sure it's flushed */
 		cont_flush();
 	}