linux-kernel - Re: [PATCH v2] panic: add an option to replay all the printk message in buffer

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Date:   Wed, 17 Apr 2019 18:46:14 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Feng Tang <feng.tang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Borislav Petkov <bp@...e.de>, ying.huang@...el.com
Subject: Re: [PATCH v2] panic: add an option to replay all the printk message
 in buffer

On (04/17/19 11:18), Petr Mladek wrote:
> > My instant thought would be put the console_unlcok() and similar funcs
> > under CONFIG_PRINTK protection, while adding nop functions in the "else"
> > segment.
> > 
> > But complexer question will be when CONFIG_PRINTK=n, how those console_xxx
> > functions should consider these to make compiled binary smaller (though it
> > rarely happens). would wait for Petr/Sergey/Steven's insights.
> 
> I guess that it is because console_sem is historically used to
> synchronize some unrelated things, espcially in tty code.
> Unfortunately, it is not easy to clean this up.
> 
> For this patch, the best solution seems to be using scnprintf()
> instead of sprintf().

OK, since we have another chance to re-iterate this. Do we really
want to complicate console_unlock() printing loop any further?
Asking off chance, can we return back to the idea of adding enum
FLUSH_PENDING/FLUSH_ALL to console_flush_on_panic()?

Does not look too complex/ugly.

---
-void console_flush_on_panic(void)
+void console_flush_on_panic(enum console_flush_mode flush_mode)
 {
 	/*
 	 * If someone else is holding the console lock, trylock will fail
@@ -2549,6 +2549,10 @@ void console_flush_on_panic(void)
 	 */
 	console_trylock();
 	console_may_schedule = 0;
+	if (flush_mode == FLUSH_ALL) {
+		console_seq = log_first_seq;
+		console_idx = log_first_idx;
+	}
 	console_unlock();
 }
---

	-ss