linux-kernel - Re: [PATCH] printk: inject caller information into the body of message

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Date:   Mon, 1 Oct 2018 17:37:21 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Petr Mladek <pmladek@...e.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Alexander Potapenko <glider@...gle.com>,
        Dmitriy Vyukov <dvyukov@...gle.com>,
        kbuild test robot <fengguang.wu@...el.com>,
        syzkaller <syzkaller@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH] printk: inject caller information into the body of
 message

On (10/01/18 14:52), Sergey Senozhatsky wrote:
> On (09/29/18 20:13), Sergey Senozhatsky wrote:
> > We used to flush "incomplete" cont lines (fragments) from console_unlock().
> > 
> > void console_unlock(void)
> > {
> > ...
> >         /* flush buffered message fragment immediately to console */
> >         console_cont_flush(text, sizeof(text));
> > again:
> >         for (;;) {
> > 	...
> > 	}
> > ...
> > }
> > 
> > Unless I'm missing something, we don't anymore.
> > Since 5c2992ee7fd8a29d04125dc0aa3522784c5fa5eb.
> > Now we print only log_buf entries. So we either wait for a \n to flush
> > a complete cont buffer, or for a race to preliminary flush cont buffer.
> 
> BTW, it just crossed my mind:

One more thing.

Since we don't print cont fragments to the consoles anymore, do we still
need the "extended consoles disable kernel cont support" thing?

cont lines are proper log_buf entries now, there is nothing to reassemble
according to "consecutive continuation flags". Or am I wrong?

---

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 308497194bd4..e72cb793aff1 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -192,16 +192,7 @@ int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write,
 	return 0;
 }
 
-/*
- * Number of registered extended console drivers.
- *
- * If extended consoles are present, in-kernel cont reassembly is disabled
- * and each fragment is stored as a separate log entry with proper
- * continuation flag so that every emitted message has full metadata.  This
- * doesn't change the result for regular consoles or /proc/kmsg.  For
- * /dev/kmsg, as long as the reader concatenates messages according to
- * consecutive continuation flags, the end result should be the same too.
- */
+/* Number of registered extended console drivers. */
 static int nr_ext_console_drivers;
 
 /*
@@ -1806,12 +1797,8 @@ static void cont_flush(void)
 
 static bool cont_add(int facility, int level, enum log_flags flags, const char *text, size_t len)
 {
-	/*
-	 * If ext consoles are present, flush and skip in-kernel
-	 * continuation.  See nr_ext_console_drivers definition.  Also, if
-	 * the line gets too long, split it up in separate records.
-	 */
-	if (nr_ext_console_drivers || cont.len + len > sizeof(cont.buf)) {
+	/* If the line gets too long, split it up in separate records. */
+	if (cont.len + len > sizeof(cont.buf)) {
 		cont_flush();
 		return false;
 	}
@@ -2731,8 +2718,7 @@ void register_console(struct console *newcon)
 	}
 
 	if (newcon->flags & CON_EXTENDED)
-		if (!nr_ext_console_drivers++)
-			pr_info("printk: continuation disabled due to ext consoles, expect more fragments in /dev/kmsg\n");
+		nr_ext_console_drivers++;
 
 	if (newcon->flags & CON_PRINTBUFFER) {
 		/*
---

	-ss