linux-kernel - [PATCH printk v4 05/39] printk: fix setting first seq for consoles

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Date:   Mon, 14 Nov 2022 17:34:58 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: [PATCH printk v4 05/39] printk: fix setting first seq for consoles

It used to be that all consoles were synchronized with respect to
which message they were printing. After commit a699449bb13b ("printk:
refactor and rework printing logic"), all consoles have their own
@seq for tracking which message they are on. That commit also changed
how the initial sequence number was chosen. Instead of choosing the
next non-printed message, it chose the sequence number of the next
message that will be added to the ringbuffer.

That change created a possibility that a non-boot console taking over
for a boot console might skip messages if the boot console was behind
and did not have a chance to catch up before being unregistered.

Since it is not possible to know which boot console a console is
taking over, use the lowest @seq of all the enabled boot consoles. If
no boot consoles are available/enabled, begin with the next message
that will be added to the ringbuffer.

Also, since boot consoles are meant to be used at boot time, handle
them the same as CON_PRINTBUFFER to ensure that no initial messages
are skipped.

Signed-off-by: John Ogness <john.ogness@...utronix.de>
---
 kernel/printk/printk.c | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 3d449dfb1ed6..8dbf3461cba0 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3222,14 +3222,31 @@ void register_console(struct console *newcon)
 	}

 	newcon->dropped = 0;
-	if (newcon->flags & CON_PRINTBUFFER) {
+	if (newcon->flags & (CON_PRINTBUFFER | CON_BOOT)) {
 		/* Get a consistent copy of @syslog_seq. */
 		mutex_lock(&syslog_lock);
 		newcon->seq = syslog_seq;
 		mutex_unlock(&syslog_lock);
 	} else {
-		/* Begin with next message. */
+		/* Begin with next message added to ringbuffer. */
 		newcon->seq = prb_next_seq(prb);
+
+		/*
+		 * If any enabled boot consoles are due to be unregistered
+		 * shortly, some may not be caught up and may be the same
+		 * device as @newcon. Since it is not known which boot console
+		 * is the same device, start with the message of the enabled
+		 * boot console that is the furthest behind.
+		 */
+		if (bootcon_registered && !keep_bootcon) {
+			for_each_console(con) {
+				if ((con->flags & CON_BOOT) &&
+				    (con->flags & CON_ENABLED) &&
+				    con->seq < newcon->seq) {
+					newcon->seq = con->seq;
+				}
+			}
+		}
 	}

 	/*
-- 
2.30.2