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Subject[PATCH printk v3 05/40] printk: fix setting first seq for consoles
Date
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@linutronix.de>
---
kernel/printk/printk.c | 16 ++++++++++++++--
1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 173f46a29252..8974523f3107 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3222,14 +3222,26 @@ 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 an enabled boot console is not caught up, start with
+ * that message instead. That boot console will be
+ * unregistered shortly and may be the same device.
+ */
+ for_each_console(con) {
+ if ((con->flags & (CON_BOOT | CON_ENABLED)) == (CON_BOOT | CON_ENABLED) &&
+ con->seq < newcon->seq) {
+ newcon->seq = con->seq;
+ }
+ }
}

/*
--
2.30.2
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