printk: flush consoles before checking progress

Commit 9e70a5e1 ("printk: Add per-console suspended state")
removed console lock usage during resume and replaced it with
the clearly defined console_list_lock and srcu mechanisms.

However, the console lock usage had an important side-effect
of flushing the consoles. After its removal, consoles were no
longer flushed before checking their progress.

Add the console_lock/console_unlock dance to the beginning
of __pr_flush() to actually flush the consoles before checking
their progress. Also add comments to clarify this additional
usage of the console lock.

Note that console_unlock() does not guarantee flushing all messages
since the commit dbdda842 ("printk: Add console owner and waiter
logic to load balance console writes").
Reported-by: Todd Brandt <todd.e.brandt@intel.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217955
Fixes: 9e70a5e1 ("printk: Add per-console suspended state")
Co-developed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Link: https://lore.kernel.org/r/20231006082151.6969-2-pmladek@suse.com

kernel/printk/printk.c

@@ -3738,12 +3738,18 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
 
 	seq = prb_next_seq(prb);
 
+	/* Flush the consoles so that records up to @seq are printed. */
+	console_lock();
+	console_unlock();
+
 	for (;;) {
 		diff = 0;
 
 		/*
 		 * Hold the console_lock to guarantee safe access to
-		 * console->seq.
+		 * console->seq. Releasing console_lock flushes more
+		 * records in case @seq is still not printed on all
+		 * usable consoles.
 		 */
 		console_lock();