[PATCH printk] printk: flush consoles before checking progress

[John Ogness]

Commit 9e70a5e109a4 ("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.

Finally, in case pr_flush() needs to poll-wait (for example,
because there has been a handover to another CPU), reduce the
polling interval from 100ms to 1ms. In the bug report it was
mentioned that 100ms is unnecessarily long.

Reported-by: Todd Brandt <todd.e.brandt@xxxxxxxxx>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217955
Fixes: 9e70a5e109a4 ("printk: Add per-console suspended state")
Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
---
 kernel/printk/printk.c | 23 ++++++++++++-----------
 1 file changed, 12 insertions(+), 11 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 7e0b4dd02398..cf7841fc6eef 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3740,12 +3740,18 @@ 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();
 
@@ -3775,16 +3781,11 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
 		if (diff == 0 || remaining == 0)
 			break;
 
-		if (remaining < 0) {
-			/* no timeout limit */
-			msleep(100);
-		} else if (remaining < 100) {
-			msleep(remaining);
-			remaining = 0;
-		} else {
-			msleep(100);
-			remaining -= 100;
-		}
+		msleep(1);
+
+		/* If @remaining < 0, there is no timeout limit. */
+		if (remaining > 0)
+			remaining--;
 
 		last_diff = diff;
 	}

base-commit: 4952801fc6adb5b50b8ec2bcc5aeef92fcce8730
-- 
2.39.2