printk: Wait for the global console lock when the system is going down

There are reports that the console kthreads block the global console
lock when the system is going down, for example, reboot, panic.

First part of the solution was to block kthreads in these problematic
system states so they stopped handling newly added messages.

Second part of the solution is to wait when for the kthreads when
they are actively printing. It solves the problem when a message
was printed before the system entered the problematic state and
the kthreads managed to step in.

A busy waiting has to be used because panic() can be called in any
context and in an unknown state of the scheduler.

There must be a timeout because the kthread might get stuck or sleeping
and never release the lock. The timeout 10s is an arbitrary value
inspired by the softlockup timeout.

Link: https://lore.kernel.org/r/20220610205038.GA3050413@paulmck-ThinkPad-P17-Gen-1
Link: https://lore.kernel.org/r/CAMdYzYpF4FNTBPZsEFeWRuEwSies36QM_As8osPWZSr2q-viEA@mail.gmail.com
Signed-off-by: Petr Mladek <pmladek@suse.com>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://lore.kernel.org/r/20220615162805.27962-3-pmladek@suse.com

1 files changed, 2 insertions, 0 deletions

diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index d947ca6c84f9..e7d8578860ad 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -20,6 +20,8 @@ enum printk_info_flags {
 	LOG_CONT	= 8,	/* text is a fragment of a continuation line */
 };
 
+extern bool block_console_kthreads;
+
 __printf(4, 0)
 int vprintk_store(int facility, int level,
 		  const struct dev_printk_info *dev_info,