[PATCH v2] printk/console: Enable console kthreads only when there is no boot console left

[Petr Mladek]

Threaded console printing does not take into consideration that boot
consoles may be accessing the same hardware as normal consoles and thus
must not be called in parallel.

Since it is currently not possible to identify which consoles are
accessing the same hardware, delay threaded console printing activation
until it is known that there are no boot consoles registered.

Link: https://lore.kernel.org/r/20220619204949.50d9154d@thinkpad
Link: https://lore.kernel.org/r/2a82eae7-a256-f70c-fd82-4e510750906e@xxxxxxxxxxx
Link: https://lore.kernel.org/r/20220619204949.50d9154d@thinkpad
Reported-by: Marek Behún <kabel@xxxxxxxxxx>
[john.ogness@xxxxxxxxxxxxx: Better description of the problem.]
Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
Tested-by: Marek Behún <kabel@xxxxxxxxxx>
---
Changes against v1:

+ Updated comment and commit message [Linus,John]
+ Added Tested-by [Marek]

 kernel/printk/printk.c | 39 ++++++++++++++++++++++++++-------------
 1 file changed, 26 insertions(+), 13 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index b095fb5f5f61..157a3e8c01bb 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3551,6 +3551,19 @@ void __init console_init(void)
 	}
 }
 
+static int __init printk_activate_kthreads(void)
+{
+	struct console *con;
+
+	console_lock();
+	printk_kthreads_available = true;
+	for_each_console(con)
+		printk_start_kthread(con);
+	console_unlock();
+
+	return 0;
+}
+
 /*
  * Some boot consoles access data that is in the init section and which will
  * be discarded after the initcalls have been run. To make sure that no code
@@ -3567,6 +3580,7 @@ void __init console_init(void)
  */
 static int __init printk_late_init(void)
 {
+	bool no_bootcon = true;
 	struct console *con;
 	int ret;
 
@@ -3588,7 +3602,10 @@ static int __init printk_late_init(void)
 			pr_warn("bootconsole [%s%d] uses init memory and must be disabled even before the real one is ready\n",
 				con->name, con->index);
 			unregister_console(con);
+			continue;
 		}
+
+		no_bootcon = false;
 	}
 	ret = cpuhp_setup_state_nocalls(CPUHP_PRINTK_DEAD, "printk:dead", NULL,
 					console_cpu_notify);
@@ -3597,23 +3614,19 @@ static int __init printk_late_init(void)
 					console_cpu_notify, NULL);
 	WARN_ON(ret < 0);
 	printk_sysctl_init();
-	return 0;
-}
-late_initcall(printk_late_init);
-
-static int __init printk_activate_kthreads(void)
-{
-	struct console *con;
 
-	console_lock();
-	printk_kthreads_available = true;
-	for_each_console(con)
-		printk_start_kthread(con);
-	console_unlock();
+	/*
+	 * Boot consoles may be accessing the same hardware as normal
+	 * consoles and thus must not be called in parallel. Therefore
+	 * only activate threaded console printing if it is known that
+	 * there are no boot consoles registered.
+	 */
+	if (no_bootcon)
+		printk_activate_kthreads();
 
 	return 0;
 }
-early_initcall(printk_activate_kthreads);
+late_initcall(printk_late_init);
 
 #if defined CONFIG_PRINTK
 /* If @con is specified, only wait for that console. Otherwise wait for all. */
-- 
2.35.3