Re: Regression: system freeze on resume from suspend introduced by printk per-console suspended state

[John Ogness <john.ogness@linutronix.de>]

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On 2026-01-08, pv <pierre.vignet@caramail.fr> wrote:
> The code in 6.18.2 has changed enough that I don't dare change it as I did in 6.7.
> After compiling and installing 6.19-rc1 (8f0b4cce4481fb22653697cced8d0d04027cb1e8),
> I can confirm that the freeze is still present.

Thanks for confirmation. Would you be willing to try a debugging patch?

I have attached a patch (based on 6.19-rc4). It should restore the old
console_lock behavior during suspend/resume. Assuming this works for
you, it also adds some debugging information so that we can figure out
who is locking the console.

Please compile with CONFIG_PRINTK_CALLER=y.

After suspend/resume, locate the debug lines with:

# dmesg | grep printk

It will probably look something like this:

# dmesg | grep printk
[   12.085727][  T843] printk: Suspending console(s) (use no_console_suspend to debug)
[   12.236199][  T843] printk: console_suspend
[   17.581734][   T67] printk: console_lock
[   17.583711][   T67] printk: console_unlock
[   17.644819][  T843] printk: console_lock
[   17.644823][  T843] printk: console_unlock
[   17.644827][  T843] printk: console_resume

If you have CONFIG_PRINTK_CALLER enabled (as I do here), it will show
the tasks locking the console. In my case it is PID 67 and 843. It would
be nice to know who these are. For me it is:

# cat /proc/67/comm
kworker/2:1-mm_percpu_wq

# cat /proc/843/comm
sh

So in summary, I would like to know:

1. Does suspend/resume work on 6.19-rc4 with this patch applied?

2. Output of "dmesg | grep printk" (after suspend/resume)

3. Which tasks were locking the console between console_suspend and
console_resume?

Your help in debugging this is greatly appreciated.

John Ogness


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From f3bb7d49b59b61a30f2fefc2b241d3f535b2afd2 Mon Sep 17 00:00:00 2001
From: John Ogness <john.ogness@linutronix.de>
Date: Thu, 8 Jan 2026 09:31:51 +0000
Subject: [PATCH] printk: Debug new vs. old suspend/resume behavior

This is just for debugging. It should restore the old console_lock
behavior for suspend/resume and also adds some debugging information.
Please compile with CONFIG_PRINTK_CALLER=y so that we can see which
tasks are locking/unlocking the console during suspend/resume.
---
 kernel/printk/printk.c | 61 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 61 insertions(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 1d765ad242b8..fd69cab4368e 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -356,6 +356,22 @@ static void __up_console_sem(unsigned long ip)
  */
 static int console_locked;
 
+static int console_suspended;
+
+int vprintk_store(int facility, int level,
+		  const struct dev_printk_info *dev_info,
+		  const char *fmt, va_list args);
+
+/* Helper function to store-only. */
+static void printk_store(const char *fmt, ...)
+{
+	va_list args;
+
+	va_start(args, fmt);
+	vprintk_store(0, LOGLEVEL_DEFAULT, NULL, fmt, args);
+	va_end(args);
+}
+
 /*
  *	Array of consoles built from command line options (console=)
  */
@@ -2841,6 +2857,15 @@ void console_lock(void)
 		msleep(1000);
 
 	down_console_sem();
+	if (console_suspended) {
+		printk_store(KERN_INFO "printk: %s\n", __func__);
+		/*
+		 * Keep console locked, but do not touch
+		 * @console_locked or @console_may_schedule.
+		 * (Although they will both be 1 here anyway.)
+		 */
+		return;
+	}
 	console_locked = 1;
 	console_may_schedule = 1;
 }
@@ -2861,6 +2886,15 @@ int console_trylock(void)
 		return 0;
 	if (down_trylock_console_sem())
 		return 0;
+	if (console_suspended) {
+		printk_store(KERN_INFO "printk: %s\n", __func__);
+		/*
+		 * The lock was acquired, but unlock directly and report
+		 * failure. Here console_locked=1 and console_may_schedule=1.
+		 */
+		up_console_sem();
+		return 0;
+	}
 	console_locked = 1;
 	console_may_schedule = 0;
 	return 1;
@@ -3354,6 +3388,16 @@ void console_unlock(void)
 {
 	struct console_flush_type ft;
 
+	if (console_suspended) {
+		printk_store(KERN_INFO "printk: %s\n", __func__);
+		/*
+		 * Simply unlock directly.
+		 * Here console_locked=1 and console_may_schedule=1.
+		 */
+		up_console_sem();
+		return;
+	}
+
 	printk_get_console_flush_type(&ft);
 	if (ft.legacy_direct)
 		__console_flush_and_unlock();
@@ -3559,6 +3603,7 @@ struct tty_driver *console_device(int *index)
 void console_suspend(struct console *console)
 {
 	__pr_flush(console, 1000, true);
+
 	console_list_lock();
 	console_srcu_write_flags(console, console->flags & ~CON_ENABLED);
 	console_list_unlock();
@@ -3570,6 +3615,12 @@ void console_suspend(struct console *console)
 	 * using the port.
 	 */
 	synchronize_srcu(&console_srcu);
+
+	console_lock();
+	console_suspended = 1;
+	printk_store(KERN_INFO "printk: %s\n", __func__);
+	/* Unlock directly (i.e. without clearing @console_locked). */
+	up_console_sem();
 }
 EXPORT_SYMBOL(console_suspend);
 
@@ -3597,6 +3648,16 @@ void console_resume(struct console *console)
 		defer_console_output();
 
 	__pr_flush(console, 1000, true);
+
+	down_console_sem();
+	printk_store(KERN_INFO "printk: %s\n", __func__);
+	console_suspended = 0;
+	/*
+	 * Perform a regular unlock.
+	 * Here console_locked=1 and console_may_schedule=1.
+	 * @console_unlocked will be cleared.
+	 */
+	console_unlock();
 }
 EXPORT_SYMBOL(console_resume);
 
-- 
2.30.2