[DEBUG v2] printk: Debug new vs. old suspend/resume behavior

[John Ogness]

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.

Changes against v1:

- Set/Clear the global "console_suspended" variable in
  console_suspend_all()/console_restore_all() instead of
  console_suspend()/console_resume().

  The functions have been renamed recently, see
  https://lore.kernel.org/all/20250226-printk-renaming-v1-0-0b878577f2e6@xxxxxxxx/

- Do not call synchronize_srcu() in the suspend/resume functions.
  They are another potentially blocking operation added by
  the problematic commit 9e70a5e109a4a2336 ("printk: Add per-console
  suspended state").

Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
---
 kernel/printk/printk.c | 64 ++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 62 insertions(+), 2 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 1d765ad242b8..23fddc4006d3 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=)
  */
@@ -2748,6 +2764,12 @@ void console_suspend_all(void)
 	if (!console_suspend_enabled)
 		return;
 
+	console_lock();
+	console_suspended = 1;
+	printk_store(KERN_INFO "printk: %s\n", __func__);
+	/* Unlock directly (i.e. without clearing @console_locked). */
+	up_console_sem();
+
 	console_list_lock();
 	for_each_console(con)
 		console_srcu_write_flags(con, con->flags | CON_SUSPENDED);
@@ -2759,7 +2781,7 @@ void console_suspend_all(void)
 	 * is guaranteed that all printing has stopped when this function
 	 * completes.
 	 */
-	synchronize_srcu(&console_srcu);
+//	synchronize_srcu(&console_srcu);
 }
 
 void console_resume_all(void)
@@ -2785,7 +2807,17 @@ void console_resume_all(void)
 		 * contexts must be able to see they are no longer suspended so
 		 * that they are guaranteed to wake up and resume printing.
 		 */
-		synchronize_srcu(&console_srcu);
+//		synchronize_srcu(&console_srcu);
+
+		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();
 	}
 
 	printk_get_console_flush_type(&ft);
@@ -2841,6 +2873,15 @@ 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 +2902,15 @@ 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 +3404,16 @@ 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();
-- 
2.52.0