Subject: [PATCH 04/24] printk: Add per-console suspended state
From: John Ogness <[email-masked]>
Date: Fri, 17 Feb 2023 15:43:37 +0000

Currently the global @console_suspended is used to determine if
consoles are in a suspended state. Its primary purpose is to allow
usage of the console_lock when suspended without causing console
printing. It is synchronized by the console_lock.

Rather than relying on the console_lock to determine suspended
state, make it an official per-console state that is set within
console->flags. This allows the state to be queried via SRCU.

@console_suspended will continue to exist, but now only to implement
the console_lock/console_unlock trickery and _not_ to represent
the suspend state of a particular console.

Signed-off-by: John Ogness <[email-masked]>
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
---
 include/linux/console.h |    3 +++
 kernel/printk/printk.c  |   46 ++++++++++++++++++++++++++++++++++++----------
 2 files changed, 39 insertions(+), 10 deletions(-)

Index: linux-6.3.0-rt11/include/linux/console.h
===================================================================
--- linux-6.3.0-rt11.orig/include/linux/console.h
+++ linux-6.3.0-rt11/include/linux/console.h
@@ -154,6 +154,8 @@ static inline int con_debug_leave(void)
  *			receiving the printk spam for obvious reasons.
  * @CON_EXTENDED:	The console supports the extended output format of
  *			/dev/kmesg which requires a larger output buffer.
+ * @CON_SUSPENDED:	Indicates if a console is suspended. If true, the
+ *			printing callbacks must not be called.
  */
 enum cons_flags {
 	CON_PRINTBUFFER		= BIT(0),
@@ -163,6 +165,7 @@ enum cons_flags {
 	CON_ANYTIME		= BIT(4),
 	CON_BRL			= BIT(5),
 	CON_EXTENDED		= BIT(6),
+	CON_SUSPENDED		= BIT(7),
 };
 
 /**
Index: linux-6.3.0-rt11/kernel/printk/printk.c
===================================================================
--- linux-6.3.0-rt11.orig/kernel/printk/printk.c
+++ linux-6.3.0-rt11/kernel/printk/printk.c
@@ -2559,10 +2559,26 @@ MODULE_PARM_DESC(console_no_auto_verbose
  */
 void suspend_console(void)
 {
+	struct console *con;
+
 	if (!console_suspend_enabled)
 		return;
 	pr_info("Suspending console(s) (use no_console_suspend to debug)\n");
 	pr_flush(1000, true);
+
+	console_list_lock();
+	for_each_console(con)
+		console_srcu_write_flags(con, con->flags | CON_SUSPENDED);
+	console_list_unlock();
+
+	/*
+	 * Ensure that all SRCU list walks have completed. All printing
+	 * contexts must be able to see that they are suspended so that it
+	 * is guaranteed that all printing has stopped when this function
+	 * completes.
+	 */
+	synchronize_srcu(&console_srcu);
+
 	console_lock();
 	console_suspended = 1;
 	up_console_sem();
@@ -2570,11 +2586,26 @@ void suspend_console(void)
 
 void resume_console(void)
 {
+	struct console *con;
+
 	if (!console_suspend_enabled)
 		return;
 	down_console_sem();
 	console_suspended = 0;
 	console_unlock();
+
+	console_list_lock();
+	for_each_console(con)
+		console_srcu_write_flags(con, con->flags & ~CON_SUSPENDED);
+	console_list_unlock();
+
+	/*
+	 * Ensure that all SRCU list walks have completed. All printing
+	 * 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);
+
 	pr_flush(1000, true);
 }
 
@@ -2677,6 +2708,9 @@ static inline bool console_is_usable(str
 	if (!(flags & CON_ENABLED))
 		return false;
 
+	if ((flags & CON_SUSPENDED))
+		return false;
+
 	if (!con->write)
 		return false;
 
@@ -3700,8 +3734,7 @@ static bool __pr_flush(struct console *c
 
 		/*
 		 * Hold the console_lock to guarantee safe access to
-		 * console->seq and to prevent changes to @console_suspended
-		 * until all consoles have been processed.
+		 * console->seq.
 		 */
 		console_lock();
 
@@ -3717,14 +3750,7 @@ static bool __pr_flush(struct console *c
 		}
 		console_srcu_read_unlock(cookie);
 
-		/*
-		 * If consoles are suspended, it cannot be expected that they
-		 * make forward progress, so timeout immediately. @diff is
-		 * still used to return a valid flush status.
-		 */
-		if (console_suspended)
-			remaining = 0;
-		else if (diff != last_diff && reset_on_progress)
+		if (diff != last_diff && reset_on_progress)
 			remaining = timeout_ms;
 
 		console_unlock();