console: introduce wrappers to read/write console flags

After switching to SRCU for console list iteration, some readers
will begin readings console->flags as a data race. Locklessly
reading console->flags provides a consistent value because there
is at most one CPU modifying console->flags and that CPU is
using only read-modify-write operations.

Introduce a wrapper for SRCU iterators to read console flags.
Introduce a matching wrapper to write to flags of registered
consoles. Writing to flags of registered consoles is synchronized
by the console_list_lock.
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20221116162152.193147-13-john.ogness@linutronix.de

include/linux/console.h

@@ -183,6 +183,51 @@ extern void console_list_unlock(void) __releases(console_mutex);
 
 extern struct hlist_head console_list;
 
+/**
+ * console_srcu_read_flags - Locklessly read the console flags
+ * @con:	struct console pointer of console to read flags from
+ *
+ * This function provides the necessary READ_ONCE() and data_race()
+ * notation for locklessly reading the console flags. The READ_ONCE()
+ * in this function matches the WRITE_ONCE() when @flags are modified
+ * for registered consoles with console_srcu_write_flags().
+ *
+ * Only use this function to read console flags when locklessly
+ * iterating the console list via srcu.
+ *
+ * Context: Any context.
+ */
+static inline short console_srcu_read_flags(const struct console *con)
+{
+	WARN_ON_ONCE(!console_srcu_read_lock_is_held());
+
+	/*
+	 * Locklessly reading console->flags provides a consistent
+	 * read value because there is at most one CPU modifying
+	 * console->flags and that CPU is using only read-modify-write
+	 * operations to do so.
+	 */
+	return data_race(READ_ONCE(con->flags));
+}
+
+/**
+ * console_srcu_write_flags - Write flags for a registered console
+ * @con:	struct console pointer of console to write flags to
+ * @flags:	new flags value to write
+ *
+ * Only use this function to write flags for registered consoles. It
+ * requires holding the console_list_lock.
+ *
+ * Context: Any context.
+ */
+static inline void console_srcu_write_flags(struct console *con, short flags)
+{
+	lockdep_assert_console_list_lock_held();
+
+	/* This matches the READ_ONCE() in console_srcu_read_flags(). */
+	WRITE_ONCE(con->flags, flags);
+}
+
 /**
  * for_each_console_srcu() - Iterator over registered consoles
  * @con:	struct console pointer used as loop cursor

kernel/printk/printk.c

@@ -3069,7 +3069,7 @@ void console_stop(struct console *console)
 	__pr_flush(console, 1000, true);
 	console_list_lock();
 	console_lock();
-	console->flags &= ~CON_ENABLED;
+	console_srcu_write_flags(console, console->flags & ~CON_ENABLED);
 	console_unlock();
 	console_list_unlock();
 
@@ -3087,7 +3087,7 @@ void console_start(struct console *console)
 {
 	console_list_lock();
 	console_lock();
-	console->flags |= CON_ENABLED;
+	console_srcu_write_flags(console, console->flags | CON_ENABLED);
 	console_unlock();
 	console_list_unlock();
 	__pr_flush(console, 1000, true);
@@ -3343,7 +3343,7 @@ void register_console(struct console *newcon)
 
 	} else if (newcon->flags & CON_CONSDEV) {
 		/* Only the new head can have CON_CONSDEV set. */
-		console_first()->flags &= ~CON_CONSDEV;
+		console_srcu_write_flags(console_first(), console_first()->flags & ~CON_CONSDEV);
 		hlist_add_head_rcu(&newcon->node, &console_list);
 
 	} else {
@@ -3400,7 +3400,7 @@ static int unregister_console_locked(struct console *console)
 	console_lock();
 
 	/* Disable it unconditionally */
-	console->flags &= ~CON_ENABLED;
+	console_srcu_write_flags(console, console->flags & ~CON_ENABLED);
 
 	if (hlist_unhashed(&console->node)) {
 		console_unlock();
@@ -3419,7 +3419,7 @@ static int unregister_console_locked(struct console *console)
 	 * console has any device attached. Oh well....
 	 */
 	if (!hlist_empty(&console_list) && console->flags & CON_CONSDEV)
-		console_first()->flags |= CON_CONSDEV;
+		console_srcu_write_flags(console_first(), console_first()->flags | CON_CONSDEV);
 
 	console_unlock();