[PATCH printk v3 22/40] console: introduce console_is_registered()

[John Ogness]

Currently it is not possible for drivers to detect if they have
already successfully registered their console. Several drivers
have multiple paths that lead to console registration. To avoid
attempting a 2nd registration (which leads to a WARN), drivers
are implementing their own solution.

Introduce console_is_registered() so drivers can easily identify
if their console is currently registered. A _locked() variant
is also provided if the caller is already holding the
console_list_lock.

Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
---
 include/linux/console.h | 28 ++++++++++++++++++++++++++++
 kernel/printk/printk.c  |  2 +-
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/include/linux/console.h b/include/linux/console.h
index 47be23be8a88..cdae70e27377 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -210,6 +210,34 @@ static inline bool console_is_enabled(const struct console *con)
 	return (data_race(READ_ONCE(con->flags)) & CON_ENABLED);
 }
 
+/* Variant of console_is_registered() when the console_list_lock is held. */
+static inline bool console_is_registered_locked(const struct console *con)
+{
+	lockdep_assert_console_list_lock_held();
+	return !hlist_unhashed(&con->node);
+}
+
+/*
+ * console_is_registered - Check if the console is registered
+ * @con:	struct console pointer of console to check
+ *
+ * Context: Process context. May sleep while acquiring console list lock.
+ * Return: true if the console is in the console list, otherwise false.
+ *
+ * If false is returned for a console that was previously registered, it
+ * can be assumed that the console's unregistration is fully completed,
+ * including the exit() callback after console list removal.
+ */
+static inline bool console_is_registered(const struct console *con)
+{
+	bool ret;
+
+	console_list_lock();
+	ret = console_is_registered_locked(con);
+	console_list_unlock();
+	return ret;
+}
+
 /**
  * for_each_console_srcu() - Iterator over registered consoles
  * @con:	struct console pointer used as loop cursor
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 31387ba3fa1a..be40a9688403 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3410,7 +3410,7 @@ static int unregister_console_locked(struct console *console)
 	/* Disable it unconditionally */
 	WRITE_ONCE(console->flags, console->flags & ~CON_ENABLED);
 
-	if (hlist_unhashed(&console->node)) {
+	if (!console_is_registered_locked(console)) {
 		console_unlock();
 		return -ENODEV;
 	}
-- 
2.30.2