Re: [PATCH 02/19] printk: Introduce console_is_nbcon

Besides checking if the current console is NBCON or not, console->flags
is also being read in order to serve as argument of the console_is_usable
function.

But CON_NBCON flag is unique: it's set just once in the console
registration and never cleared. In this case it can be possible to read
the flag when console_srcu_lock is held (which is the case when using
for_each_console).

This change makes possible to remove the flags argument from
console_is_usable in the next patches.

Signed-off-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Marcos Paulo de Souza <redacted>
---
 include/linux/console.h   | 27 +++++++++++++++++++++++++++
 kernel/debug/kdb/kdb_io.c |  2 +-
 kernel/printk/nbcon.c     |  2 +-
 kernel/printk/printk.c    | 15 ++++++---------
 4 files changed, 35 insertions(+), 11 deletions(-)

diff --git a/include/linux/console.h b/include/linux/console.h
index 35c03fc4ed51..dd4ec7a5bff9 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h

@@ -561,6 +561,33 @@ static inline void console_srcu_write_flags(struct console *con, short flags)
 	WRITE_ONCE(con->flags, flags);
 }
 
+/**
+ * console_srcu_is_nbcon - Locklessly check whether the console is nbcon

+ * @con:	struct console pointer of console to check
+ *
+ * Requires console_srcu_read_lock to be held, which implies that @con might
+ * be a registered console. The purpose of holding console_srcu_read_lock is
+ * to guarantee that no exit/cleanup routines will run if the console
+ * is currently undergoing unregistration.
+ *
+ * If the caller is holding the console_list_lock or it is _certain_ that
+ * @con is not and will not become registered, the caller may read
+ * @con->flags directly instead.
+ *
+ * Context: Any context.
+ * Return: True when CON_NBCON flag is set.
+ */
+static inline bool console_is_nbcon(const struct console *con)

+{
+	WARN_ON_ONCE(!console_srcu_read_lock_is_held());
+
+	/*
+	 * The CON_NBCON flag is statically initialized and is never
+	 * set or cleared at runtime.
+	 */
+	return data_race(con->flags & CON_NBCON);
+}
+
 /* Variant of console_is_registered() when the console_list_lock is held. */
 static inline bool console_is_registered_locked(const struct console *con)
 {