console: Improve console_srcu_read_flags() comments

It was not clear when exactly console_srcu_read_flags() must be
used vs. directly reading @console->flags.

Refactor and clarify that console_srcu_read_flags() is only
needed if the console is registered or the caller is in a
context where the registration status of the console may change
(due to another context).

The function requires the caller holds @console_srcu, which will
ensure that the caller sees an appropriate @flags value for the
registered console and that exit/cleanup routines will not run
if the console is in the process of unregistration.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240820063001.36405-13-john.ogness@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>

1 files changed, 17 insertions, 11 deletions

diff --git a/include/linux/console.h b/include/linux/console.h
index 46b3c210b931..aafe3121b74e 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -446,28 +446,34 @@ extern void console_list_unlock(void) __releases(console_mutex);
 extern struct hlist_head console_list;
 
 /**
- * console_srcu_read_flags - Locklessly read the console flags
+ * console_srcu_read_flags - Locklessly read flags of a possibly registered
+ *				console
  * @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().
+ * Locklessly reading @con->flags provides a consistent read value because
+ * there is at most one CPU modifying @con->flags and that CPU is using only
+ * read-modify-write operations to do so.
  *
- * Only use this function to read console flags when locklessly
- * iterating the console list via srcu.
+ * 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 the console state is valid (CON_SUSPENDED/CON_ENABLED)
+ * and 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: The current value of the @con->flags field.
  */
 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.
+	 * The READ_ONCE() matches the WRITE_ONCE() when @flags are modified
+	 * for registered consoles with console_srcu_write_flags().
 	 */
 	return data_race(READ_ONCE(con->flags));
 }