[PATCH printk v5 10/30] console: Improve console_srcu_read_flags() comments

John Ogness
2024-05-02T21:38:49Z

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>
---
include/linux/console.h | 28 +++++++++++++++++-----------
1 file changed, 17 insertions(+), 11 deletions(-)

diff --git a/include/linux/console.h b/include/linux/console.h
index 33a029d976c3..aea4f5aa4a45 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -428,28 +428,34 @@ 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));
}
--
2.39.2