[PATCH printk v5 12/40] console: introduce wrappers to read/write console flags

From: John Ogness
Date: Wed Nov 16 2022 - 11:23:20 EST


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@xxxxxxxxxxxxx>
Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
---
include/linux/console.h | 45 +++++++++++++++++++++++++++++++++++++++++
kernel/printk/printk.c | 10 ++++-----
2 files changed, 50 insertions(+), 5 deletions(-)

diff --git a/include/linux/console.h b/include/linux/console.h
index 24d83e24840b..c1ca461d088a 100644
--- a/include/linux/console.h
+++ b/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
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index f7479fd73114..35018f18f5aa 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3068,7 +3068,7 @@ 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();

@@ -3086,7 +3086,7 @@ 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);
@@ -3342,7 +3342,7 @@ 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 {
@@ -3399,7 +3399,7 @@ 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();
@@ -3418,7 +3418,7 @@ 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();

--
2.30.2