console: introduce wrappers to read/write console flags

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@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20221116162152.193147-13-john.ogness@linutronix.de

diff --git a/include/linux/console.h b/include/linux/console.h
index 24d83e2..c1ca461 100644 (file)
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -184,6 +184,51 @@ 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 2b45066..cad65d3 100644 (file)
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3069,7 +3069,7 @@ 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();
 
@@ -3087,7 +3087,7 @@ 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);
@@ -3343,7 +3343,7 @@ 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 {
@@ -3400,7 +3400,7 @@ 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();
@@ -3419,7 +3419,7 @@ 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();