[patch RFC 12/29] printk: Prepare for SCRU console list protection

[Thomas Gleixner <tglx@linutronix.de>]

Provide a SRCU protected variant to walk the console list.

Preperatory change for a new console infrastructure which operates
independent of console BKL.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 include/linux/console.h |   14 +++++++++++++-
 kernel/printk/printk.c  |   16 +++++++++++++---
 2 files changed, 26 insertions(+), 4 deletions(-)

--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -15,7 +15,7 @@
 #define _LINUX_CONSOLE_H_ 1
 
 #include <linux/atomic.h>
-#include <linux/list.h>
+#include <linux/rculist.h>
 #include <linux/types.h>
 
 struct vc_data;
@@ -161,6 +161,7 @@ struct console {
 #ifdef CONFIG_LOCKDEP
 extern void lockdep_assert_console_lock_held(void);
 extern void lockdep_assert_console_list_lock_held(void);
+extern bool console_srcu_read_lock_is_held(void);
 #else
 static inline void lockdep_assert_console_lock_held(void) { }
 static inline void lockdep_assert_console_list_lock_held(void) { }
@@ -172,6 +173,17 @@ extern void console_list_lock(void) __ac
 extern void console_list_unlock(void) __releases(console_mutex);
 
 /**
+ * for_each_console_srcu() - Iterator over registered consoles
+ * @con:	struct console pointer used as loop cursor
+ *
+ * Requires console_srcu_read_lock to be held. Can be invoked from
+ * any context.
+ */
+#define for_each_console_srcu(con)					\
+	hlist_for_each_entry_srcu(con, &console_list, node,		\
+				  console_srcu_read_lock_is_held())
+
+/**
  * for_each_registered_console() - Iterator over registered consoles
  * @con:	struct console pointer used as loop cursor
  *
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -90,6 +90,7 @@ static DEFINE_MUTEX(console_mutex);
 static DEFINE_SEMAPHORE(console_sem);
 HLIST_HEAD(console_list);
 EXPORT_SYMBOL_GPL(console_list);
+DEFINE_STATIC_SRCU(console_srcu);
 
 /*
  * System may need to suppress printk message under certain
@@ -118,6 +119,10 @@ void lockdep_assert_console_list_lock_he
 	lockdep_assert_held(&console_mutex);
 }
 
+bool console_srcu_read_lock_is_held(void)
+{
+	return srcu_read_lock_held(&console_srcu);
+}
 #endif
 
 enum devkmsg_log_bits {
@@ -3227,9 +3232,9 @@ void register_console(struct console *ne
 	 */
 	console_lock();
 	if (newcon->flags & CON_CONSDEV || hlist_empty(&console_list))
-		hlist_add_head(&newcon->node, &console_list);
+		hlist_add_head_rcu(&newcon->node, &console_list);
 	else
-		hlist_add_behind(&newcon->node, console_list.first);
+		hlist_add_behind_rcu(&newcon->node, console_list.first);
 
 	/* Ensure this flag is always set for the head of the list */
 	cons_first()->flags |= CON_CONSDEV;
@@ -3245,6 +3250,7 @@ void register_console(struct console *ne
 		newcon->seq = prb_next_seq(prb);
 	}
 	console_unlock();
+	/* No need to synchronize SRCU here! */
 	console_sysfs_notify();
 
 	/*
@@ -3290,7 +3296,7 @@ static int console_unregister_locked(str
 	if (hlist_unhashed(&console->node))
 		goto out_unlock;
 
-	hlist_del_init(&console->node);
+	hlist_del_init_rcu(&console->node);
 
 	/*
 	 * <HISTORICAL>
@@ -3305,6 +3311,10 @@ static int console_unregister_locked(str
 		cons_first()->flags |= CON_CONSDEV;
 
 	console_unlock();
+
+	/* Ensure that all SRCU list walks have completed */
+	synchronize_srcu(&console_srcu);
+
 	console_sysfs_notify();
 
 	if (console->exit)