[PATCH printk 12/18] printk: Prepare for SCRU console list protection

[John Ogness]

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

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@xxxxxxxxxxxxx>
Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
---
 include/linux/console.h | 14 +++++++++++++-
 kernel/printk/printk.c  | 16 +++++++++++++---
 2 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/include/linux/console.h b/include/linux/console.h
index 1e3d0a50cef1..dc0df9d9e7d9 100644
--- 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) { }
@@ -171,6 +172,17 @@ extern struct hlist_head console_list;
 extern void console_list_lock(void) __acquires(console_mutex);
 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
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index f1d31dcbd6ba..7e6d1cd34452 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -93,6 +93,7 @@ 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
@@ -121,6 +122,10 @@ void lockdep_assert_console_list_lock_held(void)
 	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 {
@@ -3232,9 +3237,9 @@ void register_console(struct console *newcon)
 	 */
 	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;
@@ -3250,6 +3255,7 @@ void register_console(struct console *newcon)
 		newcon->seq = prb_next_seq(prb);
 	}
 	console_unlock();
+	/* No need to synchronize SRCU here! */
 	console_sysfs_notify();
 
 	/*
@@ -3295,7 +3301,7 @@ static int console_unregister_locked(struct console *console)
 	if (hlist_unhashed(&console->node))
 		goto out_unlock;
 
-	hlist_del_init(&console->node);
+	hlist_del_init_rcu(&console->node);
 
 	/*
 	 * <HISTORICAL>
@@ -3310,6 +3316,10 @@ static int console_unregister_locked(struct console *console)
 		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)
-- 
2.30.2