[PATCHv2 3/4] printk: factor out register_console() code

[Sergey Senozhatsky]

We need to take console_sem lock when we iterate console drivers
list. Otherwise, another CPU can concurrently modify console drivers
list or console drivers. Current register_console() has several
race conditions - for_each_console() must be done under console_sem.

Factor out console registration code and hold console_sem throughout
entire registration process. Note that we need to unlock console_sem
and lock it again after we added new console to the list and before
we unregister boot consoles. This might look a bit weird, but this
is how we print pending logbuf messages to all registered and
available consoles.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
---
 kernel/printk/printk.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 3ac71701afa3..3b36e26d4a51 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2666,7 +2666,7 @@ static int __unregister_console(struct console *console)
  *  - Once a "real" console is registered, any attempt to register a
  *    bootconsoles will be rejected
  */
-void register_console(struct console *newcon)
+static void __register_console(struct console *newcon)
 {
 	int i;
 	unsigned long flags;
@@ -2771,7 +2771,6 @@ void register_console(struct console *newcon)
 	 *	Put this console in the list - keep the
 	 *	preferred driver at the head of the list.
 	 */
-	console_lock();
 	if ((newcon->flags & CON_CONSDEV) || console_drivers == NULL) {
 		newcon->next = console_drivers;
 		console_drivers = newcon;
@@ -2818,6 +2817,7 @@ void register_console(struct console *newcon)
 
 	console_unlock();
 	console_sysfs_notify();
+	console_lock();
 
 	if (keep_bootcon)
 		return;
@@ -2830,14 +2830,19 @@ void register_console(struct console *newcon)
 	 * went to the bootconsole (that they do not see on the real console)
 	 */
 	if (bcon && (newcon->flags & (CON_CONSDEV|CON_BOOT)) == CON_CONSDEV) {
-		console_lock();
 		for_each_console(bcon)
 			if (bcon->flags & CON_BOOT)
 				__unregister_console(bcon);
-		console_unlock();
-		console_sysfs_notify();
 	}
 }
+
+void register_console(struct console *newcon)
+{
+	console_lock();
+	__register_console(newcon);
+	console_unlock();
+	console_sysfs_notify();
+}
 EXPORT_SYMBOL(register_console);
 
 int unregister_console(struct console *console)
-- 
2.21.0