[PATCH printk v4 19/39] printk: console_device: use srcu console list iterator

[John Ogness]

Use srcu console list iteration for console list traversal. It is
acceptable because the consoles might come and go at any time.
Strict synchronizing with console registration code would not bring
any advantage over srcu.

Document why the console_lock is still necessary. Note that this
is a preparatory change for when console_lock no longer provides
synchronization for the console list.

Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
---
 kernel/printk/printk.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index f536233d8234..fb609a19f797 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3076,15 +3076,25 @@ struct tty_driver *console_device(int *index)
 {
 	struct console *c;
 	struct tty_driver *driver = NULL;
+	int cookie;
 
+	/*
+	 * Take console_lock to serialize device() callback with
+	 * other console operations. For example, fg_console is
+	 * modified under console_lock when switching vt.
+	 */
 	console_lock();
-	for_each_console(c) {
+
+	cookie = console_srcu_read_lock();
+	for_each_console_srcu(c) {
 		if (!c->device)
 			continue;
 		driver = c->device(c, index);
 		if (driver)
 			break;
 	}
+	console_srcu_read_unlock(cookie);
+
 	console_unlock();
 	return driver;
 }
-- 
2.30.2