[PATCH printk v5 18/40] printk: console_unblank: use srcu console list iterator

[John Ogness <john.ogness@linutronix.de>]

Use srcu console list iteration for console list traversal.

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@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
 kernel/printk/printk.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 75951c4bda05..b4be3b08d909 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2999,10 +2999,14 @@ EXPORT_SYMBOL(console_conditional_schedule);
 void console_unblank(void)
 {
 	struct console *c;
+	int cookie;
 
 	/*
-	 * console_unblank can no longer be called in interrupt context unless
-	 * oops_in_progress is set to 1..
+	 * Stop console printing because the unblank() callback may
+	 * assume the console is not within its write() callback.
+	 *
+	 * If @oops_in_progress is set, this may be an atomic context.
+	 * In that case, attempt a trylock as best-effort.
 	 */
 	if (oops_in_progress) {
 		if (down_trylock_console_sem() != 0)
@@ -3012,9 +3016,14 @@ void console_unblank(void)
 
 	console_locked = 1;
 	console_may_schedule = 0;
-	for_each_console(c)
-		if ((c->flags & CON_ENABLED) && c->unblank)
+
+	cookie = console_srcu_read_lock();
+	for_each_console_srcu(c) {
+		if ((console_srcu_read_flags(c) & CON_ENABLED) && c->unblank)
 			c->unblank();
+	}
+	console_srcu_read_unlock(cookie);
+
 	console_unlock();
 
 	if (!oops_in_progress)
-- 
2.30.2