printk: console_unblank: use srcu console list iterator

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>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20221116162152.193147-19-john.ogness@linutronix.de

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 853a86c94ee088a2eb08d275640ebfe4bf459644..8e37a35764f2da62c513bd65e8a3f7d6ad83276d 100644 (file)
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3000,10 +3000,14 @@ 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)
@@ -3013,9 +3017,14 @@ 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)