tty: serial: kgdboc: document console_lock usage

kgdboc_earlycon_init() uses the console_lock to ensure that no consoles
are unregistered until the kgdboc_earlycon is setup. This is necessary
because the trapping of the exit() callback assumes that the exit()
callback is not called before the trap is setup.

Explicitly document this non-typical console_lock usage.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20221116162152.193147-9-john.ogness@linutronix.de

diff --git a/drivers/tty/serial/kgdboc.c b/drivers/tty/serial/kgdboc.c
index e76f0186c3355b6b437832aeffa0fc64c66cd86e..5be381003e585eb609816cdef29e041718a10c6b 100644 (file)
--- a/drivers/tty/serial/kgdboc.c
+++ b/drivers/tty/serial/kgdboc.c
@@ -530,6 +530,14 @@ static int __init kgdboc_earlycon_init(char *opt)
         * Look for a matching console, or if the name was left blank just
         * pick the first one we find.
         */
+
+       /*
+        * Hold the console_lock to guarantee that no consoles are
+        * unregistered until the kgdboc_earlycon setup is complete.
+        * Trapping the exit() callback relies on exit() not being
+        * called until the trap is setup. This also allows safe
+        * traversal of the console list and race-free reading of @flags.
+        */
        console_lock();
        for_each_console(con) {
                if (con->write && con->read &&