tty: serial: kgdboc: fix mutex locking order for configure_kgdboc()

Several mutexes are taken while setting up console serial ports. In
particular, the tty_port->mutex and @console_mutex are taken:

  serial_pnp_probe
    serial8250_register_8250_port
      uart_add_one_port (locks tty_port->mutex)
        uart_configure_port
          register_console (locks @console_mutex)

In order to synchronize kgdb's tty_find_polling_driver() with
register_console(), commit 6193bc90 ("tty: serial: kgdboc:
synchronize tty_find_polling_driver() and register_console()") takes
the @console_mutex. However, this leads to the following call chain
(with locking):

  platform_probe
    kgdboc_probe
      configure_kgdboc (locks @console_mutex)
        tty_find_polling_driver
          uart_poll_init (locks tty_port->mutex)
            uart_set_options

This is clearly deadlock potential due to the reverse lock ordering.

Since uart_set_options() requires holding @console_mutex in order to
serialize early initialization of the serial-console lock, take the
@console_mutex in uart_poll_init() instead of configure_kgdboc().

Since configure_kgdboc() was using @console_mutex for safe traversal
of the console list, change it to use the SRCU iterator instead.

Add comments to uart_set_options() kerneldoc mentioning that it
requires holding @console_mutex (aka the console_list_lock).

Fixes: 6193bc90 ("tty: serial: kgdboc: synchronize tty_find_polling_driver() and register_console()")
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
[pmladek@suse.com: Export console_srcu_read_lock_is_held() to fix build kgdboc as a module.]
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20230112161213.1434854-1-john.ogness@linutronix.de

drivers/tty/serial/kgdboc.c

@@ -171,6 +171,7 @@ static int configure_kgdboc(void)
 	int err = -ENODEV;
 	char *cptr = config;
 	struct console *cons;
+	int cookie;
 
 	if (!strlen(config) || isspace(config[0])) {
 		err = 0;
@@ -189,20 +190,9 @@ static int configure_kgdboc(void)
 	if (kgdboc_register_kbd(&cptr))
 		goto do_register;
 
-	/*
-	 * tty_find_polling_driver() can call uart_set_options()
-	 * (via poll_init) to configure the uart. Take the console_list_lock
-	 * in order to synchronize against register_console(), which can also
-	 * configure the uart via uart_set_options(). This also allows safe
-	 * traversal of the console list.
-	 */
-	console_list_lock();
-
 	p = tty_find_polling_driver(cptr, &tty_line);
-	if (!p) {
-		console_list_unlock();
+	if (!p)
 		goto noconfig;
-	}
 
 	/*
 	 * Take console_lock to serialize device() callback with
@@ -211,7 +201,8 @@ static int configure_kgdboc(void)
 	 */
 	console_lock();
 
-	for_each_console(cons) {
+	cookie = console_srcu_read_lock();
+	for_each_console_srcu(cons) {
 		int idx;
 		if (cons->device && cons->device(cons, &idx) == p &&
 		    idx == tty_line) {
@@ -219,11 +210,10 @@ static int configure_kgdboc(void)
 			break;
 		}
 	}
+	console_srcu_read_unlock(cookie);
 
 	console_unlock();
 
-	console_list_unlock();
-
 	kgdb_tty_driver = p;
 	kgdb_tty_line = tty_line;
 

drivers/tty/serial/serial_core.c

@@ -2212,6 +2212,9 @@ EXPORT_SYMBOL_GPL(uart_parse_options);
  * @parity: parity character - 'n' (none), 'o' (odd), 'e' (even)
  * @bits: number of data bits
  * @flow: flow control character - 'r' (rts)
+ *
+ * Locking: Caller must hold console_list_lock in order to serialize
+ * early initialization of the serial-console lock.
  */
 int
 uart_set_options(struct uart_port *port, struct console *co,
@@ -2619,7 +2622,9 @@ static int uart_poll_init(struct tty_driver *driver, int line, char *options)
 
 	if (!ret && options) {
 		uart_parse_options(options, &baud, &parity, &bits, &flow);
+		console_list_lock();
 		ret = uart_set_options(port, NULL, baud, parity, bits, flow);
+		console_list_unlock();
 	}
 out:
 	mutex_unlock(&tport->mutex);

kernel/printk/printk.c

@@ -123,6 +123,7 @@ bool console_srcu_read_lock_is_held(void)
 {
 	return srcu_read_lock_held(&console_srcu);
 }
+EXPORT_SYMBOL(console_srcu_read_lock_is_held);
 #endif
 
 enum devkmsg_log_bits {