diff mbox series

[printk,v2,22/38] serial: kgdboc: document console_lock usage

Message ID 20221019145600.1282823-23-john.ogness@linutronix.de
State New
Headers
Series reduce console_lock scope |

Commit Message

John Ogness Oct. 19, 2022, 2:55 p.m. UTC 
  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>
---
 drivers/tty/serial/kgdboc.c | 8 ++++++++
 1 file changed, 8 insertions(+)

Comments

Greg KH Oct. 20, 2022, 7:42 a.m. UTC | #1 
On Wed, Oct 19, 2022 at 05:01:44PM +0206, John Ogness wrote:
> 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>
> ---
>  drivers/tty/serial/kgdboc.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/tty/serial/kgdboc.c b/drivers/tty/serial/kgdboc.c
> index e9d3f8c6e3dc..48000666789a 100644
> --- a/drivers/tty/serial/kgdboc.c
> +++ b/drivers/tty/serial/kgdboc.c
> @@ -545,6 +545,14 @@ 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.
> +	 */
>  	console_lock();
>  	for_each_console(con) {
>  		if (con->write && con->read &&
> -- 
> 2.30.2
>

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Doug Anderson Oct. 25, 2022, 12:36 a.m. UTC | #2 
Hi,

On Wed, Oct 19, 2022 at 7:56 AM John Ogness <john.ogness@linutronix.de> wrote:
>
> 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>
> ---
>  drivers/tty/serial/kgdboc.c | 8 ++++++++
>  1 file changed, 8 insertions(+)

Reviewed-by: Douglas Anderson <dianders@chromium.org>
Petr Mladek Oct. 25, 2022, 10:09 a.m. UTC | #3 
On Wed 2022-10-19 17:01:44, John Ogness wrote:
> 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.

Great catch!

Note: This behavior might be dangerous. printk_late_init() checks if
      the early console code is in the init section. It unregisters
      such consoles even when keep_bootcon parameter is used.

      Well, it is "not serious" and might be improved later.
      If the early console has this code in the early section
      then it is a bug and should be fixed. printk_late_init() does
      the best effort because this problem would block all consoles
      and would be hard to debug. It is the same with kgdb but
      it is still only a corner case and just the best effort.

> Explicitly document this non-typical console_lock usage.
> 
> Signed-off-by: John Ogness <john.ogness@linutronix.de>

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr
diff mbox series

Patch

diff --git a/drivers/tty/serial/kgdboc.c b/drivers/tty/serial/kgdboc.c
index e9d3f8c6e3dc..48000666789a 100644
--- a/drivers/tty/serial/kgdboc.c
+++ b/drivers/tty/serial/kgdboc.c
@@ -545,6 +545,14 @@  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.
+	 */
 	console_lock();
 	for_each_console(con) {
 		if (con->write && con->read &&