Re: [PATCH printk v5 35/40] tty: serial: kgdboc: use srcu console list iterator

[Doug Anderson]

Hi,

On Wed, Nov 16, 2022 at 8:22 AM John Ogness <john.ogness@xxxxxxxxxxxxx> wrote:
>
> Use srcu console list iteration for safe console list traversal.
> 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@xxxxxxxxxxxxx>
> Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
> ---
>  drivers/tty/serial/kgdboc.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/tty/serial/kgdboc.c b/drivers/tty/serial/kgdboc.c
> index 5be381003e58..c6df9ef34099 100644
> --- a/drivers/tty/serial/kgdboc.c
> +++ b/drivers/tty/serial/kgdboc.c
> @@ -451,6 +451,7 @@ static void kgdboc_earlycon_pre_exp_handler(void)
>  {
>         struct console *con;
>         static bool already_warned;
> +       int cookie;
>
>         if (already_warned)
>                 return;
> @@ -463,9 +464,14 @@ static void kgdboc_earlycon_pre_exp_handler(void)
>          * serial drivers might be OK with this, print a warning once per
>          * boot if we detect this case.
>          */
> -       for_each_console(con)
> +       cookie = console_srcu_read_lock();
> +       for_each_console_srcu(con) {
>                 if (con == kgdboc_earlycon_io_ops.cons)
> -                       return;
> +                       break;
> +       }
> +       console_srcu_read_unlock(cookie);
> +       if (con)
> +               return;

Is there truly any guarantee that "con" will be NULL if
for_each_console_srcu() finishes naturally (AKA without a "break"
being executed)?

It looks as if currently this will be true but nothing in the comments
of for_each_console_srcu() nor hlist_for_each_entry_srcu() (which it
calls) guarantees this, right? It would be nice if that was
documented, but I guess it's not a huge deal.

 Also: wasn't there just some big issue about people using loop
iteration variables after the loop finished?

https://lwn.net/Articles/885941/

Ah, I guess that's a slightly different problem and probably not relevant here.

So it seems like this is fine.

Reviewed-by: Douglas Anderson <dianders@xxxxxxxxxxxx>