linux-kernel - Re: [PATCH printk v2 26/38] kdb: use srcu console list iterator

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Date:   Mon, 24 Oct 2022 17:47:25 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Jason Wessel <jason.wessel@...driver.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Aaron Tomlin <atomlin@...hat.com>,
        kgdb-bugreport@...ts.sourceforge.net
Subject: Re: [PATCH printk v2 26/38] kdb: use srcu console list iterator

Hi,

On Wed, Oct 19, 2022 at 7:56 AM John Ogness <john.ogness@...utronix.de> wrote:
>
> Guarantee safe iteration of the console list by using SRCU.
>
> Signed-off-by: John Ogness <john.ogness@...utronix.de>
> ---
>  kernel/debug/kdb/kdb_io.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
> index 550fe8b456ec..5c0bd93c3574 100644
> --- a/kernel/debug/kdb/kdb_io.c
> +++ b/kernel/debug/kdb/kdb_io.c
> @@ -545,6 +545,7 @@ static void kdb_msg_write(const char *msg, int msg_len)
>  {
>         struct console *c;
>         const char *cp;
> +       int cookie;
>         int len;
>
>         if (msg_len == 0)
> @@ -558,7 +559,8 @@ static void kdb_msg_write(const char *msg, int msg_len)
>                 cp++;
>         }
>
> -       for_each_console(c) {
> +       cookie = console_srcu_read_lock();
> +       for_each_console_srcu(c) {

Maybe it wouldn't hurt to also have a comment saying that normally the
console_srcu_read_lock() wouldn't be enough given that we're poking
into each individual console and calling ->write() but that we're
relying on the fact that all the other CPUs are stopped at the moment
and thus we should be safe.

-Doug