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

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Date:   Thu, 17 Nov 2022 10:38:07 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Doug Anderson <dianders@...omium.org>
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>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        kgdb-bugreport@...ts.sourceforge.net, linux-serial@...r.kernel.org
Subject: Re: [PATCH printk v5 35/40] tty: serial: kgdboc: use srcu console
 list iterator

Hi Doug,

On 2022-11-16, Doug Anderson <dianders@...omium.org> wrote:
>> @@ -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)?

Right now it is true because @con becoming NULL is the exit criteria for
the loop.

> 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.

Yes, if it is frowned upon that the iterator is used outside the loop,
it would be nice if the for_each macros explicitly provided some hints
in their documentation.

> Also: wasn't there just some big issue about people using loop
> iteration variables after the loop finished?
>
> https://lwn.net/Articles/885941/

Thanks for referencing that article! Indeed if the macros are changed so
that the iterator is defined in the loop, then code like this will
break. But I would expect that making such macro changes will also
require updating the call sites to avoid unused variables outside the
loops. And then this code could receive the appropriate fixup.

I feel like if I add extra code to guarantee a NULL without relying on
the macro implementation, I'll get more resistance due to unnecessarily
adding code and variables. But I may be wrong.

> Reviewed-by: Douglas Anderson <dianders@...omium.org>

Thanks.

John