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From:   John Ogness <john.ogness@linutronix.de>
To:     Doug Anderson <dianders@chromium.org>
Cc:     Petr Mladek <pmladek@suse.com>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org,
        Jason Wessel <jason.wessel@windriver.com>,
        Daniel Thompson <daniel.thompson@linaro.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jirislaby@kernel.org>,
        kgdb-bugreport@lists.sourceforge.net, linux-serial@vger.kernel.org
Subject: Re: [PATCH printk v5 38/40] tty: serial: kgdboc: use
 console_list_lock to trap exit
In-Reply-To: <CAD=FV=VouG3ihFCg4gC3dNFs7TqzpDgowHd38y8fR8qiTfRw_A@mail.gmail.com>
References: <20221116162152.193147-1-john.ogness@linutronix.de>
 <20221116162152.193147-39-john.ogness@linutronix.de>
 <CAD=FV=VouG3ihFCg4gC3dNFs7TqzpDgowHd38y8fR8qiTfRw_A@mail.gmail.com>
Date:   Thu, 17 Nov 2022 12:14:09 +0106
Message-ID: <87mt8pon3a.fsf@jogness.linutronix.de>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On 2022-11-16, Doug Anderson <dianders@chromium.org> wrote:
>>         /*
>> -        * Hold the console_lock to guarantee that no consoles are
>> +        * Hold the console_list_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 and race-free reading of @flags.
>>          */
>> -       console_lock();
>> +       console_list_lock();
>>         for_each_console(con) {
>>                 if (con->write && con->read &&
>>                     (con->flags & (CON_BOOT | CON_ENABLED)) &&
>
> Officially don't we need both the list lock and normal lock here since
> we're reaching into the consoles?

AFAICT the only synchronization we need here is iterating the console
list, reading con->flags of a registered console, and modifying
con->exit of a registered console. The console_list_lock provides
synchronization for all of these things. By the end of this series the
console_lock does not provide synchronization for any of these things.

Is there something else that requires synchronization here?

After this series the console_lock is still responsible for:

- serializing console->write() callbacks
- stopping console->write() callbacks
- stopping console->device() callbacks
- synchronizing console->seq
- synchronizing console->dropped
- synchronizing the global @console_suspended
- providing various unclear protection for vt consoles
- some bizarre misuses in bcache

The scope may be larger than the above list. The investigation is still
ongoing.

John