linux-kernel - Re: [PATCH printk v2 21/38] serial: kgdboc: use srcu console list iterator

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Date:   Mon, 24 Oct 2022 17:33:16 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        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 v2 21/38] serial: kgdboc: use srcu console list iterator

Hi,

On Fri, Oct 21, 2022 at 8:09 AM Petr Mladek <pmladek@...e.com> wrote:
>
> On Wed 2022-10-19 17:01:43, John Ogness wrote:
> > Use srcu console list iteration for safe console list traversal.
> >
> > Note that configure_kgdboc() still requires the console_lock in
> > order to ensure that no console is in its write() callback when
> > its direct() callback is called. Add comments to clarify this.
>
> s/direct()/device()/
>
> Do you know about such requirements or is it just
> a conservative approach, please?
>
> I ask because the comment in the code says "may assume".
>
>
> Anyway, this would deserve a comment why the SRCU list iteration is
> needed even when console_lock() is needed as well.
>
> The reason is that further patches are going to synchronize
> console_list manipulation with another lock and console_lock()
> will be used only to serialize accessing con->write() callbacks.

I had the same concern. I'll note that at the end of the series the
documentation for console_lock() still says:

 * Acquires a lock which guarantees that the caller has
 * exclusive access to the console system.

That seems to imply (at least to me) that if you're holding
console_lock() there's no need to hold the SRCU lock.

-Doug