linux-kernel - Re: [PATCH printk v3 15/40] kdb: use srcu console list iterator

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Date:   Wed, 9 Nov 2022 16:27:44 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Daniel Thompson <daniel.thompson@...aro.org>,
        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>,
        Douglas Anderson <dianders@...omium.org>,
        Aaron Tomlin <atomlin@...hat.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        kgdb-bugreport@...ts.sourceforge.net
Subject: Re: [PATCH printk v3 15/40] kdb: use srcu console list iterator

On Wed 2022-11-09 10:33:55, John Ogness wrote:
> Hi Daniel,
> 
> On 2022-11-09, Daniel Thompson <daniel.thompson@...aro.org> wrote:
> >> +	/*
> >> +	 * The console_srcu_read_lock() only provides safe console list
> >> +	 * traversal. The use of the ->write() callback relies on all other
> >> +	 * CPUs being stopped at the moment and console drivers being able to
> >> +	 * handle reentrance when @oops_in_progress is set. (Note that there
> >> +	 * is no guarantee for either criteria.)
> >> +	 */
> >
> > The debugger entry protocol does ensure that other CPUs are either
> > stopped or unresponsive. In the case where the other CPU is unresponsive
> > (e.g. timed out after being asked to stop) then there is a "real" printk()
> > issued prior to any of the above interference with the console system to
> > the developer driving the debugger gets as much clue as we can offer them
> > about what is going on (typically this is emitted from regular interrupt
> > context).
> >
> > Given this comment is part of the debugger code then for the
> > oops_in_progress hack it might be more helpful to describe what
> > the developer in front the debugger needs to do to have the most
> > reliable debug session possible.
> >
> >   There is no guarantee that every console drivers can handle reentrance
> >   in this way; the developer deploying the debugger is responsible for
> >   ensuring that the console drivers they have selected handle reentrance
> >   appropriately.
> 
> Thanks for the explanation. I will change the comment to:
> 
> 	/*
> 	 * The console_srcu_read_lock() only provides safe console list
> 	 * traversal. The use of the ->write() callback relies on all other
> 	 * CPUs being stopped at the moment and console drivers being able to
> 	 * handle reentrance when @oops_in_progress is set.
> 	 *
> 	 * There is no guarantee that every console driver can handle
> 	 * reentrance in this way; the developer deploying the debugger
> 	 * is responsible for ensuring that the console drivers they
> 	 * have selected handle reentrance appropriately.
> 	 */

Looks good to me.

After merging this with the 10th patch that adds the data_race() annotated
check of CON_ENABLED flag:

Reviewed-by: Petr Mladek <pmladek@...e.com>

Best Regards,
Petr