linux-kernel - Re: [PATCH v5 4/4] kdb: Switch to use safer dbg_io

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Date:   Wed, 3 Jun 2020 10:18:30 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sumit Garg <sumit.garg@...aro.org>,
        kgdb-bugreport@...ts.sourceforge.net, jason.wessel@...driver.com,
        dianders@...omium.org, sergey.senozhatsky@...il.com,
        gregkh@...uxfoundation.org, jslaby@...e.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 4/4] kdb: Switch to use safer dbg_io_ops over console
 APIs

On Wed, Jun 03, 2020 at 10:25:04AM +0200, Petr Mladek wrote:
> On Wed 2020-06-03 12:52:15, Sumit Garg wrote:
> > In kgdb context, calling console handlers aren't safe due to locks used
> > in those handlers which could in turn lead to a deadlock. Although, using
> > oops_in_progress increases the chance to bypass locks in most console
> > handlers but it might not be sufficient enough in case a console uses
> > more locks (VT/TTY is good example).
> > 
> > Currently when a driver provides both polling I/O and a console then kdb
> > will output using the console. We can increase robustness by using the
> > currently active polling I/O driver (which should be lockless) instead
> > of the corresponding console. For several common cases (e.g. an
> > embedded system with a single serial port that is used both for console
> > output and debugger I/O) this will result in no console handler being
> > used.
> > 
> > In order to achieve this we need to reverse the order of preference to
> > use dbg_io_ops (uses polling I/O mode) over console APIs. So we just
> > store "struct console" that represents debugger I/O in dbg_io_ops and
> > while emitting kdb messages, skip console that matches dbg_io_ops
> > console in order to avoid duplicate messages. After this change,
> > "is_console" param becomes redundant and hence removed.
> > 
> > diff --git a/drivers/tty/serial/kgdboc.c b/drivers/tty/serial/kgdboc.c
> > index 4139698..6e182aa 100644
> > --- a/drivers/tty/serial/kgdboc.c
> > +++ b/drivers/tty/serial/kgdboc.c
> > @@ -558,6 +557,7 @@ static int __init kgdboc_earlycon_init(char *opt)
> >  	}
> >  
> >  	earlycon = con;
> > +	kgdboc_earlycon_io_ops.cons = con;
> >  	pr_info("Going to register kgdb with earlycon '%s'\n", con->name);
> >  	if (kgdb_register_io_module(&kgdboc_earlycon_io_ops) != 0) {
> >  		earlycon = NULL;
> 
> Should we clear kgdboc_earlycon_io_ops.cons here when
> kgdb_register_io_module() failed?
> 
> > diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h
> > index c62d764..529116b 100644
> > --- a/include/linux/kgdb.h
> > +++ b/include/linux/kgdb.h
> > @@ -276,8 +276,7 @@ struct kgdb_arch {
> >   * the I/O driver.
> >   * @post_exception: Pointer to a function that will do any cleanup work
> >   * for the I/O driver.
> > - * @is_console: 1 if the end device is a console 0 if the I/O device is
> > - * not a console
> > + * @cons: valid if the I/O device is a console; else NULL.
> >   */
> >  struct kgdb_io {
> >  	const char		*name;
> > @@ -288,7 +287,7 @@ struct kgdb_io {
> >  	void			(*deinit) (void);
> >  	void			(*pre_exception) (void);
> >  	void			(*post_exception) (void);
> > -	int			is_console;
> > +	struct console		*cons;
> 
> Nit: I would call it "con". The trailing 's' makes me feel that that the
>      variable points to an array or list of consoles.

How strongly do you feel about it?


I'd probably agree with you except that the uart subsystem, which is by
far the most prolific supplier of consoles for kgdb to use, calls
pointers to single consoles "cons" so I'd prefer to be aligned on
terminology.


Daniel.