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

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Message-ID: <20200603114242.khv5yi5yweq3e2jz@holly.lan>
Date:   Wed, 3 Jun 2020 12:42:42 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Sumit Garg <sumit.garg@...aro.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        kgdb-bugreport@...ts.sourceforge.net,
        Jason Wessel <jason.wessel@...driver.com>,
        Douglas Anderson <dianders@...omium.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Linux Kernel Mailing List <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 03:02:02PM +0530, Sumit Garg wrote:
> On Wed, 3 Jun 2020 at 13:55, Petr Mladek <pmladek@...e.com> 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?
> >
> 
> AFAIK, kgdboc_earlycon_io_ops won't be used at any later stage in case
> registration fails. So IMO, it would be a redundant assignment unless
> I missed something.

Or, putting it another way, earlycon is a redundant (albeit better
maintained) copy of kgdboc_earlycon_io_ops.cons. So I think the best
thing to do is entirely replace earlycon with
kgdboc_earlycon_io_ops.cons and then properly set it to NULL!


Daniel.