From: Sumit Garg <sumit.garg@linaro.org>
To: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Petr Mladek <pmladek@suse.com>,
kgdb-bugreport@lists.sourceforge.net,
Jason Wessel <jason.wessel@windriver.com>,
Douglas Anderson <dianders@chromium.org>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Jiri Slaby <jslaby@suse.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v5 4/4] kdb: Switch to use safer dbg_io_ops over console APIs
Date: Wed, 3 Jun 2020 18:35:29 +0530 [thread overview]
Message-ID: <CAFA6WYOqJ-7Ctr46ATqvH2zRz+9biehmL85cuE+tbHRgxxpZkg@mail.gmail.com> (raw)
In-Reply-To: <20200603114242.khv5yi5yweq3e2jz@holly.lan>
On Wed, 3 Jun 2020 at 17:12, Daniel Thompson <daniel.thompson@linaro.org> wrote:
>
> On Wed, Jun 03, 2020 at 03:02:02PM +0530, Sumit Garg wrote:
> > On Wed, 3 Jun 2020 at 13:55, Petr Mladek <pmladek@suse.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!
>
Sounds reasonable, will replace earlycon instead.
-Sumit
>
> Daniel.
prev parent reply other threads:[~2020-06-03 13:05 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-03 7:22 [PATCH v5 0/4] kdb: Improve console handling Sumit Garg
2020-06-03 7:22 ` [PATCH v5 1/4] kdb: Re-factor kdb_printf() message write code Sumit Garg
2020-06-03 8:07 ` Petr Mladek
2020-06-03 7:22 ` [PATCH v5 2/4] kdb: Check status of console prior to invoking handlers Sumit Garg
2020-06-03 8:08 ` Petr Mladek
2020-06-03 7:22 ` [PATCH v5 3/4] kdb: Make kdb_printf() console handling more robust Sumit Garg
2020-06-03 8:10 ` Petr Mladek
2020-06-03 7:22 ` [PATCH v5 4/4] kdb: Switch to use safer dbg_io_ops over console APIs Sumit Garg
2020-06-03 8:25 ` Petr Mladek
2020-06-03 9:18 ` Daniel Thompson
2020-06-03 11:59 ` Petr Mladek
2020-06-03 9:32 ` Sumit Garg
2020-06-03 11:42 ` Daniel Thompson
2020-06-03 13:05 ` Sumit Garg [this message]
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