linux-kernel - Re: [PATCH v2 2/7] serial: qcom-geni: fix shutdown race

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Message-ID: <CAD=FV=XuEPGtDCe4ssXPy2avigqviTBAycc0Q_U_Pwi9x6t23g@mail.gmail.com>
Date: Fri, 11 Oct 2024 07:30:30 -0700
From: Doug Anderson <dianders@...omium.org>
To: Johan Hovold <johan@...nel.org>
Cc: Johan Hovold <johan+linaro@...nel.org>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jiri Slaby <jirislaby@...nel.org>, 
	Bjorn Andersson <andersson@...nel.org>, Konrad Dybcio <konradybcio@...nel.org>, 
	linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-serial@...r.kernel.org, stable@...r.kernel.org, 
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH v2 2/7] serial: qcom-geni: fix shutdown race

Hi,

On Thu, Oct 10, 2024 at 11:51 PM Johan Hovold <johan@...nel.org> wrote:
>
> > > Not sure how your "console process" works, but this should only happen
> > > if you do not enable the serial console (console=ttyMSM0) and then try
> > > to use a polled console (as enabling the console will prevent port
> > > shutdown from being called).
> >
> > That simply doesn't seem to be the case for me. The port shutdown
> > seems to be called. To confirm, I put a printout at the start of
> > qcom_geni_serial_shutdown(). I see in my /proc/cmdline:
> >
> > console=ttyMSM0,115200n8
> >
> > ...and I indeed verify that I see console messages on my UART. I then run:
> >
> > stop console-ttyMSM0
> >
> > ...and I see on the UART:
> >
> > [   92.916964] DOUG: qcom_geni_serial_shutdown
> > [   92.922703] init: console-ttyMSM0 main process (611) killed by TERM signal
> >
> > Console messages keep coming out the UART even though the agetty isn't
> > there.
>
> And this is with a Chromium kernel, not mainline?

Who do you take me for?!?!  :-P :-P :-P Of course it's with mainline.


> If you take a look at tty_port_shutdown() there's a hack in there for
> consoles that was added back in 2010 and that prevents shutdown() from
> called for console ports.
>
> Put perhaps you manage to hit shutdown() via some other path. Serial
> core is not yet using tty_port_hangup() so a hangup might trigger
> that...
>
> Could you check that with a dump_stack()?

Sure. Typed from the agetty itself, here ya go. Git hash is not a
mainline git hash because I have your patches applied. "dirty" is
because of the printout / dump_stack().

lazor-rev9 ~ # stop console-ttyMSM0
[   68.772828] DOUG: qcom_geni_serial_shutdown
[   68.777365] CPU: 2 UID: 0 PID: 589 Comm: login Not tainted
6.12.0-rc1-g0bde0d120d58-dirty #1
ac8ed1a05abcc73f4fafe0543cbc76768ea594e1
[   68.789737] Hardware name: Google Lazor (rev9) with LTE (DT)
[   68.795581] Call trace:
[   68.798124]  dump_backtrace+0xf8/0x120
[   68.802025]  show_stack+0x24/0x38
[   68.805463]  dump_stack_lvl+0x40/0xc8
[   68.809265]  dump_stack+0x18/0x38
[   68.812702]  qcom_geni_serial_shutdown+0x38/0x110
[   68.817578]  uart_port_shutdown+0x48/0x68
[   68.821736]  uart_shutdown+0xcc/0x170
[   68.825530]  uart_hangup+0x54/0x158
[   68.829154]  __tty_hangup+0x20c/0x318
[   68.832954]  tty_vhangup_session+0x20/0x38
[   68.837195]  disassociate_ctty+0xe8/0x1a8
[   68.841355]  do_exit+0x10c/0x358
[   68.844716]  do_group_exit+0x9c/0xa8
[   68.848441]  get_signal+0x408/0x4d8
[   68.852071]  do_signal+0xa8/0x770
[   68.855526]  do_notify_resume+0x78/0x118
[   68.859605]  el0_svc+0x64/0x68
[   68.862790]  el0t_64_sync_handler+0x20/0x128
[   68.867218]  el0t_64_sync+0x1a8/0x1b0
[   68.872933] init: console-ttyMSM0 main process (589) killed by TERM signal


> > Now I (via ssh) drop into the debugger:
> >
> > echo g > /proc/sysrq-trigger
> >
> > I see the "kgdb" prompt but I can't interact with it because
> > qcom_geni_serial_shutdown() stopped RX.
>
> How about simply amending poll_get_char() so that it enables the
> receiver if it's not already enabled?

Yeah, this would probably work.

-Doug