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Date:   Thu, 13 Aug 2020 07:37:57 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Sumit Garg <sumit.garg@...aro.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        linux-serial@...r.kernel.org, kgdb-bugreport@...ts.sourceforge.net,
        Jiri Slaby <jslaby@...e.com>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Jason Wessel <jason.wessel@...driver.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC 2/5] serial: core: Add framework to allow NMI aware serial drivers

Hi,

On Thu, Aug 13, 2020 at 7:19 AM Sumit Garg <sumit.garg@...aro.org> wrote:
>
> On Thu, 13 Aug 2020 at 05:29, Doug Anderson <dianders@...omium.org> wrote:
> >
> > Hi,
> >
> > On Tue, Jul 21, 2020 at 5:11 AM Sumit Garg <sumit.garg@...aro.org> wrote:
> > >
> > > Add NMI framework APIs in serial core which can be leveraged by serial
> > > drivers to have NMI driven serial transfers. These APIs are kept under
> > > CONFIG_CONSOLE_POLL as currently kgdb initializing uart in polling mode
> > > is the only known user to enable NMI driven serial port.
> > >
> > > The general idea is to intercept RX characters in NMI context, if those
> > > are specific to magic sysrq then allow corresponding handler to run in
> > > NMI context. Otherwise defer all other RX and TX operations to IRQ work
> > > queue in order to run those in normal interrupt context.
> > >
> > > Also, since magic sysrq entry APIs will need to be invoked from NMI
> > > context, so make those APIs NMI safe via deferring NMI unsafe work to
> > > IRQ work queue.
> > >
> > > Signed-off-by: Sumit Garg <sumit.garg@...aro.org>
> > > ---
> > >  drivers/tty/serial/serial_core.c | 120 ++++++++++++++++++++++++++++++++++++++-
> > >  include/linux/serial_core.h      |  67 ++++++++++++++++++++++
> > >  2 files changed, 185 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> > > index 57840cf..6342e90 100644
> > > --- a/drivers/tty/serial/serial_core.c
> > > +++ b/drivers/tty/serial/serial_core.c
> > > @@ -3181,8 +3181,14 @@ static bool uart_try_toggle_sysrq(struct uart_port *port, unsigned int ch)
> > >                 return true;
> > >         }
> > >
> > > +#ifdef CONFIG_CONSOLE_POLL
> > > +       if (in_nmi())
> > > +               irq_work_queue(&port->nmi_state.sysrq_toggle_work);
> > > +       else
> > > +               schedule_work(&sysrq_enable_work);
> > > +#else
> > >         schedule_work(&sysrq_enable_work);
> > > -
> > > +#endif
> >
> > It should be a very high bar to have #ifdefs inside functions.  I
> > don't think this meets it.  Instead maybe something like this
> > (untested and maybe slightly wrong syntax, but hopefully makes
> > sense?):
> >
> > Outside the function:
> >
> > #ifdef CONFIG_CONSOLE_POLL
> > #define queue_port_nmi_work(port, work_type)
> > irq_work_queue(&port->nmi_state.work_type)
> > #else
> > #define queue_port_nmi_work(port, work_type)
> > #endif
> >
> > ...and then:
> >
> > if (IS_ENABLED(CONFIG_CONSOLE_POLL) && in_nmi())
> >   queue_port_nmi_work(port, sysrq_toggle_work);
> > else
> >   schedule_work(&sysrq_enable_work);
> >
> > ---
> >
> > The whole double-hopping is really quite annoying.  I guess
> > schedule_work() can't be called from NMI context but can be called
> > from IRQ context?  So you need to first transition from NMI context to
> > IRQ context and then go and schedule the work?  Almost feels like we
> > should just fix schedule_work() to do this double-hop for you if
> > called from NMI context.  Seems like you could even re-use the list
> > pointers in the work_struct to keep the queue of people who need to be
> > scheduled from the next irq_work?  Worst case it seems like you could
> > add a schedule_work_nmi() that would do all the hoops for you.  ...but
> > I also know very little about NMI so maybe I'm being naive.
> >
>
> Thanks for this suggestion and yes indeed we could make
> schedule_work() NMI safe and in turn get rid of all this #ifdefs. Have
> a look at below changes:
>
> diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
> index 26de0ca..1daf1b4 100644
> --- a/include/linux/workqueue.h
> +++ b/include/linux/workqueue.h
> @@ -14,6 +14,7 @@
>  #include <linux/atomic.h>
>  #include <linux/cpumask.h>
>  #include <linux/rcupdate.h>
> +#include <linux/irq_work.h>
>
>  struct workqueue_struct;
>
> @@ -106,6 +107,7 @@ struct work_struct {
>  #ifdef CONFIG_LOCKDEP
>         struct lockdep_map lockdep_map;
>  #endif
> +       struct irq_work iw;

Hrm, I was thinking you could just have a single queue per CPU then
you don't need to add all this extra data to every single "struct
work_struct".  I was thinking you could use the existing list node in
the "struct work_struct" to keep track of the list of things.  ...but
maybe my idea this isn't actually valid because the linked list might
be in use if we're scheduling work that's already pending / running?

In any case, I worry that people won't be happy with the extra
overhead per "struct work_struct".  Can we reduce it at all?  It still
does feel like you could get by with a single global queue and thus
you wouldn't need to store the function pointer and flags with every
"struct work_struct", right?  So all you'd need is a single pointer
for the linked list?  I haven't actually tried implementing this,
though, so I could certainly be wrong.


-Doug

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