linux-kernel - Re: [RFC 2/5] serial: core: Add framework to allow NMI aware serial drivers

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Message-ID: <CAFA6WYOww8T7fmP-LtzS-EXG0iBbKEUOS5KrUU_7i+yqd_rFfQ@mail.gmail.com>
Date:   Mon, 17 Aug 2020 19:53:55 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Daniel Thompson <daniel.thompson@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.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

On Mon, 17 Aug 2020 at 19:27, Doug Anderson <dianders@...omium.org> wrote:
>
> Hi,
>
> On Mon, Aug 17, 2020 at 5:27 AM Sumit Garg <sumit.garg@...aro.org> wrote:
> >
> > Thanks for your suggestion, irq_work_schedule() looked even better
> > without any overhead, see below:
> >
> > diff --git a/include/linux/irq_work.h b/include/linux/irq_work.h
> > index 3082378..1eade89 100644
> > --- a/include/linux/irq_work.h
> > +++ b/include/linux/irq_work.h
> > @@ -3,6 +3,7 @@
> >  #define _LINUX_IRQ_WORK_H
> >
> >  #include <linux/smp_types.h>
> > +#include <linux/workqueue.h>
> >
> >  /*
> >   * An entry can be in one of four states:
> > @@ -24,6 +25,11 @@ struct irq_work {
> >         void (*func)(struct irq_work *);
> >  };
> >
> > +struct irq_work_schedule {
> > +       struct irq_work work;
> > +       struct work_struct *sched_work;
> > +};
> > +
> >  static inline
> >  void init_irq_work(struct irq_work *work, void (*func)(struct irq_work *))
> >  {
> >  {
> > @@ -39,6 +45,7 @@ void init_irq_work(struct irq_work *work, void
> > (*func)(struct irq_work *))
> >
> >  bool irq_work_queue(struct irq_work *work);
> >  bool irq_work_queue_on(struct irq_work *work, int cpu);
> > +bool irq_work_schedule(struct work_struct *sched_work);
> >
> >  void irq_work_tick(void);
> >  void irq_work_sync(struct irq_work *work);
> > diff --git a/kernel/irq_work.c b/kernel/irq_work.c
> > index eca8396..3880316 100644
> > --- a/kernel/irq_work.c
> > +++ b/kernel/irq_work.c
> > @@ -24,6 +24,8 @@
> >  static DEFINE_PER_CPU(struct llist_head, raised_list);
> >  static DEFINE_PER_CPU(struct llist_head, lazy_list);
> >
> > +static struct irq_work_schedule irq_work_sched;
> > +
> >  /*
> >   * Claim the entry so that no one else will poke at it.
> >   */
> > @@ -79,6 +81,25 @@ bool irq_work_queue(struct irq_work *work)
> >  }
> >  EXPORT_SYMBOL_GPL(irq_work_queue);
> >
> > +static void irq_work_schedule_fn(struct irq_work *work)
> > +{
> > +       struct irq_work_schedule *irq_work_sched =
> > +               container_of(work, struct irq_work_schedule, work);
> > +
> > +       if (irq_work_sched->sched_work)
> > +               schedule_work(irq_work_sched->sched_work);
> > +}
> > +
> > +/* Schedule work via irq work queue */
> > +bool irq_work_schedule(struct work_struct *sched_work)
> > +{
> > +       init_irq_work(&irq_work_sched.work, irq_work_schedule_fn);
> > +       irq_work_sched.sched_work = sched_work;
> > +
> > +       return irq_work_queue(&irq_work_sched.work);
> > +}
> > +EXPORT_SYMBOL_GPL(irq_work_schedule);
>
> Wait, howzat work?  There's a single global variable that you stash
> the "sched_work" into with no locking?  What if two people schedule
> work at the same time?

This API is intended to be invoked from NMI context only, so I think
there will be a single user at a time. And we can make that explicit
as well:

+/* Schedule work via irq work queue */
+bool irq_work_schedule(struct work_struct *sched_work)
+{
+       if (in_nmi()) {
+               init_irq_work(&irq_work_sched.work, irq_work_schedule_fn);
+               irq_work_sched.sched_work = sched_work;
+
+               return irq_work_queue(&irq_work_sched.work);
+       }
+
+       return false;
+}
+EXPORT_SYMBOL_GPL(irq_work_schedule);

-Sumit

>
> -Doug