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

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Date:   Tue, 18 Aug 2020 18:36:04 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     Doug Anderson <dianders@...omium.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 19:58, Daniel Thompson
<daniel.thompson@...aro.org> wrote:
>
> On Mon, Aug 17, 2020 at 05:57:03PM +0530, Sumit Garg wrote:
> > On Fri, 14 Aug 2020 at 19:43, Daniel Thompson
> > <daniel.thompson@...aro.org> wrote:
> > > On Fri, Aug 14, 2020 at 04:47:11PM +0530, Sumit Garg wrote:
> > > Does it look better if you create a new type to map the two structures
> > > together. Alternatively are there enough existing use-cases to want to
> > > extend irq_work_queue() with irq_work_schedule() or something similar?
> > >
> >
> > 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);
> > +
>
> This is irredeemably broken.
>
> Even if we didn't care about dropping events (which we do) then when you
> overwrite irq_work_sched with a copy of another work_struct, either of
> which could currently be enqueued somewhere, then you will cause some
> very nasty corruption.
>

Okay, I see your point. I think there isn't a way to avoid caller
specific struct such as:

struct nmi_queuable_work_struct {
  struct work_struct work;
  struct irq_work iw;
};

So in that case will shift to approach as suggested by Doug to rather
have a new nmi_schedule_work() API.

-Sumit

>
> Daniel.