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

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Message-ID: <CAFA6WYO_tBUSNnuDU4yvphUb3-f7oc2GHFTTjbX3=963p=iePQ@mail.gmail.com>
Date:   Tue, 18 Aug 2020 18:48:30 +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 20:02, Daniel Thompson
<daniel.thompson@...aro.org> wrote:
>
> On Mon, Aug 17, 2020 at 07:53:55PM +0530, Sumit Garg wrote:
> > 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.
>
> How can you possibly know that?

I guess here you are referring to NMI nesting, correct?

Anyway, I am going to shift to another implementation as mentioned in
the other thread.

-Sumit

>
> This is library code, not a helper in a driver.
>
>
> Daniel.
>
>
> > 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