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Date:   Mon, 17 Aug 2020 15:32:22 +0100
From:   Daniel Thompson <daniel.thompson@linaro.org>
To:     Sumit Garg <sumit.garg@linaro.org>
Cc:     Doug Anderson <dianders@chromium.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-serial@vger.kernel.org, kgdb-bugreport@lists.sourceforge.net,
        Jiri Slaby <jslaby@suse.com>,
        Russell King - ARM Linux <linux@armlinux.org.uk>,
        Jason Wessel <jason.wessel@windriver.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [RFC 2/5] serial: core: Add framework to allow NMI aware serial
 drivers
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On Mon, Aug 17, 2020 at 07:53:55PM +0530, Sumit Garg wrote:
> On Mon, 17 Aug 2020 at 19:27, Doug Anderson <dianders@chromium.org> wrote:
> >
> > Hi,
> >
> > On Mon, Aug 17, 2020 at 5:27 AM Sumit Garg <sumit.garg@linaro.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?

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