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

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Date:   Mon, 17 Aug 2020 06:57:22 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Sumit Garg <sumit.garg@...aro.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

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?

-Doug