linux-kernel - Re: [PATCH RT 3.18] irq_work: Provide a soft-irq based queue

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Date:	Thu, 23 Apr 2015 09:19:26 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Jan Kiszka <jan.kiszka@...mens.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	RT <linux-rt-users@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RT 3.18] irq_work: Provide a soft-irq based queue

On Thu, 2015-04-23 at 09:12 +0200, Jan Kiszka wrote:
> On 2015-04-23 09:01, Mike Galbraith wrote:
> > On Thu, 2015-04-23 at 08:50 +0200, Jan Kiszka wrote:
> > > On 2015-04-23 08:11, Mike Galbraith wrote:
> > > > @@ -103,6 +98,9 @@ EXPORT_SYMBOL_GPL(irq_work_queue_on);
> > > >  /* Enqueue the irq work @work on the current CPU */
> > > >  bool irq_work_queue(struct irq_work *work)
> > > >  {
> > > > +       bool realtime = IS_ENABLED(CONFIG_PREEMPT_RT_FULL);
> > > > +       bool raise = false;
> > > > +
> > > >         /* Only queue if not already pending */
> > > >         if (!irq_work_claim(work))
> > > >                 return false;
> > > > @@ -110,25 +108,22 @@ bool irq_work_queue(struct irq_work *wor
> > > >         /* Queue the entry and raise the IPI if needed. */
> > > >         preempt_disable();
> > > >  
> > > > -#ifdef CONFIG_PREEMPT_RT_FULL
> > > > -       if (work->flags & IRQ_WORK_HARD_IRQ) {
> > > > +       if (realtime && (work->flags & IRQ_WORK_HARD_IRQ)) {
> > > >                 if (llist_add(&work->llnode, 
> > > > this_cpu_ptr(&hirq_work_list)))
> > > 
> > > This boils down to
> > > 
> > > #ifdef CONFIG_X
> > > some_type x;
> > > #endif
> > > ...
> > > 
> > > if (IS_ENABLED(CONFIG_X) && ...)
> > >       use(x);
> > > 
> > > And here we even have an indirection for IS_ENABLED via that 
> > > local 
> > > bool
> > > variable. Is that pattern OK for Linux? Does it compile in all 
> > > supported
> > > optimization levels of all supported compilers?
> > 
> > I hope it all goes away, that being what IS_ENABLED() is there for.
> 
> Hope is good - but not enough here: it breaks the build under
> !CONFIG_X, even the case without the bool var.
> 
>   CC      kernel/irq_work.o
> In file included from ../include/asm-generic/percpu.h:6:0,
>                  from ../arch/x86/include/asm/percpu.h:522,
>                  from ../arch/x86/include/asm/current.h:5,
>                  from ../arch/x86/include/asm/processor.h:15,
>                  from ../arch/x86/include/asm/irq_work.h:4,
>                  from ../include/linux/irq_work.h:47,
>                  from ../kernel/irq_work.c:11:
> ../kernel/irq_work.c: In function ‘irq_work_queue_on’:
> ../kernel/irq_work.c:85:17: error: ‘hirq_work_list’ undeclared 
> (first use in this function)
>         &per_cpu(hirq_work_list, cpu));

Aw poo, so that's just what I _thought_ it was for.

        -Mike
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