Re: [PATCH] preempt: add in_serving_irq() and apply to rcutiny and vsprintf

[Boqun Feng <boqun.feng@gmail.com>]

[Cc Thomas and Frederic since they contributed the clean-up to these
macros recently]

Background for discussion:

	https://lore.kernel.org/lkml/20210814014234.51395-1-changbin.du@gmail.com/

On Thu, Aug 19, 2021 at 07:59:16AM +0800, Changbin Du wrote:
> On Tue, Aug 17, 2021 at 12:03:16AM +0800, Boqun Feng wrote:
> > On Sat, Aug 14, 2021 at 09:42:34AM +0800, Changbin Du wrote:
> > > At some places we need to determine whether we're in nmi, hardirq or
> > > softirq context. This adds a macro in_serving_irq() as a shortcut for
> > > that.
> > > 
> > > Meanwhile, apply this new macro to existing code in rcutiny and vsprintf.
> > > 
> > > Signed-off-by: Changbin Du <changbin.du@gmail.com>
> > > ---
> > >  include/linux/preempt.h | 4 +++-
> > >  include/linux/rcutiny.h | 3 +--
> > >  lib/vsprintf.c          | 2 +-
> > >  3 files changed, 5 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/include/linux/preempt.h b/include/linux/preempt.h
> > > index 9881eac0698f..9a1c924e2c6c 100644
> > > --- a/include/linux/preempt.h
> > > +++ b/include/linux/preempt.h
> > > @@ -92,12 +92,14 @@
> > >   * in_nmi()		- We're in NMI context
> > >   * in_hardirq()		- We're in hard IRQ context
> > >   * in_serving_softirq()	- We're in softirq context
> > > + * in_serving_irq()	- We're in nmi, hardirq or softirq context
> > >   * in_task()		- We're in task context
> > >   */
> > >  #define in_nmi()		(nmi_count())
> > >  #define in_hardirq()		(hardirq_count())
> > >  #define in_serving_softirq()	(softirq_count() & SOFTIRQ_OFFSET)
> > > -#define in_task()		(!(in_nmi() | in_hardirq() | in_serving_softirq()))
> > > +#define in_serving_irq()	(in_nmi() | in_hardirq() | in_serving_softirq())
> > > +#define in_task()		(!in_serving_irq())
> > >  
> > 
> > So in_serving_irq() is !in_task(), right? If so, why not...
> > 
> Adding in_serving_irq() is to reflect the real purpose so improve readability.
> And can we preserve that !in_task() means in serving irq context in future? I don't know.
> 

Sure, no one could predict the future. But if a third context (other
than thread context and {hard,soft}irq context) comes up, which I think
is highly unlikely, we could (and should) audit all callsites of
in_task() for necessary adjustment. And introducing in_serving_irq()
won't help us in that case, because we will still need to audit usage of
in_serving_irq(), for example, let's say rcu_is_idle_cpu() for RCU_TINY
is defined as

	#define rcu_is_idle_cpu(cpu) (is_idle_task(current) && !in_serving_irq())

and we have a new type of context, and we can use in_other() to test
whether we are in it. Now even with in_serving_irq() introduced, we
still need to make sure the correct version of rcu_is_idle_cpu() is
either

	(is_idle_task(current) && (!in_serving_irq() && !in_other()))

or
	
	(is_idle_task(current) && !in_serving_irq())

Therefore, I don't see the point of introducing in_serving_irq().

Regards,
Boqun

> -- 
> Cheers,
> Changbin Du