linux-kernel - Re: PREEMPT_ACTIVE too low error with all asm-generic headers for some arches

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Date:	Wed, 24 Jun 2009 17:13:02 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Mike Frysinger <vapier.adi@...il.com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: PREEMPT_ACTIVE too low error with all asm-generic headers for
	some arches


* Mike Frysinger <vapier.adi@...il.com> wrote:

> On Wed, Jun 24, 2009 at 10:02, Arnd Bergmann wrote:
> > On Wednesday 24 June 2009, Ingo Molnar wrote:
> >> * Mike Frysinger <vapier.adi@...il.com> wrote:
> >> > if we look at linux/hardirq.h, it makes this claim:
> >> >  * - bit 28 is the PREEMPT_ACTIVE flag
> >> > if that's true, then why are we letting any arch set this define ?  a
> >> > quick survey shows that half the arches (11) are using 0x10000000 (bit
> >> > 28) while the other half (10) are using 0x4000000 (bit 26).  and then
> >> > there is the ia64 oddity which uses bit 30.  the exact value here
> >> > shouldnt really matter across arches though should it ?
> >
> > actually alpha, arm and avr32 also use bit 30 (0x40000000), there are only
> > five (or eight, depending on how you count) architectures (blackfin, h8300,
> > m68k, s390 and sparc) using bit 26.
> 
> meh, too many zeros ;)
> 
> >> Correct - what matters is to have no collision between the fields.
> >>
> >> > how about adding this to linux/thread_info.h:
> >> > #ifndef PREEMPT_ACTIVE
> >> > # ifndef PREEMPT_ACTIVE_BIT
> >> > #  define PREEMPT_ACTIVE_BIT 28
> >> > # endif
> >> > # define PREEMPT_ACTIVE (1 << PREEMPT_ACTIVE_BIT)
> >> > #endif
> >>
> >> Makes sense i guess - but do we really need that level of
> >> #ifdef nesting? PREEMPT_ACTIVE_BIT should be the main control - with
> >> a default to 28 if it's not set. PREEMPT_ACTIVE is then derived off
> >> that, without any #ifdefs.
> >
> > I think it would fit better into linux/hardirq.h instead of
> > linux/thread_info.h, because that is where the other bits of
> > the preempt count are defined.
> 
> agreed
> 
> > --- a/include/linux/hardirq.h
> > +++ b/include/linux/hardirq.h
> > @@ -62,6 +62,12 @@
> >  #define HARDIRQ_OFFSET (1UL << HARDIRQ_SHIFT)
> >  #define NMI_OFFSET     (1UL << NMI_SHIFT)
> >
> > +#ifndef PREEMPT_ACTIVE
> > +#define PREEMPT_ACTIVE_BITS    1
> > +#define PREEMPT_ACTIVE_SHIFT   (NMI_SHIFT + NMI_BITS)
> > +#define PREEMPT_ACTIVE (__IRQ_MASK(PREEMPT_ACTIVE_BITS) << PREEMPT_SHIFT)
> 
> i think you meant "<< PREEMPT_ACTIVE_SHIFT" there.  once i make 
> that change, it builds fine.

With that fix:

 Acked-by: Ingo Molnar <mingo@...e.hu>

	Ingo
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