[RFC/PATCH 5/7] ARM: Move get_thread_info macro definition to <asm/assembler.h>

[Russell King - ARM Linux]

On Thu, Oct 13, 2011 at 03:49:23PM +0100, Catalin Marinas wrote:
> On Thu, Oct 13, 2011 at 03:34:20PM +0100, Russell King - ARM Linux wrote:
> > On Wed, Oct 12, 2011 at 02:04:33AM -0400, gdavis at mvista.com wrote:
> > > diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
> > > index 78397d0..eaf4939 100644
> > > --- a/arch/arm/include/asm/assembler.h
> > > +++ b/arch/arm/include/asm/assembler.h
> > > @@ -36,6 +36,20 @@
> > >  	.endm
> > >  #endif  /* !CONFIG_THUMB2_KERNEL */
> > >  
> > > +	.macro	preempt_disable, tsk, cnt
> > > +	get_thread_info \tsk
> > > +	ldr	\cnt, [\tsk, #TI_PREEMPT]
> > > +	add	\cnt, \cnt, #1
> > > +	str	\cnt, [\tsk, #TI_PREEMPT]
> > > +	.endm
> > > +
> > > +	.macro	preempt_enable, tsk, cnt
> > > +	get_thread_info \tsk
> > > +	ldr	\cnt, [\tsk, #TI_PREEMPT]
> > > +	sub	\cnt, \cnt, #1
> > > +	str	\cnt, [\tsk, #TI_PREEMPT]
> > > +	.endm
> > > +
> > >  /*
> > >   * Endian independent macros for shifting bytes within registers.
> > >   */
> > > 
> > > 
> > > Not as efficient as it could be but I imagine the macros could
> > > be written to support optional load of \tsk and/or optional \tmp
> > > parameters to cover other common cases.
> > 
> > It's actually not that simple either: if you disable preemption, then you
> > need to check for a preempt event after re-enabling preemption.
> 
> That's not easily possible in assembly as calling a function would
> corrupt some registers. Is there any disadvantage with just doing the
> equivalent of preempt_enable_no_resched() for a few asm cases where this
> is needed?

It means you drop preemption reschedule events, which means that you can
no longer do hard-RT in any shape or form (because you're dropping the
reschedule for an indefinite amount of time.)

preempt_enable_no_resched() is fine if you're going to also put an
explicit check a bit later in the code path when it is convenient to
do so.