Re: [PATCH] powerpc/ppc64: remove __volatile__ in get_current()

[Scott Wood <scottwood@freescale.com>]

On Fri, 2013-08-23 at 18:40 -0500, James Yang wrote:
> On Sat, 10 Aug 2013, James Yang wrote:
> 
> > Uses of get_current() that normally get optimized away still result in
> > a load instruction of the current pointer in 64-bit because the inline
> > asm uses __volatile__.  This patch removes __volatile__ so that nop-ed
> > uses of get_current() don't actually result in a load of the pointer.
> > 
> > Signed-off-by: James Yang <James.Yang@freescale.com>
> > ---
> >  arch/powerpc/include/asm/current.h |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> > 
> > diff --git a/arch/powerpc/include/asm/current.h b/arch/powerpc/include/asm/current.h
> > index e2c7f06..bb250c8 100644
> > --- a/arch/powerpc/include/asm/current.h
> > +++ b/arch/powerpc/include/asm/current.h
> > @@ -19,7 +19,7 @@ static inline struct task_struct *get_current(void)
> >  {
> >  	struct task_struct *task;
> >  
> > -	__asm__ __volatile__("ld %0,%1(13)"
> > +	__asm__ ("ld %0,%1(13)"
> >  	: "=r" (task)
> >  	: "i" (offsetof(struct paca_struct, __current)));
> 
> 
> Hello, 
> 
> Scott's been able to put enough doubt in me to think that this is not 
> entirely safe, even though the testing and code generation show it to 
> work.  Please reject this patch.
> 
> I think there is still value in getting the unnecessary loads to be 
> removed since it would also allow unnecessary conditional branches to 
> be removed.  I'll think about alternate ways to do this.

Actually, I changed my mind in the other direction in parallel. :-P

I think it's probably safe.

-Scott