Re: [PATCH v2] arch/powerpc: Rework local_paca to avoid LTO warnings

["Alastair D'Silva" <alastair@au1.ibm.com>]

On Tue, 2019-03-26 at 15:58 +1000, Nicholas Piggin wrote:
> Alastair D'Silva's on March 14, 2019 12:31 pm:
> > From: Alastair D'Silva <alastair@d-silva.org>
> > 
> > When building an LTO kernel, the existing code generates warnings:
> >     ./arch/powerpc/include/asm/paca.h:37:30: warning: register of
> >         ‘local_paca’ used for multiple global register variables
> >      register struct paca_struct *local_paca asm("r13");
> >                                   ^
> >     ./arch/powerpc/include/asm/paca.h:37:30: note: conflicts with
> >         ‘local_paca’
> 
> Isn't this a bogus warning? It doesn't look like there's a way to 
> define it any other way.

There isn't any other way to define it as a global. However, the
warning is legitimate.

The compiler sees that there are multiple global register variables,
all pointing at the same register.

The compiler can only determine this when LTO is used, as otherwise it
only sees the one in the current compilation unit, whicd disappears by
the time the kernel is linked.

> 
> > This patch reworks local_paca into an inline getter & setter
> > function,
> > which addresses the warning.
> > 
> > Changelog:
> > V2
> >   - Address whitespace issues
> >   - keep new implementation close to where the old implementation
> > was
> > 
> > Signed-off-by: Alastair D'Silva <alastair@d-silva.org>
> > ---
> >  arch/powerpc/include/asm/paca.h | 37 +++++++++++++++++++++++++--
> > ------
> >  arch/powerpc/kernel/paca.c      |  2 +-
> >  2 files changed, 29 insertions(+), 10 deletions(-)
> > 
> > diff --git a/arch/powerpc/include/asm/paca.h
> > b/arch/powerpc/include/asm/paca.h
> > index e843bc5d1a0f..2fa0b43357c9 100644
> > --- a/arch/powerpc/include/asm/paca.h
> > +++ b/arch/powerpc/include/asm/paca.h
> > @@ -34,19 +34,38 @@
> >  #include <asm/cpuidle.h>
> >  #include <asm/atomic.h>
> >  
> > -register struct paca_struct *local_paca asm("r13");
> > -
> >  #if defined(CONFIG_DEBUG_PREEMPT) && defined(CONFIG_SMP)
> >  extern unsigned int debug_smp_processor_id(void); /* from
> > linux/smp.h */
> > -/*
> > - * Add standard checks that preemption cannot occur when using
> > get_paca():
> > - * otherwise the paca_struct it points to may be the wrong one
> > just after.
> > - */
> > -#define get_paca()	((void) debug_smp_processor_id(), local_paca)
> > -#else
> > -#define get_paca()	local_paca
> >  #endif
> >  
> > +static inline struct paca_struct *get_paca_no_preempt_check(void)
> > +{
> > +	register struct paca_struct *paca asm("r13");
> > +
> > +	return paca;
> > +}
> 
> Problem is it now changes the global register variable to a local 
> register variable. The compiler would presumably be within its rights
> to "cache" that return value or use another register for it, which
> is not really what we want.
> 
> 

I've confirmed that at least with GCC 8.2.0, the generated assembler is
similar, but yes, the compiler may be free to take a copy into another
register (although that would be a terrible optimisation), and then
operate on that value.

Subsequent uses would still have to call the function (ie. fetch the
data from r13) regardless, so I believe this scenario is safe.

Can you think of a scenario where this is a problem?

-- 
Alastair D'Silva
Open Source Developer
Linux Technology Centre, IBM Australia
mob: 0423 762 819