Re: [Xen-devel] [PATCH v2 15/32] powerpc: define __smp_xxx

[Boqun Feng <boqun.feng@xxxxxxxxx>]

On Tue, Jan 05, 2016 at 10:51:17AM +0200, Michael S. Tsirkin wrote:
> On Tue, Jan 05, 2016 at 09:36:55AM +0800, Boqun Feng wrote:
> > Hi Michael,
> > 
> > On Thu, Dec 31, 2015 at 09:07:42PM +0200, Michael S. Tsirkin wrote:
> > > This defines __smp_xxx barriers for powerpc
> > > for use by virtualization.
> > > 
> > > smp_xxx barriers are removed as they are
> > > defined correctly by asm-generic/barriers.h
> 
> I think this is the part that was missed in review.
> 

Yes, I realized my mistake after reread the series. But smp_lwsync() is
not defined in asm-generic/barriers.h, right?

> > > This reduces the amount of arch-specific boiler-plate code.
> > > 
> > > Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > > Acked-by: Arnd Bergmann <arnd@xxxxxxxx>
> > > ---
> > >  arch/powerpc/include/asm/barrier.h | 24 ++++++++----------------
> > >  1 file changed, 8 insertions(+), 16 deletions(-)
> > > 
> > > diff --git a/arch/powerpc/include/asm/barrier.h 
> > > b/arch/powerpc/include/asm/barrier.h
> > > index 980ad0c..c0deafc 100644
> > > --- a/arch/powerpc/include/asm/barrier.h
> > > +++ b/arch/powerpc/include/asm/barrier.h
> > > @@ -44,19 +44,11 @@
> > >  #define dma_rmb()        __lwsync()
> > >  #define dma_wmb()        __asm__ __volatile__ (stringify_in_c(SMPWMB) : 
> > > : :"memory")
> > >  
> > > -#ifdef CONFIG_SMP
> > > -#define smp_lwsync()     __lwsync()
> > > +#define __smp_lwsync()   __lwsync()
> > >  
> > 
> > so __smp_lwsync() is always mapped to lwsync, right?
> 
> Yes.
> 
> > > -#define smp_mb() mb()
> > > -#define smp_rmb()        __lwsync()
> > > -#define smp_wmb()        __asm__ __volatile__ (stringify_in_c(SMPWMB) : 
> > > : :"memory")
> > > -#else
> > > -#define smp_lwsync()     barrier()
> > > -
> > > -#define smp_mb() barrier()
> > > -#define smp_rmb()        barrier()
> > > -#define smp_wmb()        barrier()
> > > -#endif /* CONFIG_SMP */
> > > +#define __smp_mb()       mb()
> > > +#define __smp_rmb()      __lwsync()
> > > +#define __smp_wmb()      __asm__ __volatile__ (stringify_in_c(SMPWMB) : 
> > > : :"memory")
> > >  
> > >  /*
> > >   * This is a barrier which prevents following instructions from being
> > > @@ -67,18 +59,18 @@
> > >  #define data_barrier(x)  \
> > >   asm volatile("twi 0,%0,0; isync" : : "r" (x) : "memory");
> > >  
> > > -#define smp_store_release(p, v)                                          
> > > \
> > > +#define __smp_store_release(p, v)                                        
> > >         \
> > >  do {                                                                     
> > > \
> > >   compiletime_assert_atomic_type(*p);                             \
> > > - smp_lwsync();                                                   \
> > > + __smp_lwsync();                                                 \
> > 
> > , therefore this will emit an lwsync no matter SMP or UP.
> 
> Absolutely. But smp_store_release (without __) will not.
> 
> Please note I did test this: for ppc code before and after
> this patch generates exactly the same binary on SMP and UP.
> 

Yes, you're right, sorry for my mistake...

> 
> > Another thing is that smp_lwsync() may have a third user(other than
> > smp_load_acquire() and smp_store_release()):
> > 
> > http://article.gmane.org/gmane.linux.ports.ppc.embedded/89877
> > 
> > I'm OK to change my patch accordingly, but do we really want
> > smp_lwsync() get involved in this cleanup? If I understand you
> > correctly, this cleanup focuses on external API like smp_{r,w,}mb(),
> > while smp_lwsync() is internal to PPC.
> > 
> > Regards,
> > Boqun
> 
> I think you missed the leading ___ :)
> 

What I mean here was smp_lwsync() was originally internal to PPC, but
never mind ;-)

> smp_store_release is external and it needs __smp_lwsync as
> defined here.
> 
> I can duplicate some code and have smp_lwsync *not* call __smp_lwsync

You mean bringing smp_lwsync() back? because I haven't seen you defining
in asm-generic/barriers.h in previous patches and you just delete it in
this patch.

> but why do this? Still, if you prefer it this way,
> please let me know.
> 

I think deleting smp_lwsync() is fine, though I need to change atomic
variants patches on PPC because of it ;-/

Regards,
Boqun

> > >   WRITE_ONCE(*p, v);                                              \
> > >  } while (0)
> > >  
> > > -#define smp_load_acquire(p)                                              
> > > \
> > > +#define __smp_load_acquire(p)                                            
> > > \
> > >  ({                                                                       
> > > \
> > >   typeof(*p) ___p1 = READ_ONCE(*p);                               \
> > >   compiletime_assert_atomic_type(*p);                             \
> > > - smp_lwsync();                                                   \
> > > + __smp_lwsync();                                                 \
> > >   ___p1;                                                          \
> > >  })
> > >  
> > > -- 
> > > MST
> > > 
> > > --
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