Re: [Xen-devel] [PATCH v2 22/32] s390: define __smp_xxx

["Michael S. Tsirkin" <mst@xxxxxxxxxx>]

On Mon, Jan 04, 2016 at 02:45:25PM +0100, Peter Zijlstra wrote:
> On Thu, Dec 31, 2015 at 09:08:38PM +0200, Michael S. Tsirkin wrote:
> > This defines __smp_xxx barriers for s390,
> > for use by virtualization.
> > 
> > Some smp_xxx barriers are removed as they are
> > defined correctly by asm-generic/barriers.h
> > 
> > Note: smp_mb, smp_rmb and smp_wmb are defined as full barriers
> > unconditionally on this architecture.
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > Acked-by: Arnd Bergmann <arnd@xxxxxxxx>
> > ---
> >  arch/s390/include/asm/barrier.h | 15 +++++++++------
> >  1 file changed, 9 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/s390/include/asm/barrier.h 
> > b/arch/s390/include/asm/barrier.h
> > index c358c31..fbd25b2 100644
> > --- a/arch/s390/include/asm/barrier.h
> > +++ b/arch/s390/include/asm/barrier.h
> > @@ -26,18 +26,21 @@
> >  #define wmb()                              barrier()
> >  #define dma_rmb()                  mb()
> >  #define dma_wmb()                  mb()
> > -#define smp_mb()                   mb()
> > -#define smp_rmb()                  rmb()
> > -#define smp_wmb()                  wmb()
> > -
> > -#define smp_store_release(p, v)                                            
> > \
> > +#define __smp_mb()                 mb()
> > +#define __smp_rmb()                        rmb()
> > +#define __smp_wmb()                        wmb()
> > +#define smp_mb()                   __smp_mb()
> > +#define smp_rmb()                  __smp_rmb()
> > +#define smp_wmb()                  __smp_wmb()
> 
> Why define the smp_*mb() primitives here? Would not the inclusion of
> asm-generic/barrier.h do this?

No because the generic one is a nop on !SMP, this one isn't.

Pls note this patch is just reordering code without making
functional changes.
And at the moment, on s390 smp_xxx barriers are always non empty.

Some of this could be sub-optimal, but
since on s390 Linux always runs on a hypervisor,
I am not sure it's safe to use the generic version -
in other words, it just might be that for s390 smp_ and virt_
barriers must be equivalent.

If in fact this turns out to be wrong, I can pick up
a patch to change this, but I'd rather make this
a patch on top so that my patches are testable
just by compiling and comparing the binary.

-- 
MST

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