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Message-ID: <20160105180938-mutt-send-email-mst@redhat.com>
Date: Tue, 5 Jan 2016 18:16:48 +0200
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: linux-kernel@...r.kernel.org,
Peter Zijlstra <peterz@...radead.org>,
Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
Andrew Cooper <andrew.cooper3@...rix.com>,
virtualization@...ts.linux-foundation.org,
Stefano Stabellini <stefano.stabellini@...citrix.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
David Miller <davem@...emloft.net>, linux-ia64@...r.kernel.org,
linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
sparclinux@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
linux-metag@...r.kernel.org, linux-mips@...ux-mips.org,
x86@...nel.org, user-mode-linux-devel@...ts.sourceforge.net,
adi-buildroot-devel@...ts.sourceforge.net,
linux-sh@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
xen-devel@...ts.xenproject.org,
Benjamin Herrenschmidt <benh@...nel.crashing.org>,
Paul Mackerras <paulus@...ba.org>,
Michael Ellerman <mpe@...erman.id.au>,
Ingo Molnar <mingo@...nel.org>,
Davidlohr Bueso <dave@...olabs.net>,
Andrey Konovalov <andreyknvl@...gle.com>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2 15/32] powerpc: define __smp_xxx
On Tue, Jan 05, 2016 at 05:53:41PM +0800, Boqun Feng wrote:
> 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?
It isn't because as far as I could tell it is not used
outside arch/powerpc/include/asm/barrier.h
smp_store_release and smp_load_acquire.
And these are now gone.
Instead there are __smp_store_release and __smp_load_acquire
which call __smp_lwsync.
These are only used for virt and on SMP.
UP variants are generic - they just call barrier().
> > > > This reduces the amount of arch-specific boiler-plate code.
> > > >
> > > > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> > > > Acked-by: Arnd Bergmann <arnd@...db.de>
> > > > ---
> > > > 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
Sorry, I don't understand - why do you have to do anything?
I changed all users of smp_lwsync so they
use __smp_lwsync on SMP and barrier() on !SMP.
This is exactly the current behaviour, I also tested that
generated code does not change at all.
Is there a patch in your tree that conflicts with this?
> > > > 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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> > > > the body of a message to majordomo@...r.kernel.org
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