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[PATCH v2 17/32] arm: define __smp_xxx

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  [PATCH v2 17/32] arm: define __smp_xxx "Michael S. Tsirkin" <mst@redhat.com> - 2015-12-31 20:10 +0100
    Re: [PATCH v2 17/32] arm: define __smp_xxx Russell King - ARM Linux <linux@arm.linux.org.uk> - 2016-01-02 12:30 +0100
      Re: [PATCH v2 17/32] arm: define __smp_xxx "Michael S. Tsirkin" <mst@redhat.com> - 2016-01-03 10:20 +0100
        Re: [PATCH v2 17/32] arm: define __smp_xxx Peter Zijlstra <peterz@infradead.org> - 2016-01-04 14:40 +0100
          Re: [PATCH v2 17/32] arm: define __smp_xxx Peter Zijlstra <peterz@infradead.org> - 2016-01-04 15:00 +0100
            Re: [PATCH v2 17/32] arm: define __smp_xxx Russell King - ARM Linux <linux@arm.linux.org.uk> - 2016-01-04 15:10 +0100
              Re: [PATCH v2 17/32] arm: define __smp_xxx "Michael S. Tsirkin" <mst@redhat.com> - 2016-01-05 15:40 +0100
            Re: [PATCH v2 17/32] arm: define __smp_xxx "Michael S. Tsirkin" <mst@redhat.com> - 2016-01-04 21:40 +0100
          Re: [PATCH v2 17/32] arm: define __smp_xxx "Michael S. Tsirkin" <mst@redhat.com> - 2016-01-04 21:20 +0100

#1299740 — [PATCH v2 17/32] arm: define __smp_xxx

This defines __smp_xxx barriers for arm,
for use by virtualization.

smp_xxx barriers are removed as they are
defined correctly by asm-generic/barriers.h

This reduces the amount of arch-specific boiler-plate code.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
---
 arch/arm/include/asm/barrier.h | 12 +++---------
 1 file changed, 3 insertions(+), 9 deletions(-)

diff --git a/arch/arm/include/asm/barrier.h b/arch/arm/include/asm/barrier.h
index 31152e8..112cc1a 100644
--- a/arch/arm/include/asm/barrier.h
+++ b/arch/arm/include/asm/barrier.h
@@ -60,15 +60,9 @@ extern void arm_heavy_mb(void);
 #define dma_wmb()	barrier()
 #endif
 
-#ifndef CONFIG_SMP
-#define smp_mb()	barrier()
-#define smp_rmb()	barrier()
-#define smp_wmb()	barrier()
-#else
-#define smp_mb()	dmb(ish)
-#define smp_rmb()	smp_mb()
-#define smp_wmb()	dmb(ishst)
-#endif
+#define __smp_mb()	dmb(ish)
+#define __smp_rmb()	__smp_mb()
+#define __smp_wmb()	dmb(ishst)
 
 #include <asm-generic/barrier.h>
 
-- 
MST

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#1300058

On Thu, Dec 31, 2015 at 09:07:59PM +0200, Michael S. Tsirkin wrote:
> This defines __smp_xxx barriers for arm,
> for use by virtualization.
> 
> smp_xxx barriers are removed as they are
> defined correctly by asm-generic/barriers.h
> 
> This reduces the amount of arch-specific boiler-plate code.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> Acked-by: Arnd Bergmann <arnd@arndb.de>

In combination with patch 14, this looks like it should result in no
change to the resulting code.

Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>

My only concern is that it gives people an additional handle onto a
"new" set of barriers - just because they're prefixed with __*
unfortunately doesn't stop anyone from using it (been there with
other arch stuff before.)

I wonder whether we should consider making the smp memory barriers
inline functions, so these __smp_xxx() variants can be undef'd
afterwards, thereby preventing drivers getting their hands on these
new macros?

-- 
RMK's Patch system: http://www.arm.linux.org.uk/developer/patches/
FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
according to speedtest.net.
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#1300280

On Sat, Jan 02, 2016 at 11:24:38AM +0000, Russell King - ARM Linux wrote:
> On Thu, Dec 31, 2015 at 09:07:59PM +0200, Michael S. Tsirkin wrote:
> > This defines __smp_xxx barriers for arm,
> > for use by virtualization.
> > 
> > smp_xxx barriers are removed as they are
> > defined correctly by asm-generic/barriers.h
> > 
> > This reduces the amount of arch-specific boiler-plate code.
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > Acked-by: Arnd Bergmann <arnd@arndb.de>
> 
> In combination with patch 14, this looks like it should result in no
> change to the resulting code.
> 
> Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
> 
> My only concern is that it gives people an additional handle onto a
> "new" set of barriers - just because they're prefixed with __*
> unfortunately doesn't stop anyone from using it (been there with
> other arch stuff before.)
> 
> I wonder whether we should consider making the smp memory barriers
> inline functions, so these __smp_xxx() variants can be undef'd
> afterwards, thereby preventing drivers getting their hands on these
> new macros?

That'd be tricky to do cleanly since asm-generic depends on
ifndef to add generic variants where needed.

But it would be possible to add a checkpatch test for this.


> -- 
> RMK's Patch system: http://www.arm.linux.org.uk/developer/patches/
> FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
> according to speedtest.net.
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#1300749

On Sun, Jan 03, 2016 at 11:12:44AM +0200, Michael S. Tsirkin wrote:
> On Sat, Jan 02, 2016 at 11:24:38AM +0000, Russell King - ARM Linux wrote:

> > My only concern is that it gives people an additional handle onto a
> > "new" set of barriers - just because they're prefixed with __*
> > unfortunately doesn't stop anyone from using it (been there with
> > other arch stuff before.)
> > 
> > I wonder whether we should consider making the smp memory barriers
> > inline functions, so these __smp_xxx() variants can be undef'd
> > afterwards, thereby preventing drivers getting their hands on these
> > new macros?
> 
> That'd be tricky to do cleanly since asm-generic depends on
> ifndef to add generic variants where needed.
> 
> But it would be possible to add a checkpatch test for this.

Wasn't the whole purpose of these things for 'drivers' (namely
virtio/xen hypervisor interaction) to use these?

And I suppose most of virtio would actually be modules, so you cannot do
what I did with preempt_enable_no_resched() either.

But yes, it would be good to limit the use of these things.
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#1300757

On Mon, Jan 04, 2016 at 02:36:58PM +0100, Peter Zijlstra wrote:
> On Sun, Jan 03, 2016 at 11:12:44AM +0200, Michael S. Tsirkin wrote:
> > On Sat, Jan 02, 2016 at 11:24:38AM +0000, Russell King - ARM Linux wrote:
> 
> > > My only concern is that it gives people an additional handle onto a
> > > "new" set of barriers - just because they're prefixed with __*
> > > unfortunately doesn't stop anyone from using it (been there with
> > > other arch stuff before.)
> > > 
> > > I wonder whether we should consider making the smp memory barriers
> > > inline functions, so these __smp_xxx() variants can be undef'd
> > > afterwards, thereby preventing drivers getting their hands on these
> > > new macros?
> > 
> > That'd be tricky to do cleanly since asm-generic depends on
> > ifndef to add generic variants where needed.
> > 
> > But it would be possible to add a checkpatch test for this.
> 
> Wasn't the whole purpose of these things for 'drivers' (namely
> virtio/xen hypervisor interaction) to use these?

Ah, I see, you add virt_*mb() stuff later on for that use case.

So, assuming everybody does include asm-generic/barrier.h, you could
simply #undef the __smp version at the end of that, once we've generated
all the regular primitives from it, no?
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#1300761

On Mon, Jan 04, 2016 at 02:54:20PM +0100, Peter Zijlstra wrote:
> On Mon, Jan 04, 2016 at 02:36:58PM +0100, Peter Zijlstra wrote:
> > On Sun, Jan 03, 2016 at 11:12:44AM +0200, Michael S. Tsirkin wrote:
> > > On Sat, Jan 02, 2016 at 11:24:38AM +0000, Russell King - ARM Linux wrote:
> > 
> > > > My only concern is that it gives people an additional handle onto a
> > > > "new" set of barriers - just because they're prefixed with __*
> > > > unfortunately doesn't stop anyone from using it (been there with
> > > > other arch stuff before.)
> > > > 
> > > > I wonder whether we should consider making the smp memory barriers
> > > > inline functions, so these __smp_xxx() variants can be undef'd
> > > > afterwards, thereby preventing drivers getting their hands on these
> > > > new macros?
> > > 
> > > That'd be tricky to do cleanly since asm-generic depends on
> > > ifndef to add generic variants where needed.
> > > 
> > > But it would be possible to add a checkpatch test for this.
> > 
> > Wasn't the whole purpose of these things for 'drivers' (namely
> > virtio/xen hypervisor interaction) to use these?
> 
> Ah, I see, you add virt_*mb() stuff later on for that use case.
> 
> So, assuming everybody does include asm-generic/barrier.h, you could
> simply #undef the __smp version at the end of that, once we've generated
> all the regular primitives from it, no?

Not so simple - that's why I mentioned using inline functions.

The new smp_* _macros_ are:

+#define smp_mb()       __smp_mb()

which means if we simply #undef __smp_mb(), smp_mb() then points at
something which is no longer available, and we'll end up with errors
saying that __smp_mb() doesn't exist.

My suggestion was to change:

#ifndef smp_mb
#define smp_mb()	__smp_mb()
#endif

to:

#ifndef smp_mb
static inline void smp_mb(void)
{
	__smp_mb();
}
#endif

which then means __smp_mb() and friends can be #undef'd afterwards.

-- 
RMK's Patch system: http://www.arm.linux.org.uk/developer/patches/
FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
according to speedtest.net.
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#1301575

On Mon, Jan 04, 2016 at 01:59:34PM +0000, Russell King - ARM Linux wrote:
> On Mon, Jan 04, 2016 at 02:54:20PM +0100, Peter Zijlstra wrote:
> > On Mon, Jan 04, 2016 at 02:36:58PM +0100, Peter Zijlstra wrote:
> > > On Sun, Jan 03, 2016 at 11:12:44AM +0200, Michael S. Tsirkin wrote:
> > > > On Sat, Jan 02, 2016 at 11:24:38AM +0000, Russell King - ARM Linux wrote:
> > > 
> > > > > My only concern is that it gives people an additional handle onto a
> > > > > "new" set of barriers - just because they're prefixed with __*
> > > > > unfortunately doesn't stop anyone from using it (been there with
> > > > > other arch stuff before.)
> > > > > 
> > > > > I wonder whether we should consider making the smp memory barriers
> > > > > inline functions, so these __smp_xxx() variants can be undef'd
> > > > > afterwards, thereby preventing drivers getting their hands on these
> > > > > new macros?
> > > > 
> > > > That'd be tricky to do cleanly since asm-generic depends on
> > > > ifndef to add generic variants where needed.
> > > > 
> > > > But it would be possible to add a checkpatch test for this.
> > > 
> > > Wasn't the whole purpose of these things for 'drivers' (namely
> > > virtio/xen hypervisor interaction) to use these?
> > 
> > Ah, I see, you add virt_*mb() stuff later on for that use case.
> > 
> > So, assuming everybody does include asm-generic/barrier.h, you could
> > simply #undef the __smp version at the end of that, once we've generated
> > all the regular primitives from it, no?
> 
> Not so simple - that's why I mentioned using inline functions.
> 
> The new smp_* _macros_ are:
> 
> +#define smp_mb()       __smp_mb()
> 
> which means if we simply #undef __smp_mb(), smp_mb() then points at
> something which is no longer available, and we'll end up with errors
> saying that __smp_mb() doesn't exist.
> 
> My suggestion was to change:
> 
> #ifndef smp_mb
> #define smp_mb()	__smp_mb()
> #endif
> 
> to:
> 
> #ifndef smp_mb
> static inline void smp_mb(void)
> {
> 	__smp_mb();
> }
> #endif
> 
> which then means __smp_mb() and friends can be #undef'd afterwards.

Absolutely, I got it.
The issue is that e.g. tile has:
#define __smp_mb__after_atomic()        do { } while (0)

and this is cheaper than barrier().

For this reason I left
#define smp_mb__after_atomic()  __smp_mb__after_atomic()
in place there.

Now, of course I can do (in asm-generic):

#ifndef smp_mb__after_atomic
static inline void smp_mb__after_atomic(void)
{
...
}
#endif

but this seems ugly: architectures do defines, generic
version does inline.


And that is not all: APIs like smp_store_mb can take
a variety of types as arguments so they pretty much
must be implemented as macros.

Teaching checkpatch.pl to complain about it seems like the cleanest
approach.

> -- 
> RMK's Patch system: http://www.arm.linux.org.uk/developer/patches/
> FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
> according to speedtest.net.
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#1301061

On Mon, Jan 04, 2016 at 02:54:20PM +0100, Peter Zijlstra wrote:
> On Mon, Jan 04, 2016 at 02:36:58PM +0100, Peter Zijlstra wrote:
> > On Sun, Jan 03, 2016 at 11:12:44AM +0200, Michael S. Tsirkin wrote:
> > > On Sat, Jan 02, 2016 at 11:24:38AM +0000, Russell King - ARM Linux wrote:
> > 
> > > > My only concern is that it gives people an additional handle onto a
> > > > "new" set of barriers - just because they're prefixed with __*
> > > > unfortunately doesn't stop anyone from using it (been there with
> > > > other arch stuff before.)
> > > > 
> > > > I wonder whether we should consider making the smp memory barriers
> > > > inline functions, so these __smp_xxx() variants can be undef'd
> > > > afterwards, thereby preventing drivers getting their hands on these
> > > > new macros?
> > > 
> > > That'd be tricky to do cleanly since asm-generic depends on
> > > ifndef to add generic variants where needed.
> > > 
> > > But it would be possible to add a checkpatch test for this.
> > 
> > Wasn't the whole purpose of these things for 'drivers' (namely
> > virtio/xen hypervisor interaction) to use these?
> 
> Ah, I see, you add virt_*mb() stuff later on for that use case.
> 
> So, assuming everybody does include asm-generic/barrier.h, you could
> simply #undef the __smp version at the end of that, once we've generated
> all the regular primitives from it, no?

Maybe I misunderstand, but I don't think so:

------>
#define __smp_xxx FOO
#define smp_xxx __smp_xxx
#undef __smp_xxx

smp_xxx
<------

resolves to __smp_xxx, not FOO.

That's why I went the checkpatch way.


-- 
MST
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#1301026

On Mon, Jan 04, 2016 at 02:36:58PM +0100, Peter Zijlstra wrote:
> On Sun, Jan 03, 2016 at 11:12:44AM +0200, Michael S. Tsirkin wrote:
> > On Sat, Jan 02, 2016 at 11:24:38AM +0000, Russell King - ARM Linux wrote:
> 
> > > My only concern is that it gives people an additional handle onto a
> > > "new" set of barriers - just because they're prefixed with __*
> > > unfortunately doesn't stop anyone from using it (been there with
> > > other arch stuff before.)
> > > 
> > > I wonder whether we should consider making the smp memory barriers
> > > inline functions, so these __smp_xxx() variants can be undef'd
> > > afterwards, thereby preventing drivers getting their hands on these
> > > new macros?
> > 
> > That'd be tricky to do cleanly since asm-generic depends on
> > ifndef to add generic variants where needed.
> > 
> > But it would be possible to add a checkpatch test for this.
> 
> Wasn't the whole purpose of these things for 'drivers' (namely
> virtio/xen hypervisor interaction) to use these?

My take out from discussion with you was that virtualization is probably
the only valid use-case.  So at David Miller's suggestion there's a
patch later in the series that adds virt_xxxx wrappers and these are
then used by virtio xen and later maybe others.

> And I suppose most of virtio would actually be modules, so you cannot do
> what I did with preempt_enable_no_resched() either.
> 
> But yes, it would be good to limit the use of these things.

Right so the trick is checkpatch warns about use of
__smp_xxx and hopefully people are not crazy enough
to use virt_xxx variants for non-virtual drivers.

-- 
MST
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