| Message ID | 1451572003-2440-18-git-send-email-mst@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
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?
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.
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.
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?
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.
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.
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.
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.
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>