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. -- 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 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
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