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:

quoted

On Mon, Jan 04, 2016 at 02:36:58PM +0100, Peter Zijlstra wrote:

quoted

On Sun, Jan 03, 2016 at 11:12:44AM +0200, Michael S. Tsirkin wrote:

quoted

On Sat, Jan 02, 2016 at 11:24:38AM +0000, Russell King - ARM Linux wrote:

quoted

quoted

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.

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