atomic ops

[Mateusz Guzik]

On Tue, Oct 28, 2014 at 03:52:22AM +0100, Mateusz Guzik wrote:
[scratching old content so that I hopefully re-state it nicer]

I would like to reviwe the discussion about memory barriers provided in
the kernel.

The kernel (at least on amd64) lacks lightweight barriers providing only
following guarantees:
- all writes are completed prior to given point
- all reads are completed prior to given point

On amd64 such barriers require only compiler barrier, and as such
obviously beat currently used operations like load_acq (which uses
cmpxchg).

Example consumer which would benefit greatly from such barriers is
seq.h:
https://svnweb.freebsd.org/base/head/sys/sys/seq.h?view=markup

_load_acq on amd64 provides full barrier and it was noted we should not
change that in order to not break possible 3rd party consumers.
Also I don't see any alternative naming convention trying to stick to
this scheme that we could use.

As such I propose stealing naming from Linux and introduction of smp_wmb
and smp_rmb macros providing aforementioned funcionality.

So for amd64 this would be:
#define smp_wmb()	__compiler_membar()
#define smp_rmb()	__compiler_membar()

Any objections?

I'm happy to talk to arch maintainers in order to get relevant
implementations for all architectures.

-- 
Mateusz Guzik <mjguzik gmail.com>