On Fri, Nov 25, 2016 at 01:40:44PM +0100, Peter Zijlstra wrote:

On Fri, Nov 25, 2016 at 12:23:56PM +0000, Mark Rutland wrote:

quoted

Naming will be problematic; calling them ATOMIC_* makes tham sound like
they work on atomic_t. That and I have no idea how to ensure correct
usage tree-wide; I'm not sure if/how Coccinelle can help.

Peter, thoughts?

Something like so perhaps?

/*
 * Provide accessors for Single-Copy atomicy.
 *
 * That is, ensure that machine word sized loads/stores to naturally
 * aligned variables are single instructions.

Minor nit: this sounds like we *only* support the machine word size,
whereas (excluding alpha IIRC) we can generally acccess power-of-two
sizes from byte up to that.

So perhaps:

	That is, ensure that loads/stores are made with single
	instructions, where the machine can perform a tear-free access
	of that size.

 * By reason of not being able to use C11 atomic crud, use our beloved
 * volatile qualifier. Since volatile tells the compiler the value can
 * be changed behind its back, it must use Single-Copy atomic loads and
 * stores to access them, otherwise it runs the risk of load/store
 * tearing.
 */

#define SINGLE_LOAD(x)						\
{(								\
	compiletime_assert_atomic_type(typeof(x));		\
	WARN_SINGLE_COPY_ALIGNMENT(&(x));			\
	READ_ONCE(x);						\
})

#define SINGLE_STORE(x, v)					\
({								\
	compiletime_assert_atomic_type(typeof(x));		\
	WARN_SINGLE_COPY_ALIGNMENT(&(x));			\
	WRITE_ONCE(x, v);					\
})

Modulo your type comment, and mine above, this looks good to me.

Thanks,
Mark.