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:
> > 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.
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