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    SubjectRe: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE()
    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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