Re: [Xen-devel] [PATCH for 4.6 03/13] xen: Introduce ACCESS_ONCE macro

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 16.12.14 at 21:08, <julien.grall@xxxxxxxxxx> wrote:
> --- a/xen/include/xen/compiler.h
> +++ b/xen/include/xen/compiler.h
> @@ -90,4 +90,18 @@
>      __asm__ ("" : "=r"(__ptr) : "0"(ptr));      \
>      (typeof(ptr)) (__ptr + (off)); })
>  
> +/*
> + * Prevent the compiler from merging or refetching accesses.  The compiler
> + * is also forbidden from reordering successive instances of ACCESS_ONCE(),
> + * but only when the compiler is aware of some particular ordering.  One way
> + * to make the compiler aware of ordering is to put the two invocations of
> + * ACCESS_ONCE() in different C statements.
> + *
> + * This macro does absolutely -nothing- to prevent the CPU from reordering,
> + * merging, or refetching absolutely anything at any time.  Its main intended
> + * use is to mediate communication between process-level code and irq/NMI
> + * handlers, all running on the same CPU.
> + */
> +#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))

Any reason not to simply use {read,write}_atomic() instead, which we
already have?

Jan


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