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SubjectDocumentation/memory-barriers.txt: How can READ_ONCE() and WRITE_ONCE() provide cache coherence?
Date
Hi,

I just can't understand how this kind of compiler barrier macros may
provide any form of cache coherence. Sure, such kind of compiler barrier
is necessary to "reliably" access a variable from multiple CPUs. But why
it is stated that these macros *provide* cache coherence?

From Documentation/memory-barriers.txt:
> The READ_ONCE() and WRITE_ONCE() functions can prevent any number of
> optimizations that, while perfectly safe in single-threaded code, can
> be fatal in concurrent code. Here are some examples of these sorts
> of optimizations:
>
> (*) The compiler is within its rights to reorder loads and stores
> to the same variable, and in some cases, the CPU is within its
> rights to reorder loads to the same variable. This means that
> the following code:
>
> a[0] = x;
> a[1] = x;
>
> Might result in an older value of x stored in a[1] than in a[0].
> Prevent both the compiler and the CPU from doing this as follows:
>
> a[0] = READ_ONCE(x);
> a[1] = READ_ONCE(x);
>
> In short, READ_ONCE() and WRITE_ONCE() provide cache coherence for
> accesses from multiple CPUs to a single variable.

Thanks,
Sergey

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