Re: [PATCH v2 1/1] doc: Add remote CPU access details and others to this_cpu_ops.txt

[Christoph Lameter]

Regarding atomic_t in per cpu areas: I am uncomfortable especially
because both locked and unlocked RMW write operations could be acting on
values in the same cacheline. I am concerned that the unlocked operation
could have an unpredictable result.


f.e. the following per cpu data structure

struct test {
	atomic_t a;
	int b;
} onecacheline;


Local cpu does

	this_cpu_inc(onecacheline.b);

If this is racing with a remote cpus:

	atomic_inc(percpu(&a, cpu))

then we have on x86 a increment operation with locked semantics racing
with an unlocked one on the same cacheline.

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