Re: Documentation/memory-barriers.txt: How can READ_ONCE() and WRITE_ONCE() provide cache coherence?

From: Sergey Fedorov
Date: Sat Feb 27 2016 - 15:13:17 EST

On 27.02.2016 00:31, Paul E. McKenney wrote:
Without READ_ONCE(), common sub-expression elimination optimizations
can cause later reads of a given variable to see older value than
previous reads did. For a (silly) example:

a = complicated_pure_function(x);
b = x;
c = complicated_pure_function(x);

The compiler is within its rights to transform this into the following:

a = complicated_pure_function(x);
b = x;
c = a(x);

In this case, the assignment to b might see a newer value of x than did
the later assignment to c. This violates cache coherence, which states
that all reads from a given variable must agree on the order of values
taken on by that variable.

I see how READ_ONCE() and WRITE_ONCE() can prevent compiler from speculating on variable values and optimizing memory accesses. But concerning cache coherency itself, my understanding is that software can really ensure hardware cache coherency by using one of the following methods:
- by not using the caches
- by using some sort of cache maintenance instructions
- by using hardware cache coherency mechanisms (which is what normally used)

What kind of "cache coherency" do you mean?