linux-kernel - Re: Documentation/memory-barriers.txt: How can READ_ONCE() and WRITE

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Message-ID: <56D2034C.4010803@gmail.com>
Date:	Sat, 27 Feb 2016 23:13:00 +0300
From:	Sergey Fedorov <serge.fdrv@...il.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Documentation/memory-barriers.txt: How can READ_ONCE() and
 WRITE_ONCE() provide cache coherence?

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?

Thanks,
Sergey