Re: linux-next: build warnings after merge of the access_once tree

[Peter Zijlstra]

On Thu, Mar 26, 2015 at 09:15:21AM -0700, Linus Torvalds wrote:
> Notice how it is *not* about atomicitiy. The compiler can read the
> value in fifteen pieces, randomly mixing one bit or five. Nobody
> cares.

If you read Documentation/memory-barriers.txt you'll find that it very
much also is about reading it in one go.

"The ACCESS_ONCE() function 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:

 ...

 (*) For aligned memory locations whose size allows them to be accessed
     with a single memory-reference instruction, prevents "load tearing"
     and "store tearing," ..."

There are many places in the kernel where we rely and use ACCESS_ONCE()
in order to 'guarantee' single loads. Paul is the expert here, but from
what I understand the compiler is not allowed to split loads for
volatile reads (assuming the load is both naturally aligned and of
machine word size).

And the size check in READ_ONCE() helps asserting this.
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