Re: [PATCH v2 RFC 1/3] documentation: Add needed ACCESS_ONCE() calls to memory-barriers.txt

Peter Zijlstra
2013-11-23T09:04:40Z

On Fri, Nov 22, 2013 at 10:13:13AM -0800, Paul E. McKenney wrote:
> How about the following?
>
> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> COMPILER BARRIER
> ----------------
>
> The Linux kernel has an explicit compiler barrier function that prevents the
> compiler from moving the memory accesses either side of it to the other side:
>
> barrier();
>
> This is a general barrier -- there are no read-read or write-write variants
> of barrier(). Howevever, ACCESS_ONCE() can be thought of as a weak form
> for barrier() that affects only the specific accesses flagged by the
> ACCESS_ONCE().
>
> The compiler barrier has no direct effect on the CPU, which may then reorder
> things however it wishes.
>

Seems ok, however this also seems like the natural spot to put that
chunk about how a compiler can mis-transform stuff without either
barrier or ACCESS_ONC(); that currently seems spread out over the
document in some notes.

The biggest of which seems to have ended up in the GUARANTEES chapter.