On Mon, Feb 17, 2014 at 01:40:56PM -0800, Josh Triplett wrote:
> On Mon, Feb 17, 2014 at 01:26:49PM -0800, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paul...@linux.vnet.ibm.com>
> >
> > The ACCESS_ONCE() primitive provides cache coherence, but the
> > documentation does not clearly state this. This commit therefore upgrades
> > the documentation.
> >
> > Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
>
> Punctuation nit below; otherwise:
> Reviewed-by: Josh Triplett <j...@joshtriplett.org>
>
> > Documentation/memory-barriers.txt | 17 +++++++++++++++++
> > 1 file changed, 17 insertions(+)
> >
> > diff --git a/Documentation/memory-barriers.txt
> > b/Documentation/memory-barriers.txt
> > index 102dc19c4119..ad6db1d48f1f 100644
> > --- a/Documentation/memory-barriers.txt
> > +++ b/Documentation/memory-barriers.txt
> > @@ -1249,6 +1249,23 @@ 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:
> >
> > + (*) The compiler is within its rights to reorder loads and stores
> > + to the same variable, and in some cases, the CPU is within its
> > + rights to reorder loads to the same variable. This means that
> > + the following code:
> > +
> > + a[0] = x;
> > + a[1] = x;
> > +
> > + Might result in an older value of x stored in a[1] than in a[0].
> > + Prevent both the compiler and the CPU from doing this as follows:
> > +
> > + a[0] = ACCESS_ONCE(x);
> > + a[1] = ACCESS_ONCE(x);
> > +
> > + In short, ACCESS_ONCE() provides "cache coherence" for accesses from
> > + multiple CPUs to a single variable.
>
> You don't need to "quote" the well-established term "cache coherence".
Good point, fixed and applied your Reviewed-by, thank you!
Thanx, Paul
> > (*) The compiler is within its rights to merge successive loads from
> > the same variable. Such merging can cause the compiler to "optimize"
> > the following code:
> > --
> > 1.8.1.5
> >
>
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