Messages in this thread | | | Date | Mon, 17 Feb 2014 14:52:53 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 2/6] Documentation/memory-barriers.txt: ACCESS_ONCE() provides cache coherence |
| |
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" <paulmck@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 <paulmck@linux.vnet.ibm.com> > > Punctuation nit below; otherwise: > Reviewed-by: Josh Triplett <josh@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 > > >
| |