perf events ring buffer memory barrier on powerpc

[Peter Zijlstra]

On Fri, Nov 01, 2013 at 02:28:14AM -0700, Paul E. McKenney wrote:
> > This is a completely untenable position.
> 
> Indeed it is!
> 
> C/C++ never was intended to be used for parallel programming, 

And yet pretty much all kernels ever written for SMP systems are written
in it; what drugs are those people smoking?

Furthermore there's a gazillion parallel userspace programs.

> and this is
> but one of the problems that can arise when we nevertheless use it for
> parallel programming.  As compilers get smarter (for some definition of
> "smarter") and as more systems have special-purpose hardware (such as
> vector units) that are visible to the compiler, we can expect more of
> this kind of trouble.
> 
> This was one of many reasons that I decided to help with the C/C++11
> effort, whatever anyone might think about the results.

Well, I applaud your efforts, but given the results I think the C/C++
people are all completely insane.

> > How do the C/C++ people propose to deal with this?
> 
> By marking "ptr" as atomic, thus telling the compiler not to mess with it.
> And thus requiring that all accesses to it be decorated, which in the
> case of RCU could be buried in the RCU accessors.

This seems contradictory; marking it atomic would look like:

struct foo {
	unsigned long value;
	__atomic void *ptr;
	unsigned long value1;
};

Clearly we cannot hide this definition in accessors, because then
accesses to value* won't see the annotation.

That said; mandating we mark all 'shared' data with __atomic is
completely untenable and is not backwards compatible.

To be safe we must assume all data shared unless indicated otherwise.