perf events ring buffer memory barrier on powerpc

[Peter Zijlstra]

On Wed, Oct 30, 2013 at 11:40:15PM -0700, Paul E. McKenney wrote:
> > Now the whole crux of the question is if we need barrier A at all, since
> > the STORES issued by the @buf writes are dependent on the ubuf->tail
> > read.
> 
> The dependency you are talking about is via the "if" statement?
> Even C/C++11 is not required to respect control dependencies.
> 
> This one is a bit annoying.  The x86 TSO means that you really only
> need barrier(), ARM (recent ARM, anyway) and Power could use a weaker
> barrier, and so on -- but smp_mb() emits a full barrier.
> 
> Perhaps a new smp_tmb() for TSO semantics, where reads are ordered
> before reads, writes before writes, and reads before writes, but not
> writes before reads?  Another approach would be to define a per-arch
> barrier for this particular case.

I suppose we can only introduce new barrier primitives if there's more
than 1 use-case.

> > If the read shows no available space, we simply will not issue those
> > writes -- therefore we could argue we can avoid the memory barrier.
> 
> Proving that means iterating through the permitted combinations of
> compilers and architectures...  There is always hand-coded assembly
> language, I suppose.

I'm starting to think that while the C/C++ language spec says they can
wreck the world by doing these silly optimization, real world users will
push back for breaking their existing code.

I'm fairly sure the GCC people _will_ get shouted at _loudly_ when they
break the kernel by doing crazy shit like that.

Given its near impossible to write a correct program in C/C++ and
tagging the entire kernel with __atomic is equally not going to happen,
I think we must find a practical solution.

Either that, or we really need to consider forking the language and
compiler :-(