perf events ring buffer memory barrier on powerpc

[Peter Zijlstra]

On Sat, Nov 02, 2013 at 08:20:48AM -0700, Paul E. McKenney wrote:
> On Fri, Nov 01, 2013 at 11:30:17AM +0100, Peter Zijlstra wrote:
> > Furthermore there's a gazillion parallel userspace programs.
> 
> Most of which have very unaggressive concurrency designs.

pthread_mutex_t A, B;

char data_A[x];
int  counter_B = 1;

void funA(void)
{
	pthread_mutex_lock(&A);
	memset(data_A, 0, sizeof(data_A));
	pthread_mutex_unlock(&A);
}

void funB(void)
{
	pthread_mutex_lock(&B);
	counter_B++;
	pthread_mutex_unlock(&B);
}

void funC(void)
{
	pthread_mutex_lock(&B)
	printf("%d\n", counter_B);
	pthread_mutex_unlock(&B);
}

Then run: funA, funB, funC concurrently, and end with a funC.

Then explain to userman than his unaggressive program can return:
0
1

Because the memset() thought it might be a cute idea to overwrite
counter_B and fix it up 'later'. Which if I understood you right is
valid in C/C++ :-(

Not that any actual memset implementation exhibiting this trait wouldn't
be shot on the spot.

> > > 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.
> 
> #define __rcu __atomic

Yeah, except we don't use __rcu all that consistently; in fact I don't
know if I ever added it.