Re: [PATCH/RFC 1/2] jump label: make enable/disable o(1)

[Peter Zijlstra <peterz@infradead.org>]

On Thu, 2010-12-16 at 14:36 -0500, Jason Baron wrote:
> On Thu, Dec 16, 2010 at 08:33:51PM +0100, Peter Zijlstra wrote:
> > On Thu, 2010-12-16 at 14:23 -0500, Jason Baron wrote:
> > > 
> > > For the jump label disabled case, perf is using atomic_inc/dec and atomic_read
> > > to check if enabled. While other consumers (tracepoints) are just using an
> > > 'int'. I didn't want hurt the jump label disabled case for tracepoints.
> > > If we can agree to use atomic ops for tracepoints, or drop atomics from
> > > perf, that would simplify things. 
> > 
> > I had a quick look at the tracepoint stuff but got lost, but surely it
> > has a reference count somewhere as well, it needs to know when the last
> > probe goes away.. or does it check if the list is empty?
> > 
> > Anyway, tracepoint enable/disable isn't a real fast-path, surely it
> > could suffer an atomic op?
> 
> It is the atomic_read() at the tracepoint site that I am concerned
> about.

Look at the implementation :-), its just wrapper foo, its a regular read
for everything except some really weird archs (you really shouldn't care
about).

static inline int atomic_read(const atomic_t *v)
{
        return (*(volatile int *)&(v)->counter);
}

The volatile simply forces a load to be emitted.