Re: [tip:perf/core] tools/perf: Add required memory barriers

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Nov 06, 2013 at 03:00:11PM +0100, Peter Zijlstra wrote:
> On Wed, Nov 06, 2013 at 08:50:47AM -0500, Vince Weaver wrote:
> > On Wed, 6 Nov 2013, tip-bot for Peter Zijlstra wrote:
> > 
> > > +++ b/tools/perf/util/evlist.h
> > > @@ -177,7 +177,7 @@ int perf_evlist__strerror_open(struct perf_evlist *evlist, int err, char *buf, s
> > >  static inline unsigned int perf_mmap__read_head(struct perf_mmap *mm)
> > >  {
> > >  	struct perf_event_mmap_page *pc = mm->base;
> > > -	int head = pc->data_head;
> > > +	int head = ACCESS_ONCE(pc->data_head);
> > >  	rmb();
> > >  	return head;
> > 
> > so is this ACCESS_ONCE required now for proper access to the mmap buffer?
> 
> Pretty much; otherwise your C compiler is allowed to mess it up.
> 
> > remember that there are users trying to use this outside of the kernel 
> > where we don't necessarily have access to internal kernl macros.  Some of 
> > these users aren't necessarily GPLv2 compatible either (PAPI for example 
> > is more or less BSD licensed) so just cutting and pasting chunks of 
> > internal kernel macros isn't always the best route either.
> 
> Other license stuff is not my problem; that said I doubt there's much
> copyright to claim on a volatile cast.

Also, does PAPI actually use the buffer then? I thought that was
strictly self monitoring.