Messages in this thread | | | Date | Wed, 6 Nov 2013 15:00:11 +0100 | From | Peter Zijlstra <> | Subject | Re: [tip:perf/core] tools/perf: Add required memory barriers |
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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.
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