Messages in this thread | | | Date | Wed, 6 Nov 2013 15:44:56 +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 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.
long head = ((__atomic long)pc->data_head).load(memory_order_acquire);
coupled with:
((__atomic long)pc->data_tail).store(tail, memory_order_release);
might be the 'right' and proper C11 incantations to avoid having to touch kernel macros; but would obviously require a recent compiler.
Barring that, I think we're stuck with:
long head = ACCESS_ONCE(pc->data_head); smp_rmb();
...
smp_mb(); pc->data_tail = tail;
And using the right asm goo for the barriers. That said, all these asm barriers should include a compiler barriers (memory clobber) which _should_ avoid the worst compiler trickery -- although I don't think it completely obviates the need for ACCESS_ONCE() -- uncertain there.
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