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SubjectRe: perf events ring buffer memory barrier on powerpc
On 10/29, Peter Zijlstra wrote:
>
> On Tue, Oct 29, 2013 at 11:30:57AM +0100, Peter Zijlstra wrote:
> > @@ -154,9 +175,11 @@ int perf_output_begin(struct perf_output
> > * Userspace could choose to issue a mb() before updating the
> > * tail pointer. So that all reads will be completed before the
> > * write is issued.
> > + *
> > + * See perf_output_put_handle().
> > */
> > tail = ACCESS_ONCE(rb->user_page->data_tail);
> > - smp_rmb();
> > + smp_mb();
> > offset = head = local_read(&rb->head);
> > head += size;
> > if (unlikely(!perf_output_space(rb, tail, offset, head)))
>
> That said; it would be very nice to be able to remove this barrier. This
> is in every event write path :/

Yes.. And I'm afraid very much that I simply confused you. Perhaps Victor
is right and we do not need this mb(). So I am waiting for the end of
this story too.

And btw I do not understand why we need it (or smp_rmb) right after
ACCESS_ONCE(data_tail).

Oleg.



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