On Fri, 2015-07-17 at 00:32 +0200, Peter Zijlstra wrote:
> On Thu, Jul 16, 2015 at 04:41:45PM -0500, Tom Zanussi wrote:
> > On Thu, 2015-07-16 at 19:49 +0200, Peter Zijlstra wrote:
> > > On Thu, Jul 16, 2015 at 12:22:40PM -0500, Tom Zanussi wrote:
> > > > + for (i = 0; i < elt->map->n_fields; i++) {
> > > > + atomic64_set(&dup_elt->fields[i].sum,
> > > > + atomic64_read(&elt->fields[i].sum));
> > > > + dup_elt->fields[i].cmp_fn = elt->fields[i].cmp_fn;
> > > > + }
> > > > +
> > > > + return dup_elt;
> > > > +}
> > >
> > > So there is a lot of atomic64_{set,read}() in this patch set, what kind
> > > of magic properties do you assume they have?
> > >
> > > Note that atomic*_{set,read}() are weaker than {WRITE,READ}_ONCE(), so
> > > if you're assuming they do that, you're mistaken -- although it is on a
> > > TODO list someplace to go fix that.
> >
> > Not assuming any magic properties - I just need an atomic 64-bit counter
> > for the sums and that's the API for setting/reading those. When reading
> > a live trace the exact sum you get is kind of arbitrary..
>
> OK, so atomic64_read() really should provide load consistency (there are
> a few archs that lack the READ_ONCE() there).
>
> But the atomic64_set() does not provide store consistency, and in the
> above case it looks like the value you're writing is not exposed yet to
> concurrency so it doesn't matter how it issues the store.
>
Right, that's correct.
> So as long as you never atomic64_set() a value that is subject to
> concurrent modification you should be good.
Yeah, and that's the case elsewhere as well.
Thanks for clarifying,
Tom
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