On Thu, Nov 17, 2016 at 10:28:00AM +0100, Peter Zijlstra wrote:
> On Tue, Nov 15, 2016 at 10:19:09PM +0800, Boqun Feng wrote:
> > On Tue, Nov 15, 2016 at 02:01:54PM +0100, Peter Zijlstra wrote:
> > > On Tue, Nov 15, 2016 at 08:33:37PM +0800, Boqun Feng wrote:
> > > > Hi Peter,
> > > >
> > > > On Mon, Nov 14, 2016 at 06:39:53PM +0100, Peter Zijlstra wrote:
> > > > [...]
> > > > > +/*
> > > > > + * Similar to atomic_dec_and_test(), it will BUG on underflow and
> > > > > fail to
> > > > > + * decrement when saturated at UINT_MAX.
> > > > > + *
> > > > > + * Provides release memory ordering, such that prior loads and
> > > > > stores are done
> > > > > + * before a subsequent free.
> > > >
> > > > I'm not sure this is correct, the RELEASE semantics is for the STORE
> > > > part of cmpxchg, and semantically it will guarantee that memory
> > > > operations after cmpxchg won't be reordered upwards, for example, on
> > > > ARM64, the following code:
> > > >
> > > > WRITE_ONCE(x, 1)
> > > >
> > > > atomic_cmpxchg_release(&a, 1, 2);
> > > > r1 = ll(&a)
> > > > if (r1 == 1) {
> > > > sc_release(&a, 2);
> > > > }
> > > >
> > > > free()
> > > >
> > > > could be reordered as, I think:
> > > >
> > > > atomic_cmpxchg_release(&a, 1, 2);
> > > > r1 = ll(&a)
> > > > if (r1 == 1) {
> > > > free()
> > > > WRITE_ONCE(x, 1)
> > > > sc_release(&a, 2);
> > > > }
> > > >
> > > > Of course, we need to wait for Will to confirm about this. But if this
> > > > could happen, we'd better to use a smp_mb()+atomic_cmpxchg_relaxed()
> > > > here and for other refcount_dec_and_*().
> > >
> > > Can't happen I think because of the control dependency between
> > > dec_and_test() and free().
> > >
> > > That is, the cmpxchg_release() must complete to determine if it was
> > > successful or it needs a retry. The success, combined with the state of
> > > the variable will then determine if we call free().
> > >
> >
> > The thing is that determination of the variable's state(i.e.
> > store_release() succeeds) and the actual writeback to memory are two
> > separate events. So yes, free() won't execute before store_release()
> > commits successfully, but there is no barrier here to order the memory
> > effects of store_release() and free().
>
> Doesn't matter. If we dropped the refcount to 0, nobody else will be
> observing this memory anymore (unless ill-formed program). The only
> thing we need is that the free() will not be speculated.
>
> This is because all RmW on a specific variable, irrespective of their
> memory ordering on other loads/stores, are totally ordered against one
> another.
> Fair enough ;-) > > But as I said, we actually only need the pairing of orderings: > > > > 1) load part of cmpxchg -> free() > > 2) object accesses -> store part of cmpxchg > > > > Ordering #1 can be achieved via control dependency as you pointed out > > that free()s very much includes stores. And ordering #2 can be achieved > > with RELEASE. > > > > So the code is right, I just thought the comment may be misleading. The > > reason we use cmpxchg_release() is just for achieving ordering #2, and > > not to order "prior loads and stores" with "a subsequent free". > > > > Am I missing some subtle orderings here? > > I would want to further quality 1), it must be no earlier than the load > of the last / successful ll/sc round. > Great, that's more accurate! > At that point we're guaranteed a reference count of 1 that _will_ drop > to 0, and thus nobody else (should) reference that memory anymore. > > If we agree on this, I'll update the comment :-) Will, do you too agree? Agreed ;-) Control dependencies and RELEASE are totally enough for the internal correctness of refcount_t along with its interactivity with free(). People better not reply order guarantees other than this ;-) Regards, Boqun
signature.asc
Description: PGP signature
