[concurrency-interest] jdk9 VarHandle and Fence methods

Hans Boehm boehm at acm.org
Tue Sep 15 01:16:07 EDT 2015

> How does it slow down lock()?

It depends on the precise guarantee you provide, and I suspect this thread
didn't quite agree on that.  The most natural one is that the succeeding
lock acquisition happens before the failed trylock().  That implies that if
we have

x = 1;

those can't be reordered by the hardware, since a failing trylock() would
have to see the assignment to x.  That requires a fence between them on ARM
or Power.

I think the right way to think of trylock(), at least informally, is as
allowing spurious failures. I.e. trylock() is allowed to behave as though
the lock was held when it isn't.  You thus can't conclude anything about
other threads from the fact that it failed.  In this view you don't have to
think about memory ordering issues when reasoning about correctness, you
just reason about spurious failures instead.

If your code is robust against unknown, e.g. debugger, threads acquiring
the lock now and then, then it must be robust against this sort of spurious
failure.  If the lock is really used only to provide mutual exclusion, this
should not affect correctness.

On Mon, Sep 14, 2015 at 6:41 PM, Vitaly Davidovich <vitalyd at gmail.com>

> How does it slow down lock()?
> I don't necessarily disagree but I can certainly see people considering
> tryLock to have same ordering effect as (failed) CAS.  It's certainly true
> that a CAS is a lower level primitive than a lock, but I don't know if that
> resonates immediately when thinking about this.  It's also the case that on
> very popular platforms such as x86 a failing tryLock will have the same
> ordering as a successful one, and no difference is observed (and JIT
> doesn't do anything different).
> I don't understand the debugger thread example - what's the issue there?
> sent from my phone
> On Sep 14, 2015 9:07 PM, "Hans Boehm" <boehm at acm.org> wrote:
>> FWIW, this general issues is discussed in section 3 of
>> http://dl.acm.org/citation.cfm?id=1375581.1375591 .
>> Yet another argument against providing the stronger guarantees is that,
>> on many architectures, it doesn't just slow down trylock(), it more
>> importantly slows down lock().  In general, if your code cares about
>> ordering for unsuccessful trylock(), then it's not robust against, say, a
>> debugging thread unexpectedly acquiring the lock for a short period.  In my
>> view, in such a case, you're no longer using it as a lock, and you should
>> be using something else, e.g. an atomic object, with stronger guarantees.
>> On Fri, Sep 4, 2015 at 4:18 AM, Doug Lea <dl at cs.oswego.edu> wrote:
>>> On 09/03/2015 02:19 PM, Oleksandr Otenko wrote:
>>>> Has anyone come up with the answer about ordering for tryLock, or have
>>>> I missed it?
>>> You missed the dog not barking :-)
>>> The Lock specs don't require any specific HB effects here on failed
>>> tryLock. Even if we wanted to, we cannot retroactively impose any
>>> considering that anyone can implement the Lock interface (not just j.u.c)
>>> and some of these might become in violation.
>>> As you and Vitaly pointed out, there are a few fringe cases where
>>> users might want to impose ordering on failure. In jdk9, you'll
>>> me able to do this with moded VarHandle accesses and/or fences. The
>>> resulting extra fencing might be redundant here and there, but if you
>>> cared enough, you could create and rely on custom locks with stronger
>>> guarantees.
>>> -Doug
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