[concurrency-interest] jdk9 VarHandle and Fence methods

Vitaly Davidovich vitalyd at gmail.com
Tue Sep 15 08:56:21 EDT 2015


Hmm, the ordering I had in mind was unlock() happens-before a failing
tryLock.  So a thread failing on tryLock sees operations preceded by last
unlock() as ordered.  This is no different than successful tryLock or
lock() in that regard.

sent from my phone
On Sep 15, 2015 1:16 AM, "Hans Boehm" <boehm at acm.org> wrote:

> > 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;
> lock();
>
> 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>
> wrote:
>
>> 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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>>>
>>>
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