[concurrency-interest] jdk9 VarHandle and Fence methods

Peter Levart peter.levart at gmail.com
Tue Sep 15 16:52:56 EDT 2015



On 09/15/2015 02:56 PM, Vitaly Davidovich wrote:
>
> 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.
>

How can you differentiate between:
- unlock() in thread T1 followed by unsuccessful tryLock() in thread T2 and
- unsuccessfull tryLock() in T2 followed by unlock() in T1

You want T1 to already unlock before you observe tryLock() failing in T2 
(pressumably because some T3 has already obtained the lock before T2 or 
because of spurious failure).
But T2 failing tryLock() might simply be because T1 hasn't unlocked yet. 
So regardless of memory effects, you can't reason of ordering in other 
threads by observing tryLock() failing.

Do you have an example that proves me wrong?

Regards, Peter

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