[concurrency-interest] jdk9 VarHandle and Fence methods

Oleksandr Otenko oleksandr.otenko at oracle.com
Wed Sep 16 11:28:33 EDT 2015


Wow, that's some very flaky relaxation of the meaning of a lock!

I would expect the failing lock acquire to establish no sw edges, but I 
would certainly expect the order of acquires and releases (successful 
and no) to be total, and the total order of all acquires and releases 
(successful and no) to be in alignment with the total order of 
operations on volatiles. That way indeed x=1 should be visible, but only 
if it is a volatile store - no guarantees for normal stores.

Also, I would not expect the debugging thread to acquire the lock, if 
that breaks the protocol. You wouldn't encourage a debugging thread to 
write to arbitrary volatiles - so you wouldn't encourage the debugging 
thread to acquire arbitrary locks.

Alex

On 15/09/2015 06:16, Hans Boehm 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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