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Re: Question whether a problem with race conditions exists in this case

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On Fri, 16 Dec 2011, Lew wrote:

> On Thursday, December 15, 2011 6:44:39 AM UTC-8, Tom Anderson wrote:
>> On Wed, 14 Dec 2011, Saxo wrote:
>>
>>> On Dec 14, 10:26 pm, Eric Sosman <esos...@ieee-dot-org.invalid> wrote:
>>>
>>>>>    private Object lock = new Object();
>>>>
>>>>      What does `lock' buy you?  Why not just synchronize on the
>>>> Node itself?
>>>
>>> The purpose is only to indicate that some more fine-gtrained locking
>>> would be used for the real thing instead of doing a synchronized(this)
>>> { ... } thing.
>>
>> It's quite a common pattern. I'm always a bit dubious about using an
>> public object (FSVO 'public') as the victim of a synchronized block; how
>> do i know some random other bit of code in some other thread isn't going
>> to try to lock the object at some point, and cause trouble? You wouldn't
>> expose a field, would you? So why expose an object's lock? Essentially, i
>> see an object's lock as a feature, like a method or a field; it should
>> only be exposed to other classes after due consideration, and if it is,
>> its proper use should be documented.
>
> I control that by who sees the object, e.g., a 'Collections.synchronizedList()'.

That's why i said "public object (FSVO 'public')" - there are times when 
you can wrap your objects like that, but others when it doesn't make 
sense, because of the extra complexity it introduces.

Wrapping is definitely a powerful tool here, though, very good point. 
Wrapping is, generally, a way of creating a sort of neutral zone; things 
that are behind the wrapper can be public without actually being very 
exposed. You control their effective scope by controlling the availability 
of the reference.

> I see the point in what you're saying but I find it over-cautious sometimes.

Fair point. I suppose this is something you can do if you want to be able 
to make strong assumptions about when the object is locked. You don't 
always need that.

> It depends on whether you want the object to control its own internal 
> locking, which sometimes you do, or to be part of its client's thread 
> control, as the numerous standard API classes with 'synchronized' 
> methods do.

Yes, quite true. This is what i was getting at with "an object's lock 
[...] should only be exposed to other classes after due consideration, and 
if it is, its proper use should be documented". Making it part of the 
client's thread control is a valid thing to do, but it should be a 
considered act.

>> The locking analogue of a private field is an object like the above,
>> created for the sole purpose of supplying a lock that is provably only
>> accessible to code which can see the private details of the class.
>>
>> I have coined the name 'lockguffin' for these objects, and i encourage you
>> all to use it.
>
> When appropriate.

Oh, sorry, i meant that i encourage you to use the name, not the pattern! 
The name is always appropriate; the pattern certainly might not be.

tom

-- 
Get a fucking hobby that isn't breathing, browsing 4chan, or fapping. --
The Well Cultured Anonymous, on Manners

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Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-14 09:07 -0800
  Re: Question whether a problem with race conditions exists in this case Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-12-14 17:53 +0000
    Re: Question whether a problem with race conditions exists in this case Lew <lewbloch@gmail.com> - 2011-12-14 10:44 -0800
      Re: Question whether a problem with race conditions exists in this case Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2011-12-14 10:54 -0800
      Re: Question whether a problem with race conditions exists in this case Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-12-14 21:15 +0000
        Re: Question whether a problem with race conditions exists in this case Tom Anderson <twic@urchin.earth.li> - 2011-12-15 14:58 +0000
          Re: Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-15 08:40 -0800
          Re: Question whether a problem with race conditions exists in this case Gene Wirchenko <genew@ocis.net> - 2011-12-15 11:55 -0800
            Re: Question whether a problem with race conditions exists in this case Tom Anderson <twic@urchin.earth.li> - 2011-12-16 14:06 +0000
              Re: Question whether a problem with race conditions exists in this case Gene Wirchenko <genew@ocis.net> - 2011-12-16 10:09 -0800
    Re: Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-14 11:47 -0800
  Re: Question whether a problem with race conditions exists in this case Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2011-12-14 10:53 -0800
    Re: Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-14 11:54 -0800
      Re: Question whether a problem with race conditions exists in this case Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2011-12-15 16:38 -0800
        Re: Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-15 23:01 -0800
          Re: Question whether a problem with race conditions exists in this case Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2011-12-16 09:34 -0800
            Re: Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-17 11:55 -0800
              Re: Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-19 05:33 -0800
  Re: Question whether a problem with race conditions exists in this case Lew <lewbloch@gmail.com> - 2011-12-14 11:04 -0800
    Re: Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-14 12:32 -0800
      Re: Question whether a problem with race conditions exists in this case markspace <-@.> - 2011-12-14 14:13 -0800
        Re: Question whether a problem with race conditions exists in this case Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-12-14 17:44 -0500
          Re: Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-14 14:50 -0800
            Re: Question whether a problem with race conditions exists in this case markspace <-@.> - 2011-12-14 15:26 -0800
      Re: Question whether a problem with race conditions exists in this case Lew <lewbloch@gmail.com> - 2011-12-15 01:34 -0800
    Re: Question whether a problem with race conditions exists in this case Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-12-14 21:38 +0000
  Re: Question whether a problem with race conditions exists in this case Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-12-14 16:26 -0500
    Re: Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-14 13:57 -0800
      Re: Question whether a problem with race conditions exists in this case Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-12-14 18:05 -0500
        Re: Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-14 23:25 -0800
          Re: Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-14 23:28 -0800
      Re: Question whether a problem with race conditions exists in this case Tom Anderson <twic@urchin.earth.li> - 2011-12-15 14:44 +0000
        Re: Question whether a problem with race conditions exists in this case Lew <lewbloch@gmail.com> - 2011-12-16 08:27 -0800
          Re: Question whether a problem with race conditions exists in this case Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2011-12-16 09:41 -0800
            Re: Question whether a problem with race conditions exists in this case Tom Anderson <twic@urchin.earth.li> - 2011-12-16 18:51 +0000
          Re: Question whether a problem with race conditions exists in this case Tom Anderson <twic@urchin.earth.li> - 2011-12-16 18:50 +0000
    Re: Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-14 14:13 -0800
      Re: Question whether a problem with race conditions exists in this case Lew <lewbloch@gmail.com> - 2011-12-14 16:55 -0800

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