synchronized (this[.classinfo]) in druntime and phobos

[Dmitry Olshansky]

On 31.05.2012 22:34, Alex Rønne Petersen wrote:
> On 31-05-2012 16:49, Dmitry Olshansky wrote:
>>>>
>>>> But this is a protection/visibility issue, which is orthogonal on the
>>>> locking capability. It's as if you say "int is not good because anyone
>>>> can overflow it." Okay! Make it private inside a CheckedInt class.
>>>
>>> Sorry, that's a bad comparison. CheckedInt is to int, what CheckedMutex
>>> is to mutex - but I'm not suggesting anything like a CheckedMutex. I'm
>>> suggesting "mutex" but kept private inside the class /that it locks/.
>>> Yes, it's a visibility issue, the issue is that the mutex used by
>>> synchronized classes/methods is too visible/accessible and this opens it
>>> up for deadlocks which are otherwise impossible.
>>>
>>
>> Sure it's awful comparison.
>>
>>
>>>>
>>>> So where's the mutex that would be used to synchronize objects that
>>>> are not synchronizable?
>>>
>>> In the wrapper class/struct/object which derives a synchronized
>>> class/struct from the original. My D foo is not strong enough to just
>>> come up with valid D code for the idiom on the fly, but essentially you
>>> wrap the original object in a new object using a template which adds the
>>> mutex member and the interface methods (lock, tryLock, and unlock)
>>> required. No, this doesn't work with "final" classes.. but it shouldn't,
>>> they're final after all. For them you need to add/manage the mutex
>>> manually - the price you pay for "final".
>>>
>>
>> OK let me land you a hand here. My proposal, that I think fits your
>> ideas quite favorably.
>>
>> I'll enumerate certain assumptions beforehand since it's one of most
>> confusing threads I ever followed.
>>
>> 1. synchronized class means: always allocate hidden monitor mutex, lock
>> it on every public method of this class.
>
> If this means that the monitor field in regular objects goes away, then
> I'm all for this, at least.
>

Yes, it removes monitor in non-synchronized object. Glad you we are in 
agreement here.

And all public methods lower to :

<signature>
{
this.hidden__monitor.lock();
scope(exit) this.hidden__monitor.unlock();
... //code
}


-- 
Dmitry Olshansky