Subj : Re: double-checked locking in C
To   : comp.programming.threads
From : Laurent Deniau
Date : Wed Jul 06 2005 04:53 pm

Joe Seigh wrote:
> Laurent Deniau wrote:
> 
>> Joe Seigh wrote:
>>
>>>
>>> Most people don't realize that it's not just reads that can be out of
>>> order but writes as well.  So you need release semantics on the store
>>> of the init flag as well as acquire semantics on the load of that flag.
>>
>>
>>
>> This is done by the mutext lock and unlock. They unsure 
>> synchronization of the memory content with other threads (otherwise 
>> mutex would be completely useless). I do not have to deal with such 
>> problem (and it is not possible at the level of C).
> 
> 
> The mutex doesn't affect the order of stores as seen by other threads
> *not* using the mutex.

Right. And? I still not see any problem.

>>> There was nothing in your example showing that the initialization stores
>>> had to appear in memory before the store to the init flag did.
>>
>>
>>
>> This is the problem of unlock, not mine. Between the lock/unlock I 
>> *only* must ensure that if other threads see a value in the first test 
>> (by a premature memory commit), it is a valid value, which by the way 
>> never change after being set. The rest is done by lock/unlock.
>>
>> Remember the context: C language, singleton pattern (or unique 
>> initialization) and DCL. Nothing more.
>>
>> If you still see a problem in my code, please show me a sequence of 
>> actions (sequence points) done by two (or more) threads which breaks 
>> the semantic of the singleton pattern.
>>
> No, sorry.  The burden of proof is on you.  And the reason I posted the
> URL is so we don't have to disprove every bogus DCL implementation there
> is over and over again.

How I can I prove that there is no bogus? If the DCL code shown is so 
easy to break, show me a sequence of actions (points). I can detail all 
steps for different cases, but it will be much longer than showing a 
single bogus case, if any.

> Another URL you can check out
> http://www.nwcpp.org/Downloads/2004/DCLP_notes.pdf

Thanks, but at first quick reading, I still do not see problems.

ld.

.