Subj : Re: double-checked locking in C
To   : comp.programming.threads
From : Joe Seigh
Date : Wed Jul 06 2005 10:05 am

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.

> 
>> 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.

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

-- 
Joe Seigh

When you get lemons, you make lemonade.
When you get hardware, you make software.

.