Subj : Re: double-checked locking in C
To   : comp.programming.threads
From : David Schwartz
Date : Wed Jul 06 2005 11:19 am


"Laurent Deniau" <Laurent.Deniau@cern.ch> wrote in message 
news:dagnq8$dgi$1@sunnews.cern.ch...

> 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.

    I already showed you the sequence. A later thread sees 'initialized' as 
'true' but does not see the values that *were* initialized. The problem you 
are trying to solve is fundamental. It goes like this:

    1) The whole point of DCL is that subsequent threads don't have to 
acquire any locks and still must see the effects of the initialization.

    2) Nothing in the DCL patterns presented actually ensures that 
subsequent threads see the effects of the initialization.

    Therefore subsequent threads might not see the effects of the 
initialization.

    The fundamental error is thinking that mutexes do something even for 
threads that do not acquire them. A mutex is only guaranteed to work if it 
is accessed by a thread that holds the same mutex that was held by the 
thread that changed the value.

    DS

.