Subj : Re: posix and lock-free algorithms
To   : comp.programming.threads
From : David Schwartz
Date : Thu Aug 11 2005 05:53 am


"John Doug Reynolds" <smspiff@fastmail.fm> wrote in message 
news:1123758229.821292.319380@o13g2000cwo.googlegroups.com...
>>  Singleton* Singleton::instance () {
>>    Mutex m;
>>    if (pInstance == 0) {
>>      Lock lock;
>>      if (pInstance == 0) {
>>        Singleton* tmp = new Singleton;
>>        m.lock(); m.unlock(); // just a memory barrier
>>        pInstance = tmp;

> But the "reader" thread does place a memory barrier between its read
> of pInstance and any attempt to access *pInstance.

    Since you must lock and unlock a mutex every time you pass through this 
code, it's really not DCL. It's just normal locking with extra overhead. 
Hypothetically, there could be reduced contention.

    DS

.