Re: re-ordering across a mutex

Leigh Johnston <>
Wed, 24 Aug 2011 16:39:36 -0700 (PDT)
On 18/08/2011 10:32, wrote:

I have a follow-up question to my question on the double checked
locking problem

If I use a mutex of some kind like this
    execute code/ modify shared variables

For this to work, the compiler is not allowed to move code across the
acquire/release statements. Does the documentation for a mutex
usually say this?

Anyway, assuming the above is true, is not a simple solution to the
singleton problem something like this
         if (ptr == 0) // line1 - inexpensive
             if (ptr == 0)
                ptr1 =&singleton; // whatever
                ptr = ptr1;

At line1, if ptr is non-zero, we know for sure that the singleton
object has been fully constructed. No volatile anywhere. The mutexes
would also provide the needed memory barriers.

Would this work?

No, not on all systems. CPU read reordering could make non-null ptr
visible before the all of the singleton object is visible so you need a
barrier with acquire semantics after reading ptr. My singleton class
does the following:

    template <typename T>
    class singleton
        static T& instance()
            T* ret = sInstancePtr;
            if (ret == 0)
                lock theLock(sLock);
                static T sInstance;
                sInstancePtr = &sInstance;
                ret = sInstancePtr;
            return *ret;
        static lockable sLock;
        static T* sInstancePtr;

    template <typename T>
    lockable singleton<T>::sLock;
    template <typename T>
    T* singleton<T>::sInstancePtr;



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