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Maybe a bug , I'm not pretty sure ...


GlowingHack
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Sorry for my poor English , I am not the native English speaker.

I start using Mangos for only one day , and I also try to read some of the source files. I do not know my source code version is the newest,

my version is mangos-0.9.0 , if this bug has been fixed in the later version , just skip this topic .

I just read the file SingletonImp.h , and I found the coder using the double-lock routine for implementing the lazy instance.

However , this mechanism does not work on the SMAP cpu , which means on many multi-core , there exist potential hazard

while on the initialization .

The reason can be obtained by google the key word : Memory Barrier .

Here I just give the revision version , which I does not prove it is right , just tips.

// a memory barrier wraper

// this memory barrier indeed will lock the bus and notify all the core to flush their own cache

// in order to get the same and accurate value

// calling this function will make all the store-and-load operation before such calling strictly be finished.

static void MemoryBarrier() {

#if defined (__MSV_VER)

::MemoryBarrier();

#elif defined (__GNUG__)

__volatile __asm (""::: " memory " );

#endif

}

template

<

typename T,

class ThreadingModel,

class CreatePolicy,

class LifeTimePolicy

>

T&

MaNGOS::Singleton<T, ThreadingModel, CreatePolicy, LifeTimePolicy >::Instance()

{

//add

T& value = si_instance;

MemoryBarrier();

// here the value has been notified

if( !value )

{

// double-checked Locking pattern

Guard();

// since reach here , there exist no other thread gonna get in

// so it is safe to use the si_instance itself

if( !si_instance )

{

if( si_destroyed )

{

LifeTimePolicy::OnDeadReference();

si_destroyed = false;

}

si_instance = CreatePolicy::Create();

// we should make sure the value (address) has indeed been written into that pointer (si_instance)

// so flush this value

MemoryBarrier();

LifeTimePolicy::ScheduleCall(&DestroySingleton);

}

}

return *si_instance;

}

Since I havn't read the code about the time schedual of every singleton instance , maybe using the atexit function ,

so the si_destroy variable I do not revise it. Anyway , if there do need the revision , it is pretty easy .

Another thing is : using the key word "volatile" before the variable si_instance and si_destroy , since those two variables can

be accessed by two different threads or more , therefore it is safe to do this (however, this does not mean it is absolutely safe ) and this will

force compiler to generate better thread safe machine code.

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