Re: [boost] Review request: Boost.FSM

From: Steven Watanabe (steven_at_[hidden])
Date: 2007-01-06 18:02:10

AMDG

Andrey Semashev wrote:
> This is just wrong. It is perfectly legal
> > for the compiler to reorder it to be
> >         if (!m_fInitialized)
> >         {
> >           m_fInitialized = true;
> >           // Fill m_StatesInfo array for all states
> >           const root_type* pRoot = pStates;
> >           pStates->__init_states_info(reinterpret_cast< const char* 
>  >>(pRoot), m_StatesInfo);
>   
>> >         }
>> > then
>> > Thread A
>> >    if (!m_fInitialized)
>> >    {
>> >      m_fInitialized = true;
>> > Thread B
>> >    if (!m_fInitialized)
>> >    {
>> >    }
>> >    //use m_StatesInfo.
>> >    //die.
>>     
> I'd rather say, your scenario is possible here but not necessarily it 
> will happen. A compiler _may_ be smart enough to reorder expressions 
> if it may prove that it won't break the code behavior (which is true 
> here) and it will gain something (performance, for example) from it. 
> And even if it does so consider that Thread A in your example may be 
> interleaved by Thread B right between it checks "m_fInitialized" and 
> stores "true" in it. So it is actually possible that more than one 
> thread will execute the "true" branch of the "if" statement 
> concurrently. But we're on the safe side here because only simple 
> operations on POD types are involved in it.
I see that it is ok for  m_StatesInfo to be initialized more than once.  
My point was
that it can be used before it is initialized unless you make everything 
involved volatile.

In Christ,
Steven Watanabe