Re: [Boost-users] [general question] on threading and double checked locking pattern

From: Ovanes Markarian (om_boost_at_[hidden])
Date: 2007-01-23 04:39:02

Actually I read all your and Tony's points and may be I was misunderstood.
My first question is:

If mutex does not guarantee thread safety what then?

static boost:mutex s_m

class Singleton
{
        //everything as before in the class
};

//creative get

Singleton* Singleton::creating_singleton_getter()
{
        boost::mutex::scoped_lock   lock(s_m); //allways called when entered

                                                           //all other calls
to this function
                                                           // are blocking,
so it is not possible
                                                           // to enter this
function twice if lock is active
        if(Singleton::pInstance == NULL)
                Singleton::pInstance = new Singleton; //does not matter how
these steps are executed
                                                                  // and
reordered by compiler, since the function
                                                                  // can
only be entered when s_m is unlocked
        Singleton::getter = &non_creating_getter;   //this is still guarded
by locked mutex!!!
} //mutex unlock

//everything else as it was before

So the point is: As long as Singleton::instance is called from multiple
threads and these are not created from global vars before main is called,
this code should be thread safe.

The scenario is like this:

Threads:

  A              B              C                D
instance       instance       instance                     //only A, B or C
will get access to instance, other will wait
                                               instance    // if creating
get was successful, D calls the lightweigt version
                                                           //  of getter

Static class variables are guaranteed to be initialized before main is
entered:
C++ standard 9.4.2 states:
...
Static data members are initialized and destroyed exactly like non-local
objects (3.6.2, 3.6.3).
...

3.6.2 states:
...
Objects with static storage duration (3.7.1) shall be zero-initialized (8.5)
before any other initialization
takes place. Zero-initialization and initialization with a constant
expression are collectively called static
initialization; all other initialization is dynamic initialization.
...

So I assume, that initialization of getter with address of a (static) class
function is a constant expression and therefore is not a dynamic
initialization. (Please see 5.19 of a standard especially:
...
Other expressions are considered constant-expressions only for the purpose
of non-local static object
initialization (3.6.2). Such constant expressions shall evaluate to one of
the following:
...
- an address constant expression,
...
An address constant expression is a pointer to an lvalue designating an
object of static storage duration, a
string literal (2.13.4), or a function.)

Therefore there should be a guaranty that the Singleton static members are
initialized before main is entered. The locked mutex guarantees that only
one thread at one processor will enter the function at the same time. Isn't
it so?

Thanks for your ideas and answers.

Best Regards,
Ovanes

-----Original Message-----
From: Sohail Somani [mailto:s.somani_at_[hidden]] 
Sent: Tuesday, January 23, 2007 4:33 AM
To: Sohail Somani; boost-users_at_[hidden]
Subject: Re: [Boost-users] [general question] on
threadinganddoublecheckedlocking pattern

Actually, its not so much wrong as it is "working currently but will
probably break at some stage". Apparently, since Solaris always runs in
"total storage ordering" mode, their implementation of pthread_once is
correct for those hardware platforms.

Just thought I'd clarify that.

> -----Original Message-----
> From: Sohail Somani
> Sent: Monday, January 22, 2007 7:25 PM
> To: 'boost-users_at_[hidden]'
> Subject: RE: [Boost-users] [general question] on threading 
> anddoublecheckedlocking pattern
> 
> Oopsie:
> 
> http://bugs.opensolaris.org/view_bug.do?bug_id=6513516
> 
> So even the smart guys get it wrong.
> 
> > -----Original Message-----
> > From: boost-users-bounces_at_[hidden]
> > [mailto:boost-users-bounces_at_[hidden]] On Behalf Of Sohail 
> > Somani
> > Sent: Monday, January 22, 2007 5:12 PM
> > To: boost-users_at_[hidden]
> > Subject: Re: [Boost-users] [general question] on threading 
> > anddoublecheckedlocking pattern
> 
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