Re: [Boost-users] shared_ptr race condition

From: Eduardo Panisset (eduardo.panisset_at_[hidden])
Date: 2008-08-17 11:36:59

On Sun, Aug 17, 2008 at 1:25 AM, David Abrahams <dave_at_[hidden]> wrote:

>
> on Sat Aug 16 2008, "Eduardo Panisset" <eduardo.panisset-AT-gmail.com<http://eduardo.panisset-at-gmail.com/>>
> wrote:
>
> > On Sat, Aug 16, 2008 at 9:32 PM, David Abrahams <dave_at_[hidden]>
> wrote:
> >
> >     on Sat Aug 16 2008, "Eduardo Panisset" <
> eduardo.panisset-AT-gmail.com <http://eduardo.panisset-at-gmail.com/>>
> >     wrote:
> >
> >     > There are  two threads, each one with its own shared_ptr. Thread 2
> >     created
> >     > its shared_ptr2 by copying the shared_prt1 of thread 1. Then the
> >     reference
> >     > count is equal to 2.
> >     > Some time later, shared_ptr1 goes out of scope and then shared_prt2
> >     also
> >     > goes out of scope.
> >     > Consider the following event sequence:
> >     >
> >     > 1. Thread 1 executes release member function, and it is preempted
> >     before
> >     > executing  comparation if (new_use_count == 0)
> >     > 2. Thread 2 executes release member function until the end,
> disposing
> >     the
> >     > pointee.
> >     > 3. Thread 1 is rescheduled, executes comparation if (new_use_count
> ==
> >     0) and
> >     > also diposes the pointee!
> >     >
> >     > Is it possible, isn't it?
> >
> >     No.  Thread 1 will see new_use_count == 1, thread 2 will see
> >     new_use_count == 0 and dispose the pointee, and thread 1 will
> continue
> >     off the end of the function without disposing it.
> >
> > Ok, David !!! new_use_count is a local variable that receives the ref
> > count inside the critical region synchronizing the shared variable
> > use_count_, so there is no race condition.
> >
> > There is still a doubt left. How can I copy a shared_ptr from a different
> > thread that has created it and guarantee that the pointee did not go away
> > (race condition between function members release and add_ref_lock ) ?
>
> You make the copy of the shared_ptr in the first thread:
>
>    void thread2(shared_ptr<X> q)
>    {
>        ...
>    }
>
>    void main_thread(shared_ptr<X> p)
>    {
>        boost::thread t(
>            boost::bind(thread2, p) // <= _copies_ p into the thread
> function object
>        );
>     }
>

 Ok David,

I understand now.

Thank you, very much !