[vtk-developers] Weak pointers and thread safety

[David Cole]

I'm skeptical as well. VTK in general is not thread safe *at all* -- to use
VTK from multiple threads requires extreme care on the part of the VTK
client app...

I am philoposphically opposed to adding "thread safety" to bits of VTK
because:
it implies that VTK is generally thread safe, and it is not.


David C.


On Tue, Oct 12, 2010 at 9:54 AM, Moreland, Kenneth <kmorel at sandia.gov>wrote:

>  Oh, if I were only to believe it would be that simple.  Don’t forget that
> many objects rely on reference counting to determine whether they should
> actually be deleted.  I shudder to think what might happen if reference
> counts changed while a garbage collection was running.
>
> -Ken
>
>
>
> On 10/12/10 7:36 AM, "David Gobbi" <david.gobbi at gmail.com> wrote:
>
> Hi Ken,
>
> I see your point.  Indeed, applying a lock just to the WeakPointer
> doesn't really do anything.  There might be a way to just put a lock
> just around the object deletion decision in vtkObjectBase, without
> having it around every decref.  Or there might not.
>
>    David
>
> On Tue, Oct 12, 2010 at 7:12 AM, Moreland, Kenneth <kmorel at sandia.gov>
> wrote:
> > I’m skeptical that this code actually provides thread safety.  Your
> critical
> > section is only applied within a weak pointer.  It is not applied within
> > vtkObjectBase itself.  While the critical section is running some other
> > thread could call unreference and delete the object after the weak
> pointer
> > checks the reference but before the smart pointer increments the
> reference.
> >
> > I think the only way you are going to get true thread safety is to put a
> > critical section somewhere in the register/unregister calls.  However,
> that
> > might slow down everything appreciably.
> >
> > -Ken
> >
> >
> > On 10/11/10 10:43 PM, "Utkarsh Ayachit" <utkarsh.ayachit at kitware.com>
> wrote:
> >
> > That sounds like a good idea. You have my vote.
> >
> > Utkarsh
> >
> > On Mon, Oct 11, 2010 at 10:35 PM, David Gobbi <david.gobbi at gmail.com>
> wrote:
> >> Hi All,
> >>
> >> The VTK weak pointer implementation is not thread safe because it can
> >> be used just like a regular pointer, even though the object itself
> >> might be deleted by a separate thread at any time.
> >>
> >> By comparison, "safe" weak pointer like in boost and Qt cannot be used
> >> in place of ordinary pointers.  Instead, in order to use them their
> >> "GetPointer()"-like methods increment the reference count and then
> >> return a smart pointer, which is guaranteed to be valid until it goes
> >> out of scope:
> >>
> >> vtkSmartPointer<SomeClass> smartPtr = weakPtr.GetPointer();
> >> if (smartPtr.GetPointer() != 0)
> >>  {
> >>  // do something with smartPtr
> >>  }
> >> // reference count decremented when smartPtr goes out of scope
> >>
> >> Because VTK's vtkWeakPointer::GetPointer() does not return a smart
> >> pointer or do any thread locking, thread-safe use of vtkWeakPointer
> >> requires the following:
> >>
> >> vtkSimpleCriticalSection critSec;
> >> critSec.Lock();
> >> vtkSmartPointer<SomeClass> smartPtr = weakPtr.GetPointer();
> >> critSec.Unlock();
> >> if (smartPtr.GetPointer() != 0)
> >>  {
> >>  // do something with smartPtr
> >>  }
> >>
> >> If weakPtr.GetPointer() was changed to return a smart pointer, then it
> >> could do the locking internally.  This change would be mostly, but not
> >> completely, backwards compatible.  What do people think?
> >>
> >>  David
> >> _______________________________________________
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> >>
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> >
> >
> >
> >
> >    ****      Kenneth Moreland
> >     ***      Sandia National Laboratories
> > ***********
> > *** *** ***  email: kmorel at sandia.gov
> > **  ***  **  phone: (505) 844-8919
> >     ***      web:   http://www.cs.unm.edu/~kmorel
> >
> >
>
>
>
>
>    ****      Kenneth Moreland
>     ***      Sandia National Laboratories
> ***********
> *** *** ***  email: kmorel at sandia.gov
> **  ***  **  phone: (505) 844-8919
>     ***      web:   http://www.cs.unm.edu/~kmorel
>
>
> _______________________________________________
> Powered by www.kitware.com
>
> Visit other Kitware open-source projects at
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>
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