Re: [patch] Make std::tr1::shared_ptr thread-safe.

[Jonathan Wakely <cow at compsoc dot man dot ac dot uk>]

On Wed, Mar 30, 2005 at 09:52:10AM -0800, Ulrich Drepper wrote:

> Jonathan Wakely wrote:
> 
> >Index: include/tr1/boost_shared_ptr.h
> >===================================================================
> >RCS file: /cvs/gcc/gcc/libstdc++-v3/include/tr1/boost_shared_ptr.h,v
> >retrieving revision 1.1
> >diff -u -p -r1.1 boost_shared_ptr.h
> >--- include/tr1/boost_shared_ptr.h	24 Mar 2005 18:32:17 -0000	1.1
> >+++ include/tr1/boost_shared_ptr.h	29 Mar 2005 12:28:45 -0000
> >@@ -131,17 +131,18 @@ public:
> >   void
> >   add_ref_lock()
> >   {
> >-    if (_M_use_count <= 0) // TODO not yet MT safe XXX
> >+    __gnu_cxx::lock lock(_M_mutex);
> >+    if (_M_use_count == 0)
> >     {
> >       __throw_bad_weak_ptr();
> >     }
> >-    __gnu_cxx::__atomic_add(&_M_use_count, 1);
> >+    ++_M_use_count;
> >   }
> 
> This patch looks like a terrible idea.  If it's intended as an interim
> step, perhaps, but this should be handled using atomic operations alone.

Ideally, yes.

> For instance, above function could look like this:
> 
> void
> add_ref_lock()
> {
>   _Atomic_word _current;
>   do
>     {
>       _current = _M_use_count;
>       if (_current == 0)
>         __throw_bad_weak_ptr();
>     }
>   while (compare_and_exchange(&_M_use_countr, _current, _current + 1)
> != 0);
> }
> 
> Fill in what the existing compare_and_exchange equivalent is.  And

The existing compare_and_swap atomic was removed from v3 since it wasn't
used anywhere.  This was discussed here:
http://gcc.gnu.org/ml/libstdc++/2005-03/msg00310.html
I should have made it clearer when I started a new thread that I was
providing an initial, far-from-optimal version.

jon