Re: Patch ping

["Peter Dimov" <pdimov at mmltd dot net>]

> > Or another option would be to eliminate the possible memory leak in
> > some other way.  The possibility of the memory leak arises because
> > two threads may call operator() on a __use_cache simultaneously. When 
> > this happens, it is possible for both threads to test that
> > __caches[__i] is not set, and for both to set it to a new cache
> > object.  In a preemptive threading model, the threads could switch
> > right at the assignment in _M_install_cache, so it seems
> > theoretically possible for the value stored in _M_caches[__index] to
> > be confused by a simultaneous assignment from two threads.  I don't
> > see this can be safely avoided without using a mutex.

Can't this be solved using atomic_compare_exchange (I'm not sure of its 
exact spelling, or whether it's currently part of libstdc++)?

for(;;)
{

if( atomic_load_acquire( __caches[i] ) == 0 )
{
   p = new cache;

   if( !atomic_compare_exchange_release( &__caches[i], 0, p ) )
   {
       // another thread has written to __caches[i] in the meantime
       delete p;
   }
   else
   {
       break;
   }
}

}

or something along those lines.