id,summary,reporter,owner,description,type,status,milestone,component,version,severity,resolution,keywords,cc
5673,Quick allocator error in multi thread,Yury Podpruzhnikov <QWERTYura@…>,Peter Dimov,"Compiler: '''MSVS 2008''' 
use macros '''BOOST_SP_USE_QUICK_ALLOCATOR'''

Exception (Access vialation by reading address) occured by constructor of '''shared_ptr'''.

Cause of exception is multi thread creation of static mutex in quick allocator.

shared_ptr call quick allocator.
quick allocator use next code:
{{{
lightweight_mutex::scoped_lock lock( mutex() );
}}}

function '''mutex()''' return internal static variable:
{{{
boost_1_44_0\boost\smart_ptr\detail\quick_allocator.hpp
template<unsigned size, unsigned align_> struct allocator_impl
{
... 

     static lightweight_mutex & mutex()
     {
         static freeblock< sizeof( lightweight_mutex ), boost::alignment_of< lightweight_mutex >::value > fbm;
         static lightweight_mutex * pm = new( &fbm ) lightweight_mutex;
         return *pm;
     }
 
...
}
}}}
2 thread call mutex() (first time for this instantiation of template class allocator_impl). One start init '''pm''' and go to sleep. Other check that '''pm''' is initialized and return its value (in my case NULL) and raise exception.
[[BR]]
Initialization of static variable in C++ is not safe for multi thread. This code is incorrect. In dependence from realization of static variable you will be have access vialation or memory leak - flag of static variable initialization (construction) set before or after initialization.
[[BR]]
In MSVS 2008 flag set before and you take access vialation.
[[BR]][[BR]]
'''Decision''' [[BR]]
Not resolve, but minimize problem. [[BR]]
Idea:[[BR]]
As I understand, This problem can appear in first initialization of __every__ instantiation of shared_ptr - first use __every__ instantiation of allocator_impl.
[[BR]]
If you synchronize of initialization '''pm''' then problem can appear only first initialization of __any__ instantiation of shared_ptr. And I can create first fictive shared_ptr in non-multi thread and then normal use shared_ptr in multi thread.
[[BR]]
Realization:[[BR]]
{{{
boost_1_44_0\boost\smart_ptr\detail\quick_allocator.hpp

lightweight_mutex & global_mutex()
{
   static freeblock< sizeof( lightweight_mutex ), boost::alignment_of< lightweight_mutex >::value > fbm;
    static lightweight_mutex * pm = new( &fbm ) lightweight_mutex;
         return *pm;
}



template<unsigned size, unsigned align_> struct allocator_impl
{
     static lightweight_mutex & mutex()
     {
         static freeblock< sizeof( lightweight_mutex ), boost::alignment_of< lightweight_mutex >::value > fbm;
         static lightweight_mutex * pm = NULL;
         if(pm == NULL)
         {
            lightweight_mutex::scoped_lock lock( global_mutex() );
            if(pm == NULL)
            {
                pm = new( &fbm ) lightweight_mutex;
            }
         }
         return *pm;
     }
}
}}}
",Bugs,new,To Be Determined,smart_ptr,Boost 1.44.0,Problem,,,