[Boost-bugs] [Boost C++ Libraries] #5847: sp_counted_base_pt.hpp: appears to suffer a race conditions

Subject: [Boost-bugs] [Boost C++ Libraries] #5847: sp_counted_base_pt.hpp: appears to suffer a race conditions
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2011-08-31 02:01:02

#5847: sp_counted_base_pt.hpp: appears to suffer a race conditions
-------------------------------------------------+--------------------------
 Reporter:  Jeffrey Walton <noloader@…>          |       Owner:  pdimov   
     Type:  Bugs                                 |      Status:  new      
Milestone:  To Be Determined                     |   Component:  smart_ptr
  Version:  Boost 1.47.0                         |    Severity:  Problem  
 Keywords:                                       |  
-------------------------------------------------+--------------------------
 It appears release() suffers a race condition:


 {{{
 pthread_mutex_lock( &m_ );
 long new_use_count = --use_count_;
 pthread_mutex_unlock( &m_ );

 if( new_use_count == 0 )
 {
   dispose();
   weak_release();
 }
 }}}

 In a multithreaded and/or multicore environment, its not hard to imagine
 Thread A beginning a dispose()/weak_release(), Thread A preemption, and
 then Thread B acquiring a reference.

 multicore is more interesting, as you might need a memory barrier between
 the locked read of {{{new_use_count}}} and the subsequent test of
 {{{new_use_count}}} in the 'if' statement.

 For add_ref_copy(), imagine a scenario where Thread A reads use_count,
 Thread A preemption, Thread B read/increment/write use_count, then Thread
 A increcemt/write use_count:

 {{{
 void add_ref_copy()
 {
   ++use_count_;
 }
 }}}

 weak_add_ref() and weak_release() suffer similar problems:

 {{{
 void weak_add_ref() // nothrow
 {
   ++weak_count_;
 }
 }}}
 and
 {{{
 void weak_release() // nothrow
 {
   if( --weak_count_ == 0 )
   {
     destroy();
   }
 }
 }}}

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/5847>
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