[windows] Upgrade lock is not acquired when previous upgrade lock releases if another read lock is present

[Rodionov Andrew <camelot58@…>]

The problem is described here ​https://svn.boost.org/trac/boost/ticket/5516. Example and result are the same. The only difference is that I use <boost/thread/win32/shared_mutex.hpp> instead of <boost/thread/pthread/shared_mutex.hpp>.

I tried to compile with BOOST_THREAD_PROVIDES_GENERIC_SHARED_MUTEX_ON_WIN flag and everything goes right. So the problem is in function unlock_upgrade() in <boost/thread/win32/shared_mutex.hpp>.

My platform: Windows 7 x64, MS VS 2012.

[viboes]

Hi,

Please could you try adding

        void release_shared_waiters(state_data old_state)
        {

            if(old_state.shared_waiting || old_state.exclusive_waiting)
            {
                BOOST_VERIFY(detail::win32::ReleaseSemaphore(semaphores[unlock_sem],old_state.shared_waiting + (old_state.exclusive_waiting?1:0),0)!=0);
            }
        }

and changing the implementation to

        void unlock_upgrade()
        {
            state_data old_state=state;
            for(;;)
            {
                state_data new_state=old_state;
                new_state.upgrade=false;
                bool const last_reader=!--new_state.shared_count;

                if(last_reader)
                {
                    if(new_state.exclusive_waiting)
                    {
                        --new_state.exclusive_waiting;
                        new_state.exclusive_waiting_blocked=false;
                    }
                    new_state.shared_waiting=0;
                }

                state_data const current_state=interlocked_compare_exchange(&state,new_state,old_state);
                if(current_state==old_state)
                {
                    if(last_reader)
                    {
                        release_waiters(old_state);
                    }
                    else {
                        release_shared_waiters(old_state);
                    }
                    // #7720
                    //else {
                    //    release_waiters(old_state);
                    //}
                    break;
                }
                old_state=current_state;
            }
        }

Please could you also try the test in #7720?

[viboes]

Any new on this issue?

[viboes]

​https://github.com/boostorg/thread/commit/6f91af2154da808eb29de2e9e12c3e1774cdea31