Re: [Boost-bugs] [Boost C++ Libraries] #5516: Upgrade lock is not acquired when previous upgrade lock releases if another read lock is present

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #5516: Upgrade lock is not acquired when previous upgrade lock releases if another read lock is present
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2011-12-29 11:37:28

#5516: Upgrade lock is not acquired when previous upgrade lock releases if another
read lock is present
------------------------------------+---------------------------------------
  Reporter:  fred@…                 |       Owner:  viboes  
      Type:  Bugs                   |      Status:  assigned
 Milestone:  To Be Determined       |   Component:  thread  
   Version:  Boost 1.42.0           |    Severity:  Problem 
Resolution:                         |    Keywords:          
------------------------------------+---------------------------------------
Changes (by viboes):

  * type:  Support Requests => Bugs

Comment:

 Replying to [comment:7 fred@…]:
 > Viboes, agreed, last steps don't matter. My concern here is on the 7th
 step:
 >
 > m1 - Trying to take an shared lock[[BR]]
 > m1 - Took a shared lock[[BR]]
 > m2 - Trying to take an upgradable lock[[BR]]
 > m2 - Took an upgradable lock[[BR]]
 > m3 - Trying to take an upgradable lock[[BR]]
 > m2 - Released an upgradable lock[[BR]]
 > ** here I would expect m3 to be able to take the upgradable lock, even
 though m1 still has a read lock **[[BR]]
 >
 > m1 - Released the lock
 >
 > Because the "release_waiters()" is only called when there are no more
 readers, we ignore the case when a thread wanting an upgradable lock
 should be waken up when there are still read locks going on.
 >
 > This makes the locking non-eager.


 You are right. The fact that in


 {{{
         void unlock_upgrade()
         {
             boost::mutex::scoped_lock lk(state_change);
             state.upgrade=false;
             bool const last_reader=!--state.shared_count;

             if(last_reader)
             {
                 state.exclusive_waiting_blocked=false;
                 release_waiters();
             }
         }
 }}}

 state.upgrade has been set at least


 {{{
 shared_cond.notify_all();
 }}}


 must be called.

 Please could you try the following


 {{{
         void unlock_upgrade()
         {
             boost::mutex::scoped_lock lk(state_change);
             state.upgrade=false;
             bool const last_reader=!--state.shared_count;

             if(last_reader)
             {
                 state.exclusive_waiting_blocked=false;
                 release_waiters();
             } else {
               shared_cond.notify_all();
             }
         }
 }}}

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/5516#comment:8>
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