[Boost-users] Deadlock issue with boost::condition

From: Matt Schuckmann (matt_at_[hidden])
Date: 2006-02-01 15:48:53

I'm having a deadlock problem with boost::condition variables and I'm 
not sure if I'm miss using condition variables or if condition variables 
aren't the right tool for the job.

The basic problem appears to be that the thread calling notify_all (lets 
call it thread1) calls notify_all before the thread calling wait (lets 
call it thread2) calls wait. The result is thread2 waits forever for the 
notify that thread1 already sent.

To further complicate the matter thread1 can't call notify_all again 
until it has been signaled to run (via a different condition variable) 
which is signaled by thread2.

Does this make sense?
Here's some pseudo code for what I'm doing.

Thread1() //Worker thread
{
   while(1)
   {
     DoSomeStuff();

     condition1.notify_all()

     scoped_lock l(mutex2);
     condition2.wait(l);
   }
}

Thread2() //Master thread.
{
   while(1)
   {
     scoped_lock l(mutex1);
     condition1.wait(l)

     DoSomeOtherStuff();

     condition2.notify_all();
   }
}

StartMulitThreadedTask()
{
   startThread( thread2 );
   Sleep( 2000 )
   startThread( thread1 );
}

Here is a little time line of what I think is happening
thread2 locks mutex1
thread2 calls wait on condition1
thread1 calls DoSomeStuff
thread1 calls notify_all on condition1
thread1 locks mutex2
thread1 calls wait on condition2
thread2 calls DoSomeOtherStuff
thread2 calls notify_all on condition2
thread1 calls DoSomeStuff
thread1 calls notify_all on condition1
thread1 locks mutex2
thread1 calls wait on condition1
thread2 locks mutex1
thread2 calls wait on condition1
DEADLOCK OCCURS

I've done this sort of thing with plain old binary mutexes or single 
count semaphores and I can go back to that but it really seemed like 
boost::conditions simplified things a bit and should be right choice 
except for this one problem. Is there something I can do to make 
boost::conditions work in this case? Should I be doing additional or 
different locking of mutex1 and mutex2? do I need yet another mutex?

Thanks
Matt S.