[Threads-devel] More problems with condition

From: Anteru (newsgroups_at_[hidden])
Date: 2008-04-19 06:37:01

I hope this does not appear twice, seems Gmane ate my first message to 
this list.

Hi,

I've posted this to the users list as well, but it might have got lost 
as I replied on the original thread.

Well, I still have problems with the conditions, even with the latest 
SVN. The problem seems to be that inside a block like:

Lock lock (mutex);

while (something.empty ())
     cond.wait (mutex);

I'm still getting into a state where something.empty () == false and yet 
I'm waiting on the condition. The only other access happens via:

Lock lock (mutex);

something.push (...);
cond.notify_one ();

So I don't get it how something can go wrong, unless the transition from 
mutex.unlock () to cond.wait () is not fully atomic and the following 
happens:

Thread 1:                Thread 2:
while (something.empty ())
     cond.wait (mutex);
// starts, releaes lock but is not waiting yet
                     Lock lock ();
                     something.push (..);
                     cond.notify_one ();
// Nobody waiting
                     lock.release ();
     cond.wait (mutex); // starts waiting

I had a similar problem with my own condition implementation on Windows, 
with exactly the same syndromes (I've been following the "Strategies for 
Implementing POSIX Condition Variables on Win32" 
http://www.cs.wustl.edu/~schmidt/win32-cv-1.html - only got it working 
properly with SignalObjectAndWait).

Currently, it always leaves exactly *one* thread waiting, while 
previously, it left several threads waiting, so it has improved but 
there is still a problem.

Any ideas? Anything I can do to track this down further?

Cheers,
   Anteru