[concurrency-interest] transferring queued threads

tom strickland mr.tom.strickland at gmail.com
Mon Apr 6 11:09:42 EDT 2009

I have an interesting locking scenario and I'd appreciate some help getting
my head around it. I'm sorry if I've made it a little abstract, but
hopefully this will come across clearly.

I have some entities (A, B, C, D, E...) that can be associated in pairs.
When 2 entities are associated, only one thread may process them at a time:
if A and B are associated and a thread has come in for A, then any threads
coming in for B will be blocked until A's thread has completed. This seems
to imply a simple lock, shared by A and B, that can be acquired and

The problem comes when I want to dissociate A and B in order to associate B
with C. Once B is associated with C, triggers coming in for B should queue
on a lock shared between B and C, and triggers coming in for A should use a
separate lock (perhaps the original lock).

How might this be achieved?
My initial thoughts are that I should use wait... notify and a stored value
that tells a thread which lock it should be waiting for. At any time, a
running thread can look at this value to see which lock it should acquire.
If a lock has threads queued up for A and B and we need to split B's threads
out and queue them on a different lock:

1. create a new lock for B
2. acquire the new lock (in order to prevent races)
3. change the lock-id field for B to point to the new lock
4. wake the waiting threads
5. A woken thread will check whether it has been woken because:
   a. it has acquired the lock - nothing to do here, the lock is acquired,
proceed as normal.
   b. the lock that it should be waiting for has changed - wait on the new
6. do stuff with B and C
7. release the lock acquired in step 2

Does this make sense? I have more questions, mainly about how to implement
and encapsulate this, but I'd like to start by making sure that I've
understood and explained the problem clearly.


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