[concurrency-interest] transferring queued threads

David Holmes davidcholmes at aapt.net.au
Thu Apr 16 18:57:01 EDT 2009

Hi Tom,

This kind of scenario does arise whenever you have entities that most of the
time have to handle their exclusion constraints (locking) independently, but
then at times have to co-operate to provide atomic operations across sets of
objects. Acquiring multiple locks is the main way of extending the
independent use of locks - with ordering to avoid deadlocks.

What you describe is a little different in that an operation that initially
needs only a single lock, may turn into an operation that requires multiple
locks. In that case all I can suggest is that after acquiring l(c) the code
checks if that was sufficient, and if not it grabs l(b) - perhaps releasing
l(c) first to maintain ordering.

It really all depends on interaction between the different actions as to how
best to partition the locking and what strategies to try and use. I don't
think you are missing anything obvious.

David Holmes
  -----Original Message-----
  From: concurrency-interest-bounces at cs.oswego.edu
[mailto:concurrency-interest-bounces at cs.oswego.edu]On Behalf Of tom
  Sent: Thursday, 16 April 2009 10:43 PM
  To: Endre Stølsvik
  Cc: concurrency-interest at cs.oswego.edu
  Subject: Re: [concurrency-interest] transferring queued threads

  Thanks Endre, Christian for your replies and I'm sorry that I've been late
in replying.

  Perhaps I should have been more explicit in my original scenario. It may
be that I'm too close to the problem to see an obvious solution, but I don't
think that the proposed solution will work.

  Here's a more detailed description of the problem:
  This is a communications system in which 2 parties can be in a
communication session with each other - think of it as an IM session or a
phone call.

  Alice and Bob are in a call.
  Alice and Carol are also in a call.
  Alice's calls are being coordinated by a service. One function offered by
this service is "transfer", whereby the service hooks Bob and Carol together
and drops Alice out of the loop.

  These calls have moderately complex state machines that are guarded by
locks. Triggers can come into these state machines from either end of a call
and must acquire the lock to protect memory state and to prevent races on
the state machines.
  Each user has a state machine for their side of the call and each call has
its own higher-level state machine. To simplify modelling, the two users and
call operate under a single lock, so that a trigger into one user's state
machine flows across the call and state machines atomically, without having
to worry about races from other triggers.

  My problem is this:
  Alice and Bob have a call that is protected by a single lock L(B)
  Alice and Carol have a call that is protected by a single lock L(C)
  We need to end up with Bob and Carol in a call without running into a
deadlock between the two locks.

  If I switch both calls to having to acquire both locks, in order, perhaps
there is a race.
  Let's say that the transfer trigger comes in and acquires L(B). The system
realises that this is a transfer and acquires L(C) so that it has control of
both calls locks and thus can be sure that nothing else will race over the
state machines. After this, all triggers to the call Alice-Bob will have to
acquire locks L(B) and L(C) in that order. Similarly, all triggers to
Alice-Carol will have to acquire both locks, in the same order. The problem
is that there is a race in which a thread can start to wait for L(C) while
this changeover is happening and be left waiting for L(C) without knowing
that it should have acquired L(B) first.
  Sequence for two threads, labelled tB and tC:
  tB   transfer trigger comes in
  tB   acquire L(B)
  tB   acquire L(C)
  tC   trigger arrives, starts waiting for L(C)
  tB   sets Alice-Carol call to need to acquire both L(B) and L(C) from now
  tB   does stuff to set off the transfer and then releases L(C) and L(B)
  tC   acquires L(C), but has not acquired L(B)

  Again, I have some thoughts on how to solve this, but I'm worried that I'm
missing the point or overcomplicating matters in the statement of my
problem. Am I missing something obvious?



  2009/4/14 Endre Stølsvik <Online at stolsvik.com>

    On Mon, Apr 6, 2009 at 21:00, Christian Vest Hansen
    <karmazilla at gmail.com> wrote:
    > Have a lock for each entity; L(A), L(B), L(C) etc. such that each
    > entity at any given point in time has exactly one lock. These locks
    > have a universal and immutable ordering to them (like
    > System.identityHashCode, for instance).

    System.identityHashCode can give you the same result for two different
    objects, and you'd not be guaranteed a single order on the two
    different threads. Use a long sequencer (in addition!).


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