[concurrency-interest] transferring queued threads

David Holmes davidcholmes at aapt.net.au
Tue Apr 21 09:47:55 EDT 2009


Hi Tom,

I don't think your protocol is quite right, or perhaps is overly
complicated:

> To change a call lock:
> 1. Acquire the new call lock.

Ok.

> 2. Get the existing call lock, using the data lock.

Don't think you need to lock to do this. But the field should probably be
volatile because it will be protected by different locks at different times.

> 3. Acquire the existing call lock.

Ok.

> 4. Change the entity's reference to the call-lock to point to the new
lock, using the data lock.

Again you don't need a data-lock here, but you do need to check if the
call-lock you just acquired is still the right call-lock. If not then
release it and acquire the current call-lock - repeat as necessary

> 5. Release the old call lock.

Ok.

> To acquire a call-lock:
> 1. Get the call lock from the entity's data store, using the data lock.
> 2. Acquire the call lock.
> 3. Once acquired, get the call lock again from the entity's data store,
using the data lock.
> 4. Compare the two values for the lock.
>   a. If the lock has changed, release the old lock and acquire the new
one.
>   b. If the lock has not changed, proceed.

Again no need for the data-lock here as far as I can see. AScquire the
call-lock then check if it is still the call-lock. If not, drop it, repeat
the process.

The reference to the call-lock is protected by the call-lock itself. But
this means that you will need an initial call-lock to bootstrap the process.

David

-----Original Message-----
From: concurrency-interest-bounces at cs.oswego.edu
[mailto:concurrency-interest-bounces at cs.oswego.edu]On Behalf Of tom
strickland
Sent: Tuesday, 21 April 2009 11:28 PM
To: concurrency-interest at cs.oswego.edu
Subject: Re: [concurrency-interest] transferring queued threads


Thanks for your replies Christian & David.
I like the sound of the suggested approach, so if I can summarise to check
that I've got everything:

The data remains under a data lock per entity that does not change. The
state machines involved in a call are protected by a call lock, which is
accessed through the data lock and may change. To change a call lock:
1. Acquire the new call lock.
2. Get the existing call lock, using the data lock.
3. Acquire the existing call lock.
4. Change the entity's reference to the call-lock to point to the new lock,
using the data lock.
5. Release the old call lock.

To acquire a call-lock:
1. Get the call lock from the entity's data store, using the data lock.
2. Acquire the call lock.
3. Once acquired, get the call lock again from the entity's data store,
using the data lock.
4. Compare the two values for the lock.
   a. If the lock has changed, release the old lock and acquire the new one.
   b. If the lock has not changed, proceed.

The locks will have to be explicit locks, to enable the hand-over-hand
approach of changeover.

Only Alice can initiate such a change of call-lock and it can only come from
one of Alice's calls at a time, so I am not too worried about two threads
deadlocking by attempting this simultaneously. To address the points at the
end of David's comments, only Alice can cause Bob to be shunted off his call
and she cannot deadlock herself. The communicating-this-to-Bob bit is part
of the communications protocol itself and is straightforward, although it
would be polite to tell Bob that he was going to be shunted :-)

Thanks,

Tom


2009/4/21 David Holmes <davidcholmes at aapt.net.au>

Tom,

You could use AQS to devise something that waits for two conditions but it
is non-trivial to say the least. You could also just check to see after
acquiring a call-lock whether it is still the call-lock as described ie not
use AQS but just two normal Locks. Requiring you to hold the current
call-lock to change the call-lock should address race conditions, but you
may need to ensure you release the local lock before-hand to avoid
deadlocks.

It really depends on the details of the use cases here - eg who has the
right to shunt Bob off his call with Alice, and how is this communicated to
Bob?

David Holmes


2009/4/21 Christian Vest Hansen <karmazilla at gmail.com>

>

> What about this: once a thread changes the call lock, it releases the
> old one. Then, any thread that acquires the call lock must then
> immediately check that it is still the current active call lock and if
> not, it must be released again and the thread must start waiting on
> the new lock.

-----Original Message-----
From: tom strickland [mailto:mr.tom.strickland at gmail.com]
Sent: Monday, 20 April 2009 11:29 PM
To: dholmes at ieee.org
Cc: concurrency-interest at cs.oswego.edu
Subject: Re: [concurrency-interest] transferring queued threads


After more thought, I thought I'd ask if I could just shift threads from
lock-to-lock, as follows:

Each entity has its own data lock that does not change. It also has a second
"call" lock that can change. Thus, a call from Alice that turns into a call
between Alice and Bob will use a single call lock. All traffic between Alice
and Bob uses this lock to all serialise operations on the state machines for
Alice, Bob and their call. If Bob is to be shunted into a different call
with Carol, then a new lock will be created for that call. The problem is
that there may be races here. So, could the following approach work and what
would be the best starting point for implementing it?

1. Any thread that has acquired a call lock will be allowed to proceed to
completion. To change the call lock, a thread must first acquire it.
2. Any thread that is waiting to acquire the call lock (because it is
already owned by another thread) is essentially waiting for one of two
conditions:
   a. the lock has been acquired by the waiting thread
   b. the lock has changed and the waiting thread must go and wait on this
new changed lock

This seems feasible to me, but I am unsure of how to wait for two
conditions. Is this a situation where the AbstractQueuedSynchronizer would
prove useful? I don't think that I've missed anything subtle, but this
approach is using one lock to get/set the value of the current lock (a data
lock) and a separate lock, that may change, to coordinate actions and I
could easily have missed something.

Thanks,

Tom


2009/4/16 David Holmes <davidcholmes at aapt.net.au>

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.

Cheers,
David Holmes
-----Original Message-----
From: concurrency-interest-bounces at cs.oswego.edu
[mailto:concurrency-interest-bounces at cs.oswego.edu]On Behalf Of tom
strickland
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 on
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?

Thanks,

Tom




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