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From David Jencks <david_jen...@yahoo.com>
Subject SCR concurrency issues (cf FELIX-3456)
Date Thu, 19 Apr 2012 22:19:39 GMT
We've run into one definite concurrency problem in SCR and I've been discussing offline with
a colleague how to fix it and wanted to get the discussion out in the open.

The original symptom was when 2 mandatory service refs were satisfied on different threads
at once: the 2nd wasn't recognized so the component never got activated.

This is easily solved by synchronizing but this introduces risk of deadlocks (my first attempt,

We tried some partly asynchronous approaches such as https://issues.apache.org/jira/secure/attachment/12523313/FELIX-3456-4.diff.
 Unless there's a timeout (presumably due to deadlock) this gets all service events processed
before the thread exits from its first call into SCR.  However this can result in service
events getting processed later than one expects possibly on a different thread.  On further
thought we concluded that a service event must be processed fully before the service registration
call returns.  We therefore don't think any kind of asynchronous approach will work.

We've discovered the anti-circular-dependency clause in the spec (112.3.5) but it appears
to be overly biased towards SCR-only graphs of services.  We are leaning towards thinking
that SCR also needs to consider:

- an activate method registers a service that satisfies an optional dependency of a component
being activated by scr on the same thread.
- the same, except the activate method starts a new thread to register the service and waits
for it to complete.

Another scenario to consider is

components C1 and C2 registering as services, each with an optional dynamic dependency on
the other.  If one starts, and then the other, there is no problem, they both get references
to the other.  If they both start at the same time in separate threads (either because they
are in different bundles or because they get activated due to mandatory references being satisfied)
and register the services while the other is in the Activating state, a simple lock over the
service event processing will result in deadlock.  Furthermore, to get the correct result,
at least one of the services has to be bound while the component to which is is binding is
in the Activating state.

It looks like the situation can be simplified a bit by considering, for service events, whether
the dependency will result in a state change: if it's optional or mandatory but not the only
satisfying service, it won't, but if it's mandatory and the first satisfying service, it will.
 We can calculate this before calling any bind methods or activate methods.  After determining
this, we know the final state of the component.

We're considering whether some kind of 2-stage lock would work:

one level can change the state and blocks all other threads
the other level can't change the state and lets stuff like service events for non-state-changing
service references be processed according to the final state of the component. (e.g. activating
will let bind methods be called on the under-configuration object).

This does not yet consider bundle event driven state changes or deactivation or delayed component
creation or service factories.

Comments and more scenarios to consider are more than welcome.

david jencks

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