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From Felix Meschberger <fmesc...@gmail.com>
Subject Re: Deadlock in Felix
Date Wed, 30 Jan 2008 09:12:53 GMT
Hi Niclas,

The problem is (a) the generous synchronisation of Log4J and (b) locking
used by stuff used for class loading. In our projects we regularly face
issues between Log4J and our ClassLoader implementations synchronizing
on ClassLoader.loadClass().

The deadlock occurrs because both parties - framework and Log4J - lock
"big" parts of their code and call to code outside of their scope while
being locked: the framework calls the LogService outside of the
framework and Log4J calls into class loading outside of Log4J.

On solution, I could imagine, is not using Log4J, which may or may not
be an option. Maybe SLF4J or Logback could be an option here ? [ In
Sling we actually use Logback as a logging backend for our LogService
implementation ]

Another solution would be to enhance the framework Logger to diable the
use of a LogService. E.g. by defining a framework property, which when
set, causes the Logger to never use the LogService.

Both solutions don't sound right ...

Regards
Felix

Am Mittwoch, den 30.01.2008, 13:51 +0800 schrieb Niclas Hedhman:
> On Tuesday 29 January 2008 16:55, Karl Pauls wrote:
> > Could you have them retry using Felix 1.0.3? This might be related to
> > some of the concurrency things we fixed.
> >
> > In case they can not be bothered retrying on Felix 1.0.3 then maybe
> > they can provide a minimal config file that only uses publicly
> > available bundles and has this issue (then I can look into it).
> 
> I am looking at the code in the trunk, and it appears that the locking that 
> triggers this is in place.
> 
> As always, it is a bit tricky to setup threading problems. SO, I would like to 
> run a "head exercise" first.
> 
> 1. The Starter thread "FelixStartLevel" locks the ModuleFactoryImpl instance 
> in R4SearchPolicyCore.resolve().
> 
> 2. The Configuration Admin thread "Configuration Updater" calls the LogService 
> with the Configuration instance, which leads to Log4J locks on its own 
> RootLogger and  calls ClassLoader.loadClass() on something found in the 
> configuration. This leads to trying to acquire the ModuleFactoryImpl lock 
> either in R4SearchPolicyCore.resolve() or in the provided stack trace 
> R4SearchPolicyCore.getInUseCandidates() due to a ClassNotFound in the 
> previous step.
> 
> 3. The "FelixStartLevel" thread reaches 
> m_logger.log(Logger.LOG_DEBUG, "WIRE: " + wires[wireIdx]);
> in R4SearchPolicyCore.createWires() and he log() method will try to acquire 
> the RootLogger lock.
> 
> 4. DEADLOCK.
> 
> 
> I agree this is very special to the LogService, since Felix binds to it and 
> uses it for its internal use, and the responsibility is across two different 
> systems. Suggestions are welcome.
> 
> 
> Cheers


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