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From "Ard Schrijvers" <a.schrijv...@hippo.nl>
Subject RE: [jira] Created: (COCOON-1985) AbstractCachingProcessingPipeline locking with IncludeTransformer may hang pipeline
Date Thu, 18 Jan 2007 11:45:24 GMT
Hello,

> 
> Cocoon 2.1.9 introduced the concept of a lock in 
> AbstractCachingProcessingPipeline, an optimization to prevent 
> two concurrent requests from generating the same cached 
> content. The first request adds the pipeline key to the 
> transient cache to 'lock' the cache entry for that pipeline, 
> subsequent concurrent requests wait for the first request to 
> cache the content (by Object.lock()ing the pipeline key 
> entry) before proceeding, and can then use the newly cached content.
> 
> However, this has introduced an incompatibility with the 
> IncludeTransformer: if the inclusions access the same 
> yet-to-be-cached content as the root pipeline, the whole 
> assembly hangs, since a lock will be made on a lock already 
> held by the same thread, and which cannot be satisfied.
> 
> e.g.
> i) Root pipeline generates using sub-pipeline cocoon:/foo.xml
> ii) the cocoon:/foo.xml sub-pipeline adds it's pipeline key 
> to the transient store as a lock.
> iii) subsequently in the root pipeline, the IncludeTransformer is run.
> iv) one of the inclusions also generates with 
> cocoon:/foo.xml, this sub-pipeline locks in 
> AbstractProcessingPipeline.waitForLock() because the 
> sub-pipeline key is already present.
> v) deadlock.

I do not understand one part of it. If a sub-pipeline is called, cocoon:/foo.xml, there is
lock generated for this sub-pipeline seperately, right? (if not, I do not understand why it
is not like this. I suppose a lock is generated for the root pipeline, but as well for every
sub-pipeline individually. I suppose though, because i did not actually look at the code).


Now, if the include transformer calls this same sub-pipeline, which is having its own lock,
I do not see why a deadlock can occur? The root-pipeline is locked, the sub-pipeline is locked
as well. The include transformer wants to include the same sub-pipeline, waits untill this
one is finished, then can includes it, right? 

I most be missing something, 

Regards Ard

> 
> I've found a (partial, see below) solution for this: instead 
> of a plain Object being added to the transient store as the 
> lock object, the Thread.currentThread() is added; when 
> waitForLock() is called, if the lock object exists, it checks 
> that it is not the same thread before attempting to lock it; 
> if it is the same thread, then waitForLock() returns success, 
> which allows generation to proceed. You loose the efficiency 
> of generating the cache only once in this case, but at least 
> it doesn't hang! With JDK1.5 this can be made neater by using 
> Thread#holdsLock() instead of adding the thread object itself 
> to the transient store.
> 
> See patch file.
> 
> However, even with this fix, parallel includes (when enabled) 
> may still hang, because they pass the not-the-same-thread 
> test, but fail because the root pipeline, which holds the 
> initial lock, cannot complete (and therefore statisfy the 
> lock condition for the parallel threads), before the threads 
> themselves have completed, which then results in a deadlock again.
> 
> The complete solution is probably to avoid locking if the 
> lock is held by the same top-level Request, but that requires 
> more knowledge of Cocoon's processing than I (currently) have!
> 
> IMHO unless a complete solution is found to this, then this 
> optimization should be removed completely, or else made 
> optional by configuration, since it renders the 
> IncludeTransformer dangerous.
> 
> 
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