RFR 8029891 : Deadlock detected in java/lang/ClassLoader/deadlock/GetResource.java (original) (raw)

Peter Levart peter.levart at gmail.com
Wed May 6 07:57:43 UTC 2015

On 05/05/2015 03:25 AM, David Holmes wrote:

Hi Brent,

On 5/05/2015 2:11 AM, Brent Christian wrote: Hi,

Please review this fix, courtesy of Peter Levart (thanks!), that I would like to get in. https://bugs.openjdk.java.net/browse/JDK-8029891 http://cr.openjdk.java.net/~bchristi/8029891/webrev.0/ There is some discussion of it in the bug report, starting at 2014-12-31. The problem, as stated by Mandy: "System Properties is a hashtable that synchronizes on itself for any access. Currently System.getProperties returns the Properties instance accessed by the system in which any application code might synchronize on it (that's what the test is doing). The problem reported JDK-6977738 is about Properties.store method that was fixed not to synchronize on this instance. System property is a common way for changing the default setting and so it's impractical to expect the class loading code path not to call System.getProperty." This fix changes java.util.Properties to store its values in an internal ConcurrentHashMap, ignoring its Hashtable heritage. In this way, Properties can be "de-sychronized": all methods inherited from Hashtable are overridden, to remove synchronization, and delegate to the internal CHM. I don't think you want to de-synchronize the load* methods - you don't want two threads calling load concurrently. But that then raises the problem of concurrent modification while a load is in progress. Synchronization ensures serialization and by removing it you have done more than just avoid deadlocks. I think this needs a more careful examination of the expected/desired concurrent interactions between different methods. It may be that simply not utilizing the synchronized Hashtable methods is sufficient to resolve the deadlock, while still providing reasonable serialization via the existing synchronized Properties methods - or it may not. But allowing concurrent modifications will change behaviour in an unexpected, and incompatible way, in my opinion. David -----

Hi David,

You say: "It may be that simply not utilizing the synchronized Hashtable methods is sufficient to resolve the deadlock, while still providing reasonable serialization via the existing synchronized Properties methods - or it may not".

How do you propose to not utilize synchronized Hashtable methods? By not utilizing Properties at all? This may be difficult to achieve as most system configuration is specified as system Properties.

So what about taking a more conservative approach by making all "modification" and "bulk" methods synchronized and exposing just single-entry "read-only" methods as not synchronized (mainly Hashatable.get())?

Regards, Peter

The serialized form is unchanged.

An alternative approach considered would be for System.getProperties() to return a duplicate snapshot of the current Properties. This presents a compatibility risk to existing code that keeps a reference to the return value of System.getProperties() and expects to either read new properties added afterwards, or set properties on the cached copy. -Brent

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