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Re: [equinox-dev] buddy classloading & deadlock


What vendor/version of the VM are you using?

There has been a long standing bug open against Sun on a similar issue.  It all boils down to the native VM locking the classloader before loading a class from it using the ClassLoader.loadClassInternal method.  In your stacktrace below this happens when the VM calls ClassLoader.loadClassInternal (DefaultClassLoader(ClassLoader).loadClassInternal(String) line: 319).  The VM locks the classloader in native code before it ever calls into the ClassLoader.  This leaves us with no options, even if we override the loadClassInternal method to make it non-synchronized it will make no difference because the native code locks the ClassLoader no matter what the method signature is for loadClassInternal.  I have been told that Sun plans to fix this in a future release of the VM, it may be fixed in the latest 1.5 Sun VM.  The IBM VMs have fixed this issue, they will NOT lock the classloader before invoking it from the native VM code.

Tom



"Tom Huybrechts" <tom.huybrechts@xxxxxxxxx>
Sent by: equinox-dev-bounces@xxxxxxxxxxx

07/29/2006 01:26 PM

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Subject
[equinox-dev] buddy classloading & deadlock





Hi all,

I noticed that multithreading combined with buddy classloading can
produce some scary deadlocks. I had one in a project last week and
managed to recreate it in a toy example. Now I'm wondering if this is
just one of the dangers of breaking the classloading hierarchy or if
the framework could do something to avoid this ?

My toy example (also in attachment):

The main project has just one Application class:

public class Application implements IPlatformRunnable{
               
                public Object run(Object args) throws Exception {
                                 new Activator();
                                 return true;
                }
               
}


and the dependent project has an Activator

public class Activator implements BundleActivator {

public void start(BundleContext context) throws Exception {
 new Thread(new Runnable() {
                public void run() {
                try {
                   Class.forName("main.Application");
               } catch (ClassNotFoundException e) { }
 }).start();

 // let other thread run to induce deadlock                                  
 try { Thread.sleep(100); } catch (InterruptedException e) {}
}

public void stop(BundleContext context) throws Exception {}

}


The main project depends on dependent, and the dependent project has a
global buddy policy.


This causes the following deadlock:

Thread [main] (Suspended)                
                owns: DefaultClassLoader  (id=16)                
                waiting for: DefaultClassLoader  (id=15)                
                ClasspathManager.findLocalClassImpl(String, ClassLoadingStatsHook[]) line: 422                
                ClasspathManager.findLocalClass(String) line: 410                
                DefaultClassLoader.findLocalClass(String) line: 188                
                BundleLoader.findLocalClass(String) line: 339                
                SingleSourcePackage.loadClass(String) line: 37                
                BundleLoader.findClass(String, boolean) line: 388                
                BundleLoader.findClass(String) line: 352                
                DefaultClassLoader.loadClass(String, boolean) line: 83                
                DefaultClassLoader(ClassLoader).loadClass(String) line: 251                
                DefaultClassLoader(ClassLoader).loadClassInternal(String) line: 319                
                Application.run(Object) line: 11                
                PlatformActivator$1.run(Object) line: 78                
                EclipseAppLauncher.runApplication(Object) line: 92                
                EclipseAppLauncher.start(Object) line: 68                
                EclipseStarter.run(Object) line: 400                
                EclipseStarter.run(String[], Runnable) line: 177                
                NativeMethodAccessorImpl.invoke0(Method, Object, Object[]) line: not
available [native method]
                NativeMethodAccessorImpl.invoke(Object, Object[]) line: 39                
                DelegatingMethodAccessorImpl.invoke(Object, Object[]) line: 25                
                Method.invoke(Object, Object...) line: 585                
                Main.invokeFramework(String[], URL[]) line: 336                
                Main.basicRun(String[]) line: 280                
                Main.run(String[]) line: 977                
                Main.main(String[]) line: 952                

Thread [Thread-1] (Suspended)                
                owns: DefaultClassLoader  (id=15)                
                waiting for: DefaultClassLoader  (id=16)                
                ClasspathManager.findLocalClassImpl(String, ClassLoadingStatsHook[]) line: 422                
                ClasspathManager.findLocalClass(String) line: 410                
                DefaultClassLoader.findLocalClass(String) line: 188                
                BundleLoader.findLocalClass(String) line: 339                
                BundleLoader.findClass(String, boolean) line: 391                
                BundleLoader.findClass(String) line: 352                
                DefaultClassLoader.loadClass(String, boolean) line: 83                
                DefaultClassLoader(ClassLoader).loadClass(String) line: 251                
                BundleLoader.loadClass(String) line: 276                
                BundleHost.loadClass(String, boolean) line: 227                
                BundleHost(AbstractBundle).loadClass(String) line: 1245                
                GlobalPolicy.loadClass(String) line: 39                
                PolicyHandler.doBuddyClassLoading(String) line: 138                
                BundleLoader.findClass(String, boolean) line: 402                
                BundleLoader.findClass(String) line: 352                
                DefaultClassLoader.loadClass(String, boolean) line: 83                
                DefaultClassLoader(ClassLoader).loadClass(String) line: 251                
                DefaultClassLoader(ClassLoader).loadClassInternal(String) line: 319                
                Class<T>.forName0(String, boolean, ClassLoader) line: not available
[native method]
                Class<T>.forName(String) line: 164                
                Activator$1.run() line: 12                
                Thread.run() line: 595
[attachment "buddy-classloading-deadlock.zip" deleted by Thomas Watson/Austin/IBM] _______________________________________________
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