Re: [equinox-dev] buddy classloading & deadlock

[Thomas Watson <tjwatson@xxxxxxxxxx>]

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
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