When doing some performance test on the REST server, I've come across exceptions like this:

2010-05-26 17:05:43,025 9375 ERROR [org.infinispan.interceptors.InvocationContextInterceptor] (http-8080-7 Execution error:
java.lang.NullPointerException
at org.infinispan.util.concurrent.locks.LockManagerImpl.lockAndRecord(LockManagerImpl.java:83)
at org.infinispan.container.EntryFactoryImpl.acquireLock(EntryFactoryImpl.java:205)
at org.infinispan.container.EntryFactoryImpl.wrapEntryForWriting(EntryFactoryImpl.java:148)
at org.infinispan.container.EntryFactoryImpl.wrapEntryForWriting(EntryFactoryImpl.java:106)
at org.infinispan.interceptors.LockingInterceptor.visitPutKeyValueCommand(LockingInterceptor.java:197)
at org.infinispan.commands.write.PutKeyValueCommand.acceptVisitor(PutKeyValueCommand.java:76)
at org.infinispan.interceptors.base.CommandInterceptor.invokeNextInterceptor(CommandInterceptor.java:118)
at org.infinispan.interceptors.base.CommandInterceptor.handleDefault(CommandInterceptor.java:132)
at org.infinispan.commands.AbstractVisitor.visitPutKeyValueCommand(AbstractVisitor.java:57)
at org.infinispan.commands.write.PutKeyValueCommand.acceptVisitor(PutKeyValueCommand.java:76)
at org.infinispan.interceptors.base.CommandInterceptor.invokeNextInterceptor(CommandInterceptor.java:118)
at org.infinispan.interceptors.TxInterceptor.enlistWriteAndInvokeNext(TxInterceptor.java:183)
at org.infinispan.interceptors.TxInterceptor.visitPutKeyValueCommand(TxInterceptor.java:132)
at org.infinispan.commands.write.PutKeyValueCommand.acceptVisitor(PutKeyValueCommand.java:76)
at org.infinispan.interceptors.base.CommandInterceptor.invokeNextInterceptor(CommandInterceptor.java:118)
at org.infinispan.interceptors.InvocationContextInterceptor.handleAll(InvocationContextInterceptor.java:58)
at org.infinispan.interceptors.InvocationContextInterceptor.handleDefault(InvocationContextInterceptor.java:39)
at org.infinispan.commands.AbstractVisitor.visitPutKeyValueCommand(AbstractVisitor.java:57)
at org.infinispan.commands.write.PutKeyValueCommand.acceptVisitor(PutKeyValueCommand.java:76)
at org.infinispan.interceptors.InterceptorChain.invoke(InterceptorChain.java:271)
at org.infinispan.CacheDelegate.put(CacheDelegate.java:411)
at org.infinispan.CacheSupport.put(CacheSupport.java:28)
at org.infinispan.rest.Server.putEntry(Server.scala:86)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.jboss.resteasy.core.MethodInjectorImpl.invoke(MethodInjectorImpl.java:140)
at org.jboss.resteasy.core.ResourceMethod.invokeOnTarget(ResourceMethod.java:246)
at org.jboss.resteasy.core.ResourceMethod.invoke(ResourceMethod.java:214)
at org.jboss.resteasy.core.ResourceMethod.invoke(ResourceMethod.java:204)
at org.jboss.resteasy.core.SynchronousDispatcher.getResponse(SynchronousDispatcher.java:486)
at org.jboss.resteasy.core.SynchronousDispatcher.invoke(SynchronousDispatcher.java:463)
at org.jboss.resteasy.core.SynchronousDispatcher.invoke(SynchronousDispatcher.java:117)
at org.jboss.resteasy.plugins.server.servlet.ServletContainerDispatcher.service(ServletContainerDispatcher.java:198)
at org.jboss.resteasy.plugins.server.servlet.HttpServletDispatcher.service(HttpServletDispatcher.java:48)
at org.jboss.resteasy.plugins.server.servlet.HttpServletDispatcher.service(HttpServletDispatcher.java:43)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:852)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:588)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
at java.lang.Thread.run(Thread.java:637)

These exceptions are appearing as a result of multiple threads trying to create and operate on the same cache instance concurrently.