Re: Request for review, 6766644: Redefinition of compiled method fails with assertion "Can not load classes with the Compiler thread"

[Daniel D. Daugherty]

On 1/25/2011 12:44 PM, Kelly O'Hair wrote: On Jan 25, 2011, at 11:00 AM, Tom Rodriguez wrote: On Jan 25, 2011, at 8:49 AM, Keith McGuigan wrote: Hello, This code modifies the way that JVMTI compiled-method-load events are posted.  Previously, they were posted directly from the compiler thread, which could cause issues if the JVMTI event handling code made calls to RedefineClasses, since the compiler thread is unable to load classes if that is required.  This solution is to defer the posting of the events to the Service thread (formerly: LowMemoryDetector thread) which is a Java thread and is able to load classes. The posting appears to be asynchronous which I don't think will work.  The method could be unloaded or invalidated and freed before the event has been posted.  It has to be done synchronously I think. I have a vague memory of someone saying that there were restrictions on what a JVMTI client was allowed to do in response to a CompiledMethodLoad but I can't find any evidence of that.  So is that just a bogus memory or is there some restriction like this? It's been a while now, but there are restrictions as to what a JVMTI event handler can do, and I'm pretty sure redefining classes may be on that limitation list, but I'm not seeing it explicitly spelled out. Neither Keith or I could find any restrictions in latest (javase/6) docs... I see this:   http://download.oracle.com/javase/1.5.0/docs/guide/jvmti/jvmti.html#EventSection It says "and the JVM TI implementation does not queue events in any way". Yup. Any queueing needed by the agent is supposed to be implemented in the agent... And these compiled-method-load events can also be generated from this:   http://download.oracle.com/javase/1.5.0/docs/guide/jvmti/jvmti.html#GenerateEvents And it mentions some issues with JNI. Yup. The JNI and calling Java code issues are specific to calling the GenerateEvents() API itself. Fortunately, only a JavaThread can call GenerateEvents() and a JavaThread can call an event handler for CompiledMethodLoad where the event handler does a RedefineClasses() call also. Dan Sorry, it's been so long, my serviceability brain cells have not been maintained very well. :^( -kto tom The "Service thread" now handles both low-memory detection and JVMTI deferred event posting.   I left the door open for other types of events to be deferred and posted via this mechanism in case we find other situations where posting events from a non-Java thread causes problems. webrev:  http://cr.openjdk.java.net/~kamg/6766644/webrev.01/ -- - Keith