Some additional details on this...
The side effect of a nmethodLocker::lock_nmethod(nm) call
is the nmethod::is_locked_by_vm() returns true.
It is checked in the nmethod::flush(), nmethod::can_convert_to_zombie()
and some other places.
If it is still not safe to look at metadata when the nmethod is locked
then there is some code which does not honor this locking convention.
Thanks,
Serguei
On 11/15/19 1:45 PM, [email protected] wrote:
Hi Coleen,
I have some questions.
Both the compiler method load and unload are posted as deferred events.
Both events keep the nmethod alive until the ServiceThread processes
the event.
The implementation is:
JvmtiDeferredEvent
JvmtiDeferredEvent::compiled_method_load_event(nmethod* nm) {
. . .
// Keep the nmethod alive until the ServiceThread can process
// this deferred event.
nmethodLocker::lock_nmethod(nm);
return event;
}
JvmtiDeferredEvent
JvmtiDeferredEvent::compiled_method_unload_event(nmethod* nm,
jmethodID id, const void* code) {
. . .
// Keep the nmethod alive until the ServiceThread can process
// this deferred event. This will keep the memory for the
// generated code from being reused too early. We pass
// zombie_ok == true here so that our nmethod that was just
// made into a zombie can be locked.
nmethodLocker::lock_nmethod(nm, true /* zombie_ok */);
return event;
}
void JvmtiDeferredEvent::post() {
assert(ServiceThread::is_service_thread(Thread::current()),
"Service thread must post enqueued events");
switch(_type) {
case TYPE_COMPILED_METHOD_LOAD: {
nmethod* nm = _event_data.compiled_method_load;
JvmtiExport::post_compiled_method_load(nm);
// done with the deferred event so unlock the nmethod
nmethodLocker::unlock_nmethod(nm);
break;
}
case TYPE_COMPILED_METHOD_UNLOAD: {
nmethod* nm = _event_data.compiled_method_unload.nm;
JvmtiExport::post_compiled_method_unload(
_event_data.compiled_method_unload.method_id,
_event_data.compiled_method_unload.code_begin);
// done with the deferred event so unlock the nmethod
nmethodLocker::unlock_nmethod(nm);
break;
}
. . .
}
}
Then I wonder how is it possible for the nmethod to be not alive here?:
2168 void JvmtiExport::post_compiled_method_load(nmethod *nm) {
. . .
2173 // It's not safe to look at metadata for unloaded methods.
2174 if (!nm->is_alive()) {
2175 return;
2176 }
At least, it lokks like something else is broken.
Do I miss something important here?
Thanks,
Serguei
On 11/14/19 5:15 PM, [email protected] wrote:
Summary: Don't post information which uses metadata from unloaded
nmethods
Tested tier1-3 and 100 times with test that failed (reproduced
failure without the fix).
open webrev at
http://cr.openjdk.java.net/~coleenp/2019/8173361.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8173361
Thanks,
Coleen
