RFR: 8268364: jmethod clearing should be done during unloading

[Daniel D.Daugherty]

On Wed, 30 Jun 2021 22:48:03 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

> This patch moves the jmethod clearing to ClassLoaderData::unload() but also adds a check to Method::checked_resolved_jmethod_id() to handle the case where ZGC may be unloading a class but not have gotten to ClassLoaderData::unload() yet.  JVMTI will read a NULL method for checked_resolved_jmethod_id() in this case, and not get a Method that will shortly, or has already been reclaimed in the Metaspace destructor.
> Since I was there, I also added Method::is_valid_method() check to checked_resolve_jmethod_id. I don't think it's expensive anymore but it could be added under DEBUG.  Either way method->method_holder()->is_loader_alive() will crash if !is_valid_method so we should leave it.   As I wrote in the related issues, the bogus Method may have been because of a previous set of bugs with post_compiled_method_load events.
> 
> Tested with tiers 1-6 on linux-x64-debug and 1-3 on windows-x64-debug.
> 
> Also ran vmTestbase/nsk/{jdi,jvmti} tests with VM_OPTIONS=-XX:+UseZGC  -XX:ZCollectionInterval=0.01 -XX:ZFragmen
> tationLimit=0

Just comments for now. I need to think about this more.

src/hotspot/share/oops/method.cpp line 2257:

> 2255:   if (mid == NULL) return NULL;
> 2256:   Method* o = resolve_jmethod_id(mid);
> 2257:   if (o == NULL || o == JNIMethodBlock::_free_method) {

I need to think more about deleting the is_method() check.

src/hotspot/share/oops/method.cpp line 2261:

> 2259:   }
> 2260:   // Method should otherwise be valid. Assert for testing.
> 2261:   assert(is_valid_method(o), "should be valid jmethodid");

nit typo: s/jmethodid/jmethodID/

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PR: https://git.openjdk.java.net/jdk/pull/4643