RFR(S) 8249293: Unsafe stackwalk in VM_GetOrSetLocal::doit_prologue()

[Reingruber, Richard]

Hi,

please help review this fix for VM_GetOrSetLocal. It moves the unsafe stackwalk from the vm
operation prologue before the safepoint into the doit() method executed at the safepoint.

Webrev: http://cr.openjdk.java.net/~rrich/webrevs/8249293/webrev.0/index.html
Bug:    https://bugs.openjdk.java.net/browse/JDK-8249293

According to the JVMTI spec on local variable access it is not required to suspend the target thread
T [1]. The operation will simply fail with JVMTI_ERROR_NO_MORE_FRAMES if T is executing
bytecodes. It will succeed though if T is blocked because of synchronization or executing some native
code.

The issue is that in the latter case the stack walk in VM_GetOrSetLocal::doit_prologue() to prepare
the access to the local variable is unsafe, because it is done before the safepoint and it races
with T returning to execute bytecodes making its stack not walkable. The included test shows that
this can crash the VM if T wins the race.

Manual testing:

  - new test test/hotspot/jtreg/serviceability/jvmti/GetLocalVariable/GetLocalWithoutSuspendTest.java
  - test/hotspot/jtreg/vmTestbase/nsk/jvmti
  - test/hotspot/jtreg/serviceability/jvmti

Nightly regression tests @SAP: JCK and JTREG, also in Xcomp mode, SPECjvm2008, SPECjbb2015,
Renaissance Suite, SAP specific tests with fastdebug and release builds on all platforms

Thanks, Richard.

[1] https://docs.oracle.com/en/java/javase/14/docs/specs/jvmti.html#local