RFR: 8285794: AsyncGetCallTrace might acquire a lock via JavaThread::thread_from_jni_environment

[David Holmes]

On Thu, 28 Apr 2022 09:38:30 GMT, Johannes Bechberger <duke at openjdk.java.net> wrote:

> Calling JavaThread::thread_from_jni_environment for a terminated thread in AsyncGetCallTrace might cause the acquisition of a lock, making AsyncGetCallTrace non-signal-safe. 
> 
> AsyncGetCallTrace can only be called for the current threads (there are asserts for that), therefore using JavaThread::current directly and checking the termination status is semantically equivalent.

Changes requested by dholmes (Reviewer).

src/hotspot/share/prims/forte.cpp line 566:

> 564: void AsyncGetCallTrace(ASGCT_CallTrace *trace, jint depth, void* ucontext) {
> 565: 
> 566:   JavaThread* thread = JavaThread::current_or_null();

As this can be in a signal handling context it needs to be Thread::current_or_null_safe().

src/hotspot/share/prims/forte.cpp line 568:

> 566:   JavaThread* thread = JavaThread::current_or_null();
> 567: 
> 568:   if (trace->env_id == NULL || thread == NULL || thread->is_terminated() || thread->is_exiting()) {

`is_exiting()` also covers `is_terminated()`.

src/hotspot/share/prims/forte.cpp line 580:

> 578:   }
> 579: 
> 580:   assert(thread == JavaThread::thread_from_jni_environment(trace->env_id),

Please add  a comment before the assert:

// This is safe now as the thread has not terminated and so no VM exit check occurs.

-------------

PR: https://git.openjdk.java.net/jdk/pull/8446