RFR: 8286960: Test serviceability/jvmti/vthread/SuspendResume2 crashed: missing ThreadsListHandle in calling context

[Serguei Spitsyn]

On Wed, 25 May 2022 07:23:36 GMT, Serguei Spitsyn <sspitsyn at openjdk.org> wrote:

> A part of this issue was contributed with the following changeset:
> 
> commit ea23e7333e03abb4aca3e9f3854bab418a4b70e2
> Author: Daniel D. Daugherty <[dcubed at openjdk.org](mailto:dcubed at openjdk.org)>
> Date: Mon Nov 8 14:45:04 2021 +0000
> 
>     8249004: Reduce ThreadsListHandle overhead in relation to direct handshakes
>     Reviewed-by: coleenp, sspitsyn, dholmes, rehn
> 
> The following change in `src/hotspot/share/runtime/thread.cpp` added new assert:
> 
> bool JavaThread::java_suspend() {
> - ThreadsListHandle tlh;
> - if (!tlh.includes(this)) {
> - log_trace(thread, suspend)("JavaThread:" INTPTR_FORMAT " not on ThreadsList, no suspension", p2i(this));
> - return false;
> - }
> + guarantee(Thread::is_JavaThread_protected(this, /* checkTLHOnly */ true),
>  + "missing ThreadsListHandle in calling context.");
>   return this->handshake_state()->suspend();
> }
> 
> This new assert misses a check for target thread as being current `JavaThread`.
> 
> Also, the JVMTI SuspendThread is protected with TLH:
> 
> JvmtiEnv::SuspendThread(jthread thread) {
>   JavaThread* current = JavaThread::current();
>   ThreadsListHandle tlh(current);              <= TLS defined here!!!
> 
>    oop thread_oop = NULL;
>    {
>      JvmtiVTMSTransitionDisabler disabler(true); 
> 
> 
> However, it is possible that a new carrier thread (and an associated `JavaThread`) can be created after the `TLH` was set and the target virtual thread can be mounted on new carrier thread. Then target virtual thread will be associated with newly created `JavaThread` which is unprotected by the TLH.
> The right way to be protected from this situation it is to prevent mount state transitions with `JvmtiVTMSTransitionDisabler` before the TLH is set as in the change below:
> 
> 
> @@ -929,13 +929,13 @@ JvmtiEnv::GetAllThreads(jint* threads_count_ptr, jthread** threads_ptr) {
>  jvmtiError
>  JvmtiEnv::SuspendThread(jthread thread) {
>    JavaThread* current = JavaThread::current();
> -  ThreadsListHandle tlh(current);
> 
>    jvmtiError err;
>    JavaThread* java_thread = NULL;
>    oop thread_oop = NULL;
>    {
>      JvmtiVTMSTransitionDisabler disabler(true);
> +    ThreadsListHandle tlh(current);
> 
>      err = get_threadOop_and_JavaThread(tlh.list(), thread, &java_thread, &thread_oop);
>      if (err != JVMTI_ERROR_NONE) {
> 
> 
> 
> This problem exist in all JVMTI Suspend functions:
>  `SuspendThread`, `SuspendThreadList` and `SuspendAllVirtualThreads`.

Yes, I understand it. Thank you for clarification!

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

PR: https://git.openjdk.java.net/jdk/pull/8878