On Wed, 25 May 2022 07:23:36 GMT, Serguei Spitsyn <sspit...@openjdk.org> wrote:
> A part of this issue was contributed with the following changeset: > > commit ea23e7333e03abb4aca3e9f3854bab418a4b70e2 > Author: Daniel D. Daugherty <[dcu...@openjdk.org](mailto:dcu...@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`. These changes look good to me - as per our discussion in JBS issue. Thanks, David ------------- Marked as reviewed by dholmes (Reviewer). PR: https://git.openjdk.java.net/jdk/pull/8878
