runtime/openjdkjvmti/ti_thread.cc

/* Copyright (C) 2017 The Android Open Source Project
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This file implements interfaces from the file jvmti.h. This implementation
 * is licensed under the same terms as the file jvmti.h.  The
 * copyright and license information for the file jvmti.h follows.
 *
 * Copyright (c) 2003, 2011, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

#include "ti_thread.h"

#include "android-base/strings.h"
#include "art_field-inl.h"
#include "art_jvmti.h"
#include "base/logging.h"
#include "base/mutex.h"
#include "events-inl.h"
#include "gc/system_weak.h"
#include "gc_root-inl.h"
#include "jni_internal.h"
#include "mirror/class.h"
#include "mirror/object-inl.h"
#include "mirror/string.h"
#include "obj_ptr.h"
#include "ti_phase.h"
#include "runtime.h"
#include "runtime_callbacks.h"
#include "ScopedLocalRef.h"
#include "scoped_thread_state_change-inl.h"
#include "thread-current-inl.h"
#include "thread_list.h"
#include "well_known_classes.h"

namespace openjdkjvmti {

art::ArtField* ThreadUtil::context_class_loader_ = nullptr;

struct ThreadCallback : public art::ThreadLifecycleCallback, public art::RuntimePhaseCallback {
  jthread GetThreadObject(art::Thread* self) REQUIRES_SHARED(art::Locks::mutator_lock_) {
    if (self->GetPeer() == nullptr) {
      return nullptr;
    }
    return self->GetJniEnv()->AddLocalReference<jthread>(self->GetPeer());
  }
  template <ArtJvmtiEvent kEvent>
  void Post(art::Thread* self) REQUIRES_SHARED(art::Locks::mutator_lock_) {
    DCHECK_EQ(self, art::Thread::Current());
    ScopedLocalRef<jthread> thread(self->GetJniEnv(), GetThreadObject(self));
    art::ScopedThreadSuspension sts(self, art::ThreadState::kNative);
    event_handler->DispatchEvent<kEvent>(self,
                                         reinterpret_cast<JNIEnv*>(self->GetJniEnv()),
                                         thread.get());
  }

  void ThreadStart(art::Thread* self) OVERRIDE REQUIRES_SHARED(art::Locks::mutator_lock_) {
    if (!started) {
      // Runtime isn't started. We only expect at most the signal handler or JIT threads to be
      // started here.
      if (art::kIsDebugBuild) {
        std::string name;
        self->GetThreadName(name);
        if (name != "JDWP" &&
            name != "Signal Catcher" &&
            !android::base::StartsWith(name, "Jit thread pool")) {
          LOG(FATAL) << "Unexpected thread before start: " << name;
        }
      }
      return;
    }
    Post<ArtJvmtiEvent::kThreadStart>(self);
  }

  void ThreadDeath(art::Thread* self) OVERRIDE REQUIRES_SHARED(art::Locks::mutator_lock_) {
    Post<ArtJvmtiEvent::kThreadEnd>(self);
  }

  void NextRuntimePhase(RuntimePhase phase) OVERRIDE REQUIRES_SHARED(art::Locks::mutator_lock_) {
    if (phase == RuntimePhase::kInit) {
      // We moved to VMInit. Report the main thread as started (it was attached early, and must
      // not be reported until Init.
      started = true;
      Post<ArtJvmtiEvent::kThreadStart>(art::Thread::Current());
    }
  }

  EventHandler* event_handler = nullptr;
  bool started = false;
};

ThreadCallback gThreadCallback;

void ThreadUtil::Register(EventHandler* handler) {
  art::Runtime* runtime = art::Runtime::Current();

  gThreadCallback.started = runtime->IsStarted();
  gThreadCallback.event_handler = handler;

  art::ScopedThreadStateChange stsc(art::Thread::Current(),
                                    art::ThreadState::kWaitingForDebuggerToAttach);
  art::ScopedSuspendAll ssa("Add thread callback");
  runtime->GetRuntimeCallbacks()->AddThreadLifecycleCallback(&gThreadCallback);
  runtime->GetRuntimeCallbacks()->AddRuntimePhaseCallback(&gThreadCallback);
}

void ThreadUtil::CacheData() {
  art::ScopedObjectAccess soa(art::Thread::Current());
  art::ObjPtr<art::mirror::Class> thread_class =
      soa.Decode<art::mirror::Class>(art::WellKnownClasses::java_lang_Thread);
  CHECK(thread_class != nullptr);
  context_class_loader_ = thread_class->FindDeclaredInstanceField("contextClassLoader",
                                                                  "Ljava/lang/ClassLoader;");
  CHECK(context_class_loader_ != nullptr);
}

void ThreadUtil::Unregister() {
  art::ScopedThreadStateChange stsc(art::Thread::Current(),
                                    art::ThreadState::kWaitingForDebuggerToAttach);
  art::ScopedSuspendAll ssa("Remove thread callback");
  art::Runtime* runtime = art::Runtime::Current();
  runtime->GetRuntimeCallbacks()->RemoveThreadLifecycleCallback(&gThreadCallback);
  runtime->GetRuntimeCallbacks()->RemoveRuntimePhaseCallback(&gThreadCallback);
}

jvmtiError ThreadUtil::GetCurrentThread(jvmtiEnv* env ATTRIBUTE_UNUSED, jthread* thread_ptr) {
  art::Thread* self = art::Thread::Current();

  art::ScopedObjectAccess soa(self);

  jthread thread_peer;
  if (self->IsStillStarting()) {
    thread_peer = nullptr;
  } else {
    thread_peer = soa.AddLocalReference<jthread>(self->GetPeer());
  }

  *thread_ptr = thread_peer;
  return ERR(NONE);
}

// Get the native thread. The spec says a null object denotes the current thread.
static art::Thread* GetNativeThread(jthread thread,
                                    const art::ScopedObjectAccessAlreadyRunnable& soa)
    REQUIRES_SHARED(art::Locks::mutator_lock_) {
  if (thread == nullptr) {
    return art::Thread::Current();
  }

  art::MutexLock mu(soa.Self(), *art::Locks::thread_list_lock_);
  return art::Thread::FromManagedThread(soa, thread);
}

jvmtiError ThreadUtil::GetThreadInfo(jvmtiEnv* env, jthread thread, jvmtiThreadInfo* info_ptr) {
  if (info_ptr == nullptr) {
    return ERR(NULL_POINTER);
  }
  if (!PhaseUtil::IsLivePhase()) {
    return JVMTI_ERROR_WRONG_PHASE;
  }

  art::ScopedObjectAccess soa(art::Thread::Current());

  art::Thread* self = GetNativeThread(thread, soa);
  if (self == nullptr && thread == nullptr) {
    return ERR(INVALID_THREAD);
  }

  JvmtiUniquePtr<char[]> name_uptr;
  if (self != nullptr) {
    // Have a native thread object, this thread is alive.
    std::string name;
    self->GetThreadName(name);
    jvmtiError name_result;
    name_uptr = CopyString(env, name.c_str(), &name_result);
    if (name_uptr == nullptr) {
      return name_result;
    }
    info_ptr->name = name_uptr.get();

    info_ptr->priority = self->GetNativePriority();

    info_ptr->is_daemon = self->IsDaemon();

    art::ObjPtr<art::mirror::Object> peer = self->GetPeerFromOtherThread();

    // ThreadGroup.
    if (peer != nullptr) {
      art::ArtField* f = art::jni::DecodeArtField(art::WellKnownClasses::java_lang_Thread_group);
      CHECK(f != nullptr);
      art::ObjPtr<art::mirror::Object> group = f->GetObject(peer);
      info_ptr->thread_group = group == nullptr
                                   ? nullptr
                                   : soa.AddLocalReference<jthreadGroup>(group);
    } else {
      info_ptr->thread_group = nullptr;
    }

    // Context classloader.
    DCHECK(context_class_loader_ != nullptr);
    art::ObjPtr<art::mirror::Object> ccl = peer != nullptr
        ? context_class_loader_->GetObject(peer)
        : nullptr;
    info_ptr->context_class_loader = ccl == nullptr
                                         ? nullptr
                                         : soa.AddLocalReference<jobject>(ccl);
  } else {
    // Only the peer. This thread has either not been started, or is dead. Read things from
    // the Java side.
    art::ObjPtr<art::mirror::Object> peer = soa.Decode<art::mirror::Object>(thread);

    // Name.
    {
      art::ArtField* f = art::jni::DecodeArtField(art::WellKnownClasses::java_lang_Thread_name);
      CHECK(f != nullptr);
      art::ObjPtr<art::mirror::Object> name = f->GetObject(peer);
      std::string name_cpp;
      const char* name_cstr;
      if (name != nullptr) {
        name_cpp = name->AsString()->ToModifiedUtf8();
        name_cstr = name_cpp.c_str();
      } else {
        name_cstr = "";
      }
      jvmtiError name_result;
      name_uptr = CopyString(env, name_cstr, &name_result);
      if (name_uptr == nullptr) {
        return name_result;
      }
      info_ptr->name = name_uptr.get();
    }

    // Priority.
    {
      art::ArtField* f = art::jni::DecodeArtField(art::WellKnownClasses::java_lang_Thread_priority);
      CHECK(f != nullptr);
      info_ptr->priority = static_cast<jint>(f->GetInt(peer));
    }

    // Daemon.
    {
      art::ArtField* f = art::jni::DecodeArtField(art::WellKnownClasses::java_lang_Thread_daemon);
      CHECK(f != nullptr);
      info_ptr->is_daemon = f->GetBoolean(peer) == 0 ? JNI_FALSE : JNI_TRUE;
    }

    // ThreadGroup.
    {
      art::ArtField* f = art::jni::DecodeArtField(art::WellKnownClasses::java_lang_Thread_group);
      CHECK(f != nullptr);
      art::ObjPtr<art::mirror::Object> group = f->GetObject(peer);
      info_ptr->thread_group = group == nullptr
                                   ? nullptr
                                   : soa.AddLocalReference<jthreadGroup>(group);
    }

    // Context classloader.
    DCHECK(context_class_loader_ != nullptr);
    art::ObjPtr<art::mirror::Object> ccl = peer != nullptr
        ? context_class_loader_->GetObject(peer)
        : nullptr;
    info_ptr->context_class_loader = ccl == nullptr
                                         ? nullptr
                                         : soa.AddLocalReference<jobject>(ccl);
  }

  name_uptr.release();

  return ERR(NONE);
}

// Return the thread's (or current thread, if null) thread state. Return kStarting in case
// there's no native counterpart (thread hasn't been started, yet, or is dead).
static art::ThreadState GetNativeThreadState(jthread thread,
                                             const art::ScopedObjectAccessAlreadyRunnable& soa,
                                             art::Thread** native_thread)
    REQUIRES_SHARED(art::Locks::mutator_lock_) {
  art::Thread* self = nullptr;
  art::MutexLock mu(soa.Self(), *art::Locks::thread_list_lock_);
  if (thread == nullptr) {
    self = art::Thread::Current();
  } else {
    self = art::Thread::FromManagedThread(soa, thread);
  }
  *native_thread = self;
  if (self == nullptr || self->IsStillStarting()) {
    return art::ThreadState::kStarting;
  }
  return self->GetState();
}

static jint GetJvmtiThreadStateFromInternal(art::ThreadState internal_thread_state) {
  jint jvmti_state = JVMTI_THREAD_STATE_ALIVE;

  if (internal_thread_state == art::ThreadState::kSuspended) {
    jvmti_state |= JVMTI_THREAD_STATE_SUSPENDED;
    // Note: We do not have data about the previous state. Otherwise we should load the previous
    //       state here.
  }

  if (internal_thread_state == art::ThreadState::kNative) {
    jvmti_state |= JVMTI_THREAD_STATE_IN_NATIVE;
  }

  if (internal_thread_state == art::ThreadState::kRunnable ||
      internal_thread_state == art::ThreadState::kWaitingWeakGcRootRead ||
      internal_thread_state == art::ThreadState::kSuspended) {
    jvmti_state |= JVMTI_THREAD_STATE_RUNNABLE;
  } else if (internal_thread_state == art::ThreadState::kBlocked) {
    jvmti_state |= JVMTI_THREAD_STATE_BLOCKED_ON_MONITOR_ENTER;
  } else {
    // Should be in waiting state.
    jvmti_state |= JVMTI_THREAD_STATE_WAITING;

    if (internal_thread_state == art::ThreadState::kTimedWaiting ||
        internal_thread_state == art::ThreadState::kSleeping) {
      jvmti_state |= JVMTI_THREAD_STATE_WAITING_WITH_TIMEOUT;
    } else {
      jvmti_state |= JVMTI_THREAD_STATE_WAITING_INDEFINITELY;
    }

    if (internal_thread_state == art::ThreadState::kSleeping) {
      jvmti_state |= JVMTI_THREAD_STATE_SLEEPING;
    }

    if (internal_thread_state == art::ThreadState::kTimedWaiting ||
        internal_thread_state == art::ThreadState::kWaiting) {
      jvmti_state |= JVMTI_THREAD_STATE_IN_OBJECT_WAIT;
    }

    // TODO: PARKED. We'll have to inspect the stack.
  }

  return jvmti_state;
}

static jint GetJavaStateFromInternal(art::ThreadState internal_thread_state) {
  switch (internal_thread_state) {
    case art::ThreadState::kTerminated:
      return JVMTI_JAVA_LANG_THREAD_STATE_TERMINATED;

    case art::ThreadState::kRunnable:
    case art::ThreadState::kNative:
    case art::ThreadState::kWaitingWeakGcRootRead:
    case art::ThreadState::kSuspended:
      return JVMTI_JAVA_LANG_THREAD_STATE_RUNNABLE;

    case art::ThreadState::kTimedWaiting:
    case art::ThreadState::kSleeping:
      return JVMTI_JAVA_LANG_THREAD_STATE_TIMED_WAITING;

    case art::ThreadState::kBlocked:
      return JVMTI_JAVA_LANG_THREAD_STATE_BLOCKED;

    case art::ThreadState::kStarting:
      return JVMTI_JAVA_LANG_THREAD_STATE_NEW;

    case art::ThreadState::kWaiting:
    case art::ThreadState::kWaitingForGcToComplete:
    case art::ThreadState::kWaitingPerformingGc:
    case art::ThreadState::kWaitingForCheckPointsToRun:
    case art::ThreadState::kWaitingForDebuggerSend:
    case art::ThreadState::kWaitingForDebuggerToAttach:
    case art::ThreadState::kWaitingInMainDebuggerLoop:
    case art::ThreadState::kWaitingForDebuggerSuspension:
    case art::ThreadState::kWaitingForDeoptimization:
    case art::ThreadState::kWaitingForGetObjectsAllocated:
    case art::ThreadState::kWaitingForJniOnLoad:
    case art::ThreadState::kWaitingForSignalCatcherOutput:
    case art::ThreadState::kWaitingInMainSignalCatcherLoop:
    case art::ThreadState::kWaitingForMethodTracingStart:
    case art::ThreadState::kWaitingForVisitObjects:
    case art::ThreadState::kWaitingForGcThreadFlip:
      return JVMTI_JAVA_LANG_THREAD_STATE_WAITING;
  }
  LOG(FATAL) << "Unreachable";
  UNREACHABLE();
}

jvmtiError ThreadUtil::GetThreadState(jvmtiEnv* env ATTRIBUTE_UNUSED,
                                      jthread thread,
                                      jint* thread_state_ptr) {
  if (thread_state_ptr == nullptr) {
    return ERR(NULL_POINTER);
  }

  art::ScopedObjectAccess soa(art::Thread::Current());
  art::Thread* native_thread = nullptr;
  art::ThreadState internal_thread_state = GetNativeThreadState(thread, soa, &native_thread);

  if (internal_thread_state == art::ThreadState::kStarting) {
    if (thread == nullptr) {
      // No native thread, and no Java thread? We must be starting up. Report as wrong phase.
      return ERR(WRONG_PHASE);
    }

    // Need to read the Java "started" field to know whether this is starting or terminated.
    art::ObjPtr<art::mirror::Object> peer = soa.Decode<art::mirror::Object>(thread);
    art::ObjPtr<art::mirror::Class> klass = peer->GetClass();
    art::ArtField* started_field = klass->FindDeclaredInstanceField("started", "Z");
    CHECK(started_field != nullptr);
    bool started = started_field->GetBoolean(peer) != 0;
    constexpr jint kStartedState = JVMTI_JAVA_LANG_THREAD_STATE_NEW;
    constexpr jint kTerminatedState = JVMTI_THREAD_STATE_TERMINATED |
                                      JVMTI_JAVA_LANG_THREAD_STATE_TERMINATED;
    *thread_state_ptr = started ? kTerminatedState : kStartedState;
    return ERR(NONE);
  }
  DCHECK(native_thread != nullptr);

  // Translate internal thread state to JVMTI and Java state.
  jint jvmti_state = GetJvmtiThreadStateFromInternal(internal_thread_state);
  if (native_thread->IsInterrupted()) {
    jvmti_state |= JVMTI_THREAD_STATE_INTERRUPTED;
  }
  if (native_thread->IsSuspended()) {
    jvmti_state |= JVMTI_THREAD_STATE_SUSPENDED;
  }

  // Java state is derived from nativeGetState.
  // Note: Our implementation assigns "runnable" to suspended. As such, we will have slightly
  //       different mask. However, this is for consistency with the Java view.
  jint java_state = GetJavaStateFromInternal(internal_thread_state);

  *thread_state_ptr = jvmti_state | java_state;

  return ERR(NONE);
}

jvmtiError ThreadUtil::GetAllThreads(jvmtiEnv* env,
                                     jint* threads_count_ptr,
                                     jthread** threads_ptr) {
  if (threads_count_ptr == nullptr || threads_ptr == nullptr) {
    return ERR(NULL_POINTER);
  }

  art::Thread* current = art::Thread::Current();

  art::ScopedObjectAccess soa(current);

  art::MutexLock mu(current, *art::Locks::thread_list_lock_);
  std::list<art::Thread*> thread_list = art::Runtime::Current()->GetThreadList()->GetList();

  std::vector<art::ObjPtr<art::mirror::Object>> peers;

  for (art::Thread* thread : thread_list) {
    // Skip threads that are still starting.
    if (thread->IsStillStarting()) {
      continue;
    }

    art::ObjPtr<art::mirror::Object> peer = thread->GetPeerFromOtherThread();
    if (peer != nullptr) {
      peers.push_back(peer);
    }
  }

  if (peers.empty()) {
    *threads_count_ptr = 0;
    *threads_ptr = nullptr;
  } else {
    unsigned char* data;
    jvmtiError data_result = env->Allocate(peers.size() * sizeof(jthread), &data);
    if (data_result != ERR(NONE)) {
      return data_result;
    }
    jthread* threads = reinterpret_cast<jthread*>(data);
    for (size_t i = 0; i != peers.size(); ++i) {
      threads[i] = soa.AddLocalReference<jthread>(peers[i]);
    }

    *threads_count_ptr = static_cast<jint>(peers.size());
    *threads_ptr = threads;
  }
  return ERR(NONE);
}

jvmtiError ThreadUtil::SetThreadLocalStorage(jvmtiEnv* env ATTRIBUTE_UNUSED,
                                             jthread thread,
                                             const void* data) {
  art::ScopedObjectAccess soa(art::Thread::Current());
  art::Thread* self = GetNativeThread(thread, soa);
  if (self == nullptr && thread == nullptr) {
    return ERR(INVALID_THREAD);
  }
  if (self == nullptr) {
    return ERR(THREAD_NOT_ALIVE);
  }

  self->SetCustomTLS(data);

  return ERR(NONE);
}

jvmtiError ThreadUtil::GetThreadLocalStorage(jvmtiEnv* env ATTRIBUTE_UNUSED,
                                             jthread thread,
                                             void** data_ptr) {
  if (data_ptr == nullptr) {
    return ERR(NULL_POINTER);
  }

  art::ScopedObjectAccess soa(art::Thread::Current());
  art::Thread* self = GetNativeThread(thread, soa);
  if (self == nullptr && thread == nullptr) {
    return ERR(INVALID_THREAD);
  }
  if (self == nullptr) {
    return ERR(THREAD_NOT_ALIVE);
  }

  *data_ptr = const_cast<void*>(self->GetCustomTLS());
  return ERR(NONE);
}

struct AgentData {
  const void* arg;
  jvmtiStartFunction proc;
  jthread thread;
  JavaVM* java_vm;
  jvmtiEnv* jvmti_env;
  jint priority;
};

static void* AgentCallback(void* arg) {
  std::unique_ptr<AgentData> data(reinterpret_cast<AgentData*>(arg));
  CHECK(data->thread != nullptr);

  // We already have a peer. So call our special Attach function.
  art::Thread* self = art::Thread::Attach("JVMTI Agent thread", true, data->thread);
  CHECK(self != nullptr);
  // The name in Attach() is only for logging. Set the thread name. This is important so
  // that the thread is no longer seen as starting up.
  {
    art::ScopedObjectAccess soa(self);
    self->SetThreadName("JVMTI Agent thread");
  }

  // Release the peer.
  JNIEnv* env = self->GetJniEnv();
  env->DeleteGlobalRef(data->thread);
  data->thread = nullptr;

  // Run the agent code.
  data->proc(data->jvmti_env, env, const_cast<void*>(data->arg));

  // Detach the thread.
  int detach_result = data->java_vm->DetachCurrentThread();
  CHECK_EQ(detach_result, 0);

  return nullptr;
}

jvmtiError ThreadUtil::RunAgentThread(jvmtiEnv* jvmti_env,
                                      jthread thread,
                                      jvmtiStartFunction proc,
                                      const void* arg,
                                      jint priority) {
  if (priority < JVMTI_THREAD_MIN_PRIORITY || priority > JVMTI_THREAD_MAX_PRIORITY) {
    return ERR(INVALID_PRIORITY);
  }
  JNIEnv* env = art::Thread::Current()->GetJniEnv();
  if (thread == nullptr || !env->IsInstanceOf(thread, art::WellKnownClasses::java_lang_Thread)) {
    return ERR(INVALID_THREAD);
  }
  if (proc == nullptr) {
    return ERR(NULL_POINTER);
  }

  std::unique_ptr<AgentData> data(new AgentData);
  data->arg = arg;
  data->proc = proc;
  // We need a global ref for Java objects, as local refs will be invalid.
  data->thread = env->NewGlobalRef(thread);
  data->java_vm = art::Runtime::Current()->GetJavaVM();
  data->jvmti_env = jvmti_env;
  data->priority = priority;

  pthread_t pthread;
  int pthread_create_result = pthread_create(&pthread,
                                             nullptr,
                                             &AgentCallback,
                                             reinterpret_cast<void*>(data.get()));
  if (pthread_create_result != 0) {
    return ERR(INTERNAL);
  }
  data.release();

  return ERR(NONE);
}

// Suspends the current thread if it has any suspend requests on it.
static void SuspendCheck(art::Thread* self)
    REQUIRES(!art::Locks::mutator_lock_, !art::Locks::user_code_suspension_lock_) {
  art::ScopedObjectAccess soa(self);
  // Really this is only needed if we are in FastJNI and actually have the mutator_lock_ already.
  self->FullSuspendCheck();
}

jvmtiError ThreadUtil::SuspendOther(art::Thread* self,
                                    jthread target_jthread,
                                    art::Thread* target) {
  // Loop since we need to bail out and try again if we would end up getting suspended while holding
  // the user_code_suspension_lock_ due to a SuspendReason::kForUserCode. In this situation we
  // release the lock, wait to get resumed and try again.
  do {
    // Suspend ourself if we have any outstanding suspends. This is so we won't suspend due to
    // another SuspendThread in the middle of suspending something else potentially causing a
    // deadlock. We need to do this in the loop because if we ended up back here then we had
    // outstanding SuspendReason::kForUserCode suspensions and we should wait for them to be cleared
    // before continuing.
    SuspendCheck(self);
    art::MutexLock mu(self, *art::Locks::user_code_suspension_lock_);
    {
      art::MutexLock thread_list_mu(self, *art::Locks::thread_suspend_count_lock_);
      // Make sure we won't be suspended in the middle of holding the thread_suspend_count_lock_ by
      // a user-code suspension. We retry and do another SuspendCheck to clear this.
      if (self->GetUserCodeSuspendCount() != 0) {
        continue;
      } else if (target->GetUserCodeSuspendCount() != 0) {
        return ERR(THREAD_SUSPENDED);
      }
    }
    bool timeout = true;
    while (timeout) {
      art::ThreadState state = target->GetState();
      if (state == art::ThreadState::kTerminated || state == art::ThreadState::kStarting) {
        return ERR(THREAD_NOT_ALIVE);
      }
      target = art::Runtime::Current()->GetThreadList()->SuspendThreadByPeer(
          target_jthread,
          /* request_suspension */ true,
          art::SuspendReason::kForUserCode,
          &timeout);
      if (target == nullptr && !timeout) {
        // TODO It would be good to get more information about why exactly the thread failed to
        // suspend.
        return ERR(INTERNAL);
      }
    }
    return OK;
  } while (true);
  UNREACHABLE();
}

jvmtiError ThreadUtil::SuspendSelf(art::Thread* self) {
  CHECK(self == art::Thread::Current());
  {
    art::MutexLock mu(self, *art::Locks::user_code_suspension_lock_);
    art::MutexLock thread_list_mu(self, *art::Locks::thread_suspend_count_lock_);
    if (self->GetUserCodeSuspendCount() != 0) {
      // This can only happen if we race with another thread to suspend 'self' and we lose.
      return ERR(THREAD_SUSPENDED);
    }
    // We shouldn't be able to fail this.
    if (!self->ModifySuspendCount(self, +1, nullptr, art::SuspendReason::kForUserCode)) {
      // TODO More specific error would be nice.
      return ERR(INTERNAL);
    }
  }
  // Once we have requested the suspend we actually go to sleep. We need to do this after releasing
  // the suspend_lock to make sure we can be woken up. This call gains the mutator lock causing us
  // to go to sleep until we are resumed.
  SuspendCheck(self);
  return OK;
}

jvmtiError ThreadUtil::SuspendThread(jvmtiEnv* env ATTRIBUTE_UNUSED, jthread thread) {
  art::Thread* self = art::Thread::Current();
  art::Thread* target;
  {
    art::ScopedObjectAccess soa(self);
    target = GetNativeThread(thread, soa);
  }
  if (target == nullptr) {
    return ERR(INVALID_THREAD);
  }
  if (target == self) {
    return SuspendSelf(self);
  } else {
    return SuspendOther(self, thread, target);
  }
}

jvmtiError ThreadUtil::ResumeThread(jvmtiEnv* env ATTRIBUTE_UNUSED,
                                    jthread thread) {
  if (thread == nullptr) {
    return ERR(NULL_POINTER);
  }
  art::Thread* self = art::Thread::Current();
  art::Thread* target;
  {
    // NB This does a SuspendCheck (during thread state change) so we need to make sure we don't
    // have the 'suspend_lock' locked here.
    art::ScopedObjectAccess soa(self);
    target = GetNativeThread(thread, soa);
  }
  if (target == nullptr) {
    return ERR(INVALID_THREAD);
  } else if (target == self) {
    // We would have paused until we aren't suspended anymore due to the ScopedObjectAccess so we
    // can just return THREAD_NOT_SUSPENDED. Unfortunately we cannot do any real DCHECKs about
    // current state since it's all concurrent.
    return ERR(THREAD_NOT_SUSPENDED);
  }
  // Now that we know we aren't getting suspended ourself (since we have a mutator lock) we lock the
  // suspend_lock to start suspending.
  art::MutexLock mu(self, *art::Locks::user_code_suspension_lock_);
  {
    // The JVMTI spec requires us to return THREAD_NOT_SUSPENDED if it is alive but we really cannot
    // tell why resume failed.
    art::MutexLock thread_list_mu(self, *art::Locks::thread_suspend_count_lock_);
    if (target->GetUserCodeSuspendCount() == 0) {
      return ERR(THREAD_NOT_SUSPENDED);
    }
  }
  if (target->GetState() == art::ThreadState::kTerminated) {
    return ERR(THREAD_NOT_ALIVE);
  }
  DCHECK(target != self);
  if (!art::Runtime::Current()->GetThreadList()->Resume(target, art::SuspendReason::kForUserCode)) {
    // TODO Give a better error.
    // This is most likely THREAD_NOT_SUSPENDED but we cannot really be sure.
    return ERR(INTERNAL);
  }
  return OK;
}

// Suspends all the threads in the list at the same time. Getting this behavior is a little tricky
// since we can have threads in the list multiple times. This generally doesn't matter unless the
// current thread is present multiple times. In that case we need to suspend only once and either
// return the same error code in all the other slots if it failed or return ERR(THREAD_SUSPENDED) if
// it didn't. We also want to handle the current thread last to make the behavior of the code
// simpler to understand.
jvmtiError ThreadUtil::SuspendThreadList(jvmtiEnv* env,
                                         jint request_count,
                                         const jthread* threads,
                                         jvmtiError* results) {
  if (request_count == 0) {
    return ERR(ILLEGAL_ARGUMENT);
  } else if (results == nullptr || threads == nullptr) {
    return ERR(NULL_POINTER);
  }
  // This is the list of the indexes in 'threads' and 'results' that correspond to the currently
  // running thread. These indexes we need to handle specially since we need to only actually
  // suspend a single time.
  std::vector<jint> current_thread_indexes;
  art::Thread* self = art::Thread::Current();
  for (jint i = 0; i < request_count; i++) {
    {
      art::ScopedObjectAccess soa(self);
      if (threads[i] == nullptr || GetNativeThread(threads[i], soa) == self) {
        current_thread_indexes.push_back(i);
        continue;
      }
    }
    results[i] = env->SuspendThread(threads[i]);
  }
  if (!current_thread_indexes.empty()) {
    jint first_current_thread_index = current_thread_indexes[0];
    // Suspend self.
    jvmtiError res = env->SuspendThread(threads[first_current_thread_index]);
    results[first_current_thread_index] = res;
    // Fill in the rest of the error values as appropriate.
    jvmtiError other_results = (res != OK) ? res : ERR(THREAD_SUSPENDED);
    for (auto it = ++current_thread_indexes.begin(); it != current_thread_indexes.end(); ++it) {
      results[*it] = other_results;
    }
  }
  return OK;
}

jvmtiError ThreadUtil::ResumeThreadList(jvmtiEnv* env,
                                        jint request_count,
                                        const jthread* threads,
                                        jvmtiError* results) {
  if (request_count == 0) {
    return ERR(ILLEGAL_ARGUMENT);
  } else if (results == nullptr || threads == nullptr) {
    return ERR(NULL_POINTER);
  }
  for (jint i = 0; i < request_count; i++) {
    results[i] = env->ResumeThread(threads[i]);
  }
  return OK;
}

}  // namespace openjdkjvmti