Revert "Preempt ConcurrentMarking tasks instead of merely pausing in PauseScope."

This reverts commit e9750cb8.

Reason for revert: Several GC failures, e.g. https://build.chromium.org/p/client.v8/builders/V8%20Linux/builds/23236, https://build.chromium.org/p/client.v8/builders/V8%20Mac/builds/18390 

Original change's description:
> Preempt ConcurrentMarking tasks instead of merely pausing in PauseScope.
> 
> Follow-up to https://chromium-review.googlesource.com/c/v8/v8/+/924867
> 
> This is the core goal of the initial CL @
> https://chromium-review.googlesource.com/c/v8/v8/+/922103
> which was since split into multiple to diagnose a bots-only failure.
> 
> Bug: chromium:812178
> Change-Id: I4c4e0b517737e020862917bd89fa6ce38244e597
> Reviewed-on: https://chromium-review.googlesource.com/924031
> Commit-Queue: Gabriel Charette <gab@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#51356}

TBR=gab@chromium.org,ulan@chromium.org,mlippautz@chromium.org

Change-Id: Ic095e32708e58acbe5955bf29e65af34c59d321e
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:812178
Reviewed-on: https://chromium-review.googlesource.com/925301Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51363}

src/heap/concurrent-marking.cc

@@ -436,8 +436,13 @@ void ConcurrentMarking::Run(int task_id, TaskState* task_state) {
                       GCTracer::BackgroundScope::MC_BACKGROUND_MARKING);
   size_t kBytesUntilInterruptCheck = 64 * KB;
   int kObjectsUntilInterrupCheck = 1000;
-  ConcurrentMarkingVisitor visitor(shared_, bailout_, &task_state->live_bytes,
-                                   weak_objects_, task_id);
+  LiveBytesMap* live_bytes = nullptr;
+  {
+    base::LockGuard<base::Mutex> guard(&task_state->lock);
+    live_bytes = &task_state->live_bytes;
+  }
+  ConcurrentMarkingVisitor visitor(shared_, bailout_, live_bytes, weak_objects_,
+                                   task_id);
   double time_ms;
   size_t marked_bytes = 0;
   if (FLAG_trace_concurrent_marking) {
@@ -446,7 +451,8 @@ void ConcurrentMarking::Run(int task_id, TaskState* task_state) {
   }
   {
     TimedScope scope(&time_ms);
-
+    {
+      base::LockGuard<base::Mutex> guard(&task_state->lock);
       bool done = false;
       while (!done) {
         size_t current_marked_bytes = 0;
@@ -472,15 +478,21 @@ void ConcurrentMarking::Run(int task_id, TaskState* task_state) {
         marked_bytes += current_marked_bytes;
         base::AsAtomicWord::Relaxed_Store<size_t>(&task_state->marked_bytes,
                                                   marked_bytes);
-      if (task_state->preemption_request.Value()) {
+        if (task_state->interrupt_request.Value()) {
+          TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.gc"),
+                       "ConcurrentMarking::Run Paused");
+          task_state->interrupt_condition.Wait(&task_state->lock);
+        } else if (task_state->preemption_request.Value()) {
           TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.gc"),
                        "ConcurrentMarking::Run Preempted");
           break;
         }
       }
+      // The lock is also required to synchronize with worklist update after
+      // young generation GC.
       bailout_->FlushToGlobal(task_id);
       on_hold_->FlushToGlobal(task_id);
-
+    }
     weak_objects_->weak_cells.FlushToGlobal(task_id);
     weak_objects_->transition_arrays.FlushToGlobal(task_id);
     base::AsAtomicWord::Relaxed_Store<size_t>(&task_state->marked_bytes, 0);
@@ -522,6 +534,7 @@ void ConcurrentMarking::ScheduleTasks() {
             "Scheduling concurrent marking task %d\n", i);
       }
       task_state_[i].preemption_request.SetValue(false);
+      task_state_[i].interrupt_request.SetValue(false);
       is_pending_[i] = true;
       ++pending_task_count_;
       Task* task = new Task(heap_->isolate(), this, &task_state_[i], i);
@@ -610,12 +623,23 @@ size_t ConcurrentMarking::TotalMarkedBytes() {
 ConcurrentMarking::PauseScope::PauseScope(ConcurrentMarking* concurrent_marking)
     : concurrent_marking_(concurrent_marking) {
   if (!FLAG_concurrent_marking) return;
-  concurrent_marking_->Stop(ConcurrentMarking::StopRequest::PREEMPT_TASKS);
+  // Request task_state for all tasks.
+  for (int i = 1; i <= kMaxTasks; i++) {
+    concurrent_marking_->task_state_[i].interrupt_request.SetValue(true);
+  }
+  // Now take a lock to ensure that the tasks are waiting.
+  for (int i = 1; i <= kMaxTasks; i++) {
+    concurrent_marking_->task_state_[i].lock.Lock();
+  }
 }
 
 ConcurrentMarking::PauseScope::~PauseScope() {
   if (!FLAG_concurrent_marking) return;
-  concurrent_marking_->RescheduleTasksIfNeeded();
+  for (int i = kMaxTasks; i >= 1; i--) {
+    concurrent_marking_->task_state_[i].interrupt_request.SetValue(false);
+    concurrent_marking_->task_state_[i].interrupt_condition.NotifyAll();
+    concurrent_marking_->task_state_[i].lock.Unlock();
+  }
 }
 
 }  // namespace internal

src/heap/concurrent-marking.h

@@ -30,8 +30,7 @@ using LiveBytesMap =
 class ConcurrentMarking {
  public:
   // When the scope is entered, the concurrent marking tasks
-  // are preempted and are not looking at the heap objects, concurrent marking
-  // is resumed when the scope is exited.
+  // are paused and are not looking at the heap objects.
   class PauseScope {
    public:
     explicit PauseScope(ConcurrentMarking* concurrent_marking);
@@ -82,6 +81,16 @@ class ConcurrentMarking {
     // marker to give up the worker thread.
     base::AtomicValue<bool> preemption_request;
 
+    // When the concurrent marking task has this lock, then objects in the
+    // heap are guaranteed to not move.
+    base::Mutex lock;
+    // The main thread sets this flag to true, when it wants the concurrent
+    // maker to give up the lock.
+    base::AtomicValue<bool> interrupt_request;
+    // The concurrent marker waits on this condition until the request
+    // flag is cleared by the main thread.
+    base::ConditionVariable interrupt_condition;
+
     LiveBytesMap live_bytes;
     size_t marked_bytes = 0;
     char cache_line_padding[64];