Instrument callers of Semaphore::Signal to help with investigation of

src/heap/spaces.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/heap/spaces.h"
 
 #include "src/base/bits.h"
 #include "src/base/platform/platform.h"
 #include "src/base/platform/semaphore.h"
 #include "src/full-codegen/full-codegen.h"
@@ skipping 338 matching lines @@
 }
 
 class MemoryAllocator::Unmapper::UnmapFreeMemoryTask : public v8::Task {
  public:
   explicit UnmapFreeMemoryTask(Unmapper* unmapper) : unmapper_(unmapper) {}
 
  private:
   // v8::Task overrides.
   void Run() override {
     unmapper_->PerformFreeMemoryOnQueuedChunks();
-    unmapper_->pending_unmapping_tasks_semaphore_.Signal();
+    unmapper_->pending_unmapping_tasks_semaphore_.Signal(
+        "Unmapper::UnmapFreeMemoryTask::Run");
   }
 
   Unmapper* unmapper_;
   DISALLOW_COPY_AND_ASSIGN(UnmapFreeMemoryTask);
 };
 
 void MemoryAllocator::Unmapper::FreeQueuedChunks() {
   if (FLAG_concurrent_sweeping) {
     V8::GetCurrentPlatform()->CallOnBackgroundThread(
         new UnmapFreeMemoryTask(this), v8::Platform::kShortRunningTask);
@@ skipping 2851 matching lines @@
     object->ShortPrint();
     PrintF("\n");
   }
   printf(" --------------------------------------\n");
   printf(" Marked: %x, LiveCount: %x\n", mark_size, LiveBytes());
 }
 
 #endif  // DEBUG
 }  // namespace internal
 }  // namespace v8