Wait for concurrent unmapping tasks in GC prologue.

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index accae68a1260ce53ba5fe2b83f996e5633b2d7ee..96d0a00c863db54de117a6652c135fea46e7cd6f 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -137,7 +137,8 @@ Heap::Heap()
       current_gc_callback_flags_(GCCallbackFlags::kNoGCCallbackFlags),
       external_string_table_(this),
       chunks_queued_for_free_(NULL),
-      pending_unmap_job_semaphore_(0),
+      concurrent_unmaping_tasks_active_(0),
+      pending_unmap_tasks_semaphore_(0),
       gc_callbacks_depth_(0),
       deserialization_complete_(false),
       concurrent_sweeping_enabled_(false),
@@ -6524,7 +6525,7 @@ class Heap::UnmapFreeMemoryTask : public v8::Task {
   // v8::Task overrides.
   void Run() override {
     heap_->FreeQueuedChunks(head_);
-    heap_->pending_unmap_job_semaphore_.Signal();
+    heap_->pending_unmap_tasks_semaphore_.Signal();
   }
 
   Heap* heap_;
@@ -6536,8 +6537,13 @@ class Heap::UnmapFreeMemoryTask : public v8::Task {
 
 void Heap::WaitUntilUnmappingOfFreeChunksCompleted() {
   // We start an unmap job after sweeping and after compaction.
-  pending_unmap_job_semaphore_.Wait();
-  pending_unmap_job_semaphore_.Wait();
+  if (concurrent_unmaping_tasks_active_ > 0) {

[Michael Lippautz, 2015/08/27 11:37:41]

We could make this more general, so that we can start as many unmapping
tasks as we like:

  while(concurrent_unmapping_tasks_active_ > 0) {
    pending_unmap_tasks_semaphore_.Wait();
    concurrent_unmapping_tasks_active_--;
  }

wdyt?

[Hannes Payer (out of office), 2015/08/27 12:34:37]

The intention was to assert if we are waiting for an unexpected number of tasks.
But that may be a bit too static. I will change it to what you proposed. Done.

+    // There should be two concurrent unmapping tasks running.
+    DCHECK(concurrent_unmaping_tasks_active_ == 2);
+    pending_unmap_tasks_semaphore_.Wait();
+    pending_unmap_tasks_semaphore_.Wait();
+    concurrent_unmaping_tasks_active_ = 0;
+  }
 }
 
 
@@ -6574,8 +6580,9 @@ void Heap::FreeQueuedChunks() {
   } else {
     // If we do not have anything to unmap, we just signal the semaphore
     // that we are done.
-    pending_unmap_job_semaphore_.Signal();
+    pending_unmap_tasks_semaphore_.Signal();
   }
+  concurrent_unmaping_tasks_active_++;
 }