Make tree id sequence in LayerTreeHost thread-safe.

cc/trees/layer_tree_host.cc

Index: cc/trees/layer_tree_host.cc
diff --git a/cc/trees/layer_tree_host.cc b/cc/trees/layer_tree_host.cc
index b0eed2f63205e01af80d600a73218404ec108179..c14736a939ed6f9fb213dbbca516fb4e8b776c0b 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -11,10 +11,12 @@
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/debug/trace_event.h"
+#include "base/lazy_instance.h"
 #include "base/message_loop/message_loop.h"
 #include "base/metrics/histogram.h"
 #include "base/stl_util.h"
 #include "base/strings/string_number_conversions.h"
+#include "base/synchronization/lock.h"
 #include "cc/animation/animation_registrar.h"
 #include "cc/animation/layer_animation_controller.h"
 #include "cc/base/math_util.h"
@@ -41,7 +43,14 @@
 #include "ui/gfx/size_conversions.h"
 
 namespace {
-static int s_num_layer_tree_instances;
+static base::LazyInstance<base::Lock>::Leaky
+    s_next_tree_id_lock = LAZY_INSTANCE_INITIALIZER;
+
+inline int GetNextTreeId() {
+  static int s_next_tree_id = 1;
+  base::AutoLock lock(s_next_tree_id_lock.Get());

[caseq, 2013/11/07 09:20:03]

Any reason to not use StaticAtomicSequenceNumber here?

[dshwang, 2013/11/07 09:46:03]

Hi, Patch Set 1 used AtomicSequenceNumber, but AtomicSequenceNumber is
initialized to 0, so there was additional code to set 1 to it.
It looked a bit complicated, and this is very rare called. so we decided to use
lock.

+  return s_next_tree_id++;
+}
 }
 
 namespace cc {
@@ -89,10 +98,6 @@ UIResourceRequest& UIResourceRequest::operator=(
 
 UIResourceRequest::~UIResourceRequest() {}
 
-bool LayerTreeHost::AnyLayerTreeHostInstanceExists() {
-  return s_num_layer_tree_instances > 0;
-}
-
 scoped_ptr<LayerTreeHost> LayerTreeHost::Create(
     LayerTreeHostClient* client,
     SharedBitmapManager* manager,
@@ -105,8 +110,6 @@ scoped_ptr<LayerTreeHost> LayerTreeHost::Create(
   return layer_tree_host.Pass();
 }
 
-static int s_next_tree_id = 1;
-
 LayerTreeHost::LayerTreeHost(LayerTreeHostClient* client,
                              SharedBitmapManager* manager,
                              const LayerTreeSettings& settings)
@@ -135,12 +138,11 @@ LayerTreeHost::LayerTreeHost(LayerTreeHostClient* client,
       partial_texture_update_requests_(0),
       in_paint_layer_contents_(false),
       total_frames_used_for_lcd_text_metrics_(0),
-      tree_id_(s_next_tree_id++),
+      tree_id_(GetNextTreeId()),
       next_commit_forces_redraw_(false),
       shared_bitmap_manager_(manager) {
   if (settings_.accelerated_animation_enabled)
     animation_registrar_ = AnimationRegistrar::Create();
-  s_num_layer_tree_instances++;
   rendering_stats_instrumentation_->set_record_rendering_stats(
       debug_state_.RecordRenderingStats());
 }
@@ -182,8 +184,6 @@ LayerTreeHost::~LayerTreeHost() {
     proxy_->Stop();
   }
 
-  s_num_layer_tree_instances--;
-
   if (root_layer_.get()) {
     // The layer tree must be destroyed before the layer tree host. We've
     // made a contract with our animation controllers that the registrar