PPAPI: Remove threading options; it's always on

This also re-enables thread checking for the host side resource and var trackers. Before, checking was disabled everywhere.

BUG=159240,92909


Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=186925
Reverted: https://src.chromium.org/viewvc/chrome?view=rev&revision=186939 due to build errors

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=187340

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=187427
Reverted: https://src.chromium.org/viewvc/chrome?view=rev&revision=187668 due to a failing check in Canary, which was fixed here: https://src.chromium.org/viewvc/chrome?view=rev&revision=187681

Review URL: https://chromiumcodereview.appspot.com/12378050

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@189518 0039d316-1c4b-4281-b951-d872f2087c98

33 files changed, 261 insertions, 227 deletions

diff --git a/build/common.gypi b/build/common.gypi
index 1f4e9fa..f267679 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -322,11 +322,6 @@
       # Webrtc compilation is enabled by default. Set to 0 to disable.
       'enable_webrtc%': 1,
 
-      # PPAPI by default does not support plugins making calls off the main
-      # thread. Set to 1 to turn on experimental support for out-of-process
-      # plugins to make call of the main thread.
-      'enable_pepper_threading%': 1,
-
       # Enables use of the session service, which is enabled by default.
       # Support for disabling depends on the platform.
       'enable_session_service%': 1,
@@ -680,7 +675,6 @@
     'use_x11%': '<(use_x11)',
     'use_gnome_keyring%': '<(use_gnome_keyring)',
     'linux_fpic%': '<(linux_fpic)',
-    'enable_pepper_threading%': '<(enable_pepper_threading)',
     'chromeos%': '<(chromeos)',
     'enable_viewport%': '<(enable_viewport)',
     'enable_hidpi%': '<(enable_hidpi)',
@@ -1783,9 +1777,6 @@
       ['proprietary_codecs==1', {
         'defines': ['USE_PROPRIETARY_CODECS'],
       }],
-      ['enable_pepper_threading==1', {
-        'defines': ['ENABLE_PEPPER_THREADING'],
-      }],
       ['enable_viewport==1', {
         'defines': ['ENABLE_VIEWPORT'],
       }],
diff --git a/content/browser/ppapi_plugin_process_host.cc b/content/browser/ppapi_plugin_process_host.cc
index 236477a..e91b2b4 100644
--- a/content/browser/ppapi_plugin_process_host.cc
+++ b/content/browser/ppapi_plugin_process_host.cc
@@ -275,10 +275,9 @@ bool PpapiPluginProcessHost::Init(const PepperPluginInfo& info) {
                              arraysize(kCommonForwardSwitches));
 
   if (!is_broker_) {
-    // TODO(vtl): Stop passing flash args in the command line, on windows is
+    // TODO(vtl): Stop passing flash args in the command line, or windows is
     // going to explode.
     static const char* kPluginForwardSwitches[] = {
-      switches::kDisablePepperThreading,
       switches::kDisableSeccompFilterSandbox,
 #if defined(OS_MACOSX)
       switches::kEnableSandboxLogging,
diff --git a/content/ppapi_plugin/ppapi_thread.cc b/content/ppapi_plugin/ppapi_thread.cc
index e0bfb78..0f718af 100644
--- a/content/ppapi_plugin/ppapi_thread.cc
+++ b/content/ppapi_plugin/ppapi_thread.cc
@@ -93,8 +93,6 @@ PpapiThread::PpapiThread(const CommandLine& command_line, bool is_broker)
   globals->set_plugin_proxy_delegate(this);
   globals->set_command_line(
       command_line.GetSwitchValueASCII(switches::kPpapiFlashArgs));
-  globals->set_enable_threading(
-      !command_line.HasSwitch(switches::kDisablePepperThreading));
 
   webkit_platform_support_.reset(new PpapiWebKitPlatformSupportImpl);
   WebKit::initialize(webkit_platform_support_.get());
diff --git a/ppapi/ppapi_ipc_untrusted.gyp b/ppapi/ppapi_ipc_untrusted.gyp
index fd55a58..45ffd0f 100644
--- a/ppapi/ppapi_ipc_untrusted.gyp
+++ b/ppapi/ppapi_ipc_untrusted.gyp
@@ -23,11 +23,6 @@
             'nlib_target': 'libppapi_ipc_untrusted.a',
             'build_glibc': 0,
             'build_newlib': 1,
-            'defines': [
-              # Enable threading for the untrusted side of the proxy.
-              # TODO(bbudge) remove when this is the default.
-              'ENABLE_PEPPER_THREADING',
-            ],
           },
           'include_dirs': [
             '..',
diff --git a/ppapi/ppapi_proxy.gypi b/ppapi/ppapi_proxy.gypi
index 838bbc6..cdc222a 100644
--- a/ppapi/ppapi_proxy.gypi
+++ b/ppapi/ppapi_proxy.gypi
@@ -69,6 +69,7 @@
           'proxy/interface_list.h',
           'proxy/interface_proxy.cc',
           'proxy/interface_proxy.h',
+          'proxy/locking_resource_releaser.h',
           'proxy/plugin_array_buffer_var.cc',
           'proxy/plugin_array_buffer_var.h',
           'proxy/plugin_dispatcher.cc',
diff --git a/ppapi/ppapi_proxy_untrusted.gyp b/ppapi/ppapi_proxy_untrusted.gyp
index 49fae82..2b87d7b 100644
--- a/ppapi/ppapi_proxy_untrusted.gyp
+++ b/ppapi/ppapi_proxy_untrusted.gyp
@@ -22,11 +22,6 @@
             'nlib_target': 'libppapi_proxy_untrusted.a',
             'build_glibc': 0,
             'build_newlib': 1,
-            'defines': [
-              # Enable threading for the untrusted side of the proxy.
-              # TODO(bbudge) remove when this is the default.
-              'ENABLE_PEPPER_THREADING',
-            ],
           },
           'include_dirs': [
             '..',
diff --git a/ppapi/ppapi_shared_untrusted.gyp b/ppapi/ppapi_shared_untrusted.gyp
index e085f60..2c0f8bd 100644
--- a/ppapi/ppapi_shared_untrusted.gyp
+++ b/ppapi/ppapi_shared_untrusted.gyp
@@ -23,11 +23,6 @@
             'nlib_target': 'libppapi_shared_untrusted.a',
             'build_glibc': 0,
             'build_newlib': 1,
-            'defines': [
-              # Enable threading for the untrusted side of the proxy.
-              # TODO(bbudge) remove when this is the default.
-              'ENABLE_PEPPER_THREADING',
-            ],
           },
           'include_dirs': [
             '..',
diff --git a/ppapi/proxy/device_enumeration_resource_helper_unittest.cc b/ppapi/proxy/device_enumeration_resource_helper_unittest.cc
index 330bca3..d578a9e 100644
--- a/ppapi/proxy/device_enumeration_resource_helper_unittest.cc
+++ b/ppapi/proxy/device_enumeration_resource_helper_unittest.cc
@@ -14,6 +14,7 @@
 #include "ppapi/proxy/ppapi_messages.h"
 #include "ppapi/proxy/ppapi_proxy_test.h"
 #include "ppapi/shared_impl/ppb_device_ref_shared.h"
+#include "ppapi/shared_impl/proxy_lock.h"
 #include "ppapi/shared_impl/var.h"
 #include "ppapi/thunk/enter.h"
 #include "ppapi/thunk/ppb_device_ref_api.h"
@@ -37,7 +38,7 @@ Connection GetConnection(PluginProxyTestHarness* harness) {
 bool CompareDeviceRef(PluginVarTracker* var_tracker,
                       PP_Resource resource,
                       const DeviceRefData& expected) {
-  thunk::EnterResource<thunk::PPB_DeviceRef_API> enter(resource, true);
+  thunk::EnterResourceNoLock<thunk::PPB_DeviceRef_API> enter(resource, true);
   if (enter.failed())
     return false;
 
@@ -189,6 +190,7 @@ class TestMonitorDeviceChange {
   static void MonitorDeviceChangeCallback(void* user_data,
                                           uint32_t device_count,
                                           const PP_Resource devices[]) {
+    ProxyAutoLock lock;
     TestMonitorDeviceChange* helper =
         static_cast<TestMonitorDeviceChange*>(user_data);
     CHECK(!helper->called_);
@@ -218,6 +220,8 @@ class TestMonitorDeviceChange {
 }  // namespace
 
 TEST_F(DeviceEnumerationResourceHelperTest, EnumerateDevices) {
+  ProxyAutoLock lock;
+
   scoped_refptr<TestResource> resource(
       new TestResource(GetConnection(this), pp_instance()));
   DeviceEnumerationResourceHelper& device_enumeration =
@@ -252,11 +256,13 @@ TEST_F(DeviceEnumerationResourceHelperTest, EnumerateDevices) {
   data_item.id = "id_2";
   data.push_back(data_item);
 
-  ASSERT_TRUE(plugin_dispatcher()->OnMessageReceived(
-      PpapiPluginMsg_ResourceReply(
-          reply_params,
-          PpapiPluginMsg_DeviceEnumeration_EnumerateDevicesReply(data))));
-
+  {
+    ProxyAutoUnlock unlock;
+    ASSERT_TRUE(plugin_dispatcher()->OnMessageReceived(
+        PpapiPluginMsg_ResourceReply(
+            reply_params,
+            PpapiPluginMsg_DeviceEnumeration_EnumerateDevicesReply(data))));
+  }
   EXPECT_TRUE(callback.called());
   EXPECT_EQ(PP_OK, callback.result());
   EXPECT_EQ(2U, output.count());
@@ -265,6 +271,8 @@ TEST_F(DeviceEnumerationResourceHelperTest, EnumerateDevices) {
 }
 
 TEST_F(DeviceEnumerationResourceHelperTest, MonitorDeviceChange) {
+  ProxyAutoLock lock;
+
   scoped_refptr<TestResource> resource(
       new TestResource(GetConnection(this), pp_instance()));
   DeviceEnumerationResourceHelper& device_enumeration =
@@ -293,12 +301,15 @@ TEST_F(DeviceEnumerationResourceHelperTest, MonitorDeviceChange) {
 
   helper.SetExpectedResult(data);
 
-  // Synthesize a response with no device.
-  ASSERT_TRUE(plugin_dispatcher()->OnMessageReceived(
-      PpapiPluginMsg_ResourceReply(
-          reply_params,
-          PpapiPluginMsg_DeviceEnumeration_NotifyDeviceChange(
-              callback_id, data))));
+  {
+    ProxyAutoUnlock unlock;
+    // Synthesize a response with no device.
+    ASSERT_TRUE(plugin_dispatcher()->OnMessageReceived(
+        PpapiPluginMsg_ResourceReply(
+            reply_params,
+            PpapiPluginMsg_DeviceEnumeration_NotifyDeviceChange(
+                callback_id, data))));
+  }
   EXPECT_TRUE(helper.called() && helper.same_as_expected());
 
   DeviceRefData data_item;
@@ -313,12 +324,15 @@ TEST_F(DeviceEnumerationResourceHelperTest, MonitorDeviceChange) {
 
   helper.SetExpectedResult(data);
 
-  // Synthesize a response with some devices.
-  ASSERT_TRUE(plugin_dispatcher()->OnMessageReceived(
-      PpapiPluginMsg_ResourceReply(
-          reply_params,
-          PpapiPluginMsg_DeviceEnumeration_NotifyDeviceChange(
-              callback_id, data))));
+  {
+    ProxyAutoUnlock unlock;
+    // Synthesize a response with some devices.
+    ASSERT_TRUE(plugin_dispatcher()->OnMessageReceived(
+        PpapiPluginMsg_ResourceReply(
+            reply_params,
+            PpapiPluginMsg_DeviceEnumeration_NotifyDeviceChange(
+                callback_id, data))));
+  }
   EXPECT_TRUE(helper.called() && helper.same_as_expected());
 
   TestMonitorDeviceChange helper2(&var_tracker());
@@ -340,24 +354,30 @@ TEST_F(DeviceEnumerationResourceHelperTest, MonitorDeviceChange) {
 
   helper.SetExpectedResult(data);
   helper2.SetExpectedResult(data);
-  // |helper2| should receive the result while |helper| shouldn't.
-  ASSERT_TRUE(plugin_dispatcher()->OnMessageReceived(
-      PpapiPluginMsg_ResourceReply(
-          reply_params,
-          PpapiPluginMsg_DeviceEnumeration_NotifyDeviceChange(
-              callback_id2, data))));
+  {
+    ProxyAutoUnlock unlock;
+    // |helper2| should receive the result while |helper| shouldn't.
+    ASSERT_TRUE(plugin_dispatcher()->OnMessageReceived(
+        PpapiPluginMsg_ResourceReply(
+            reply_params,
+            PpapiPluginMsg_DeviceEnumeration_NotifyDeviceChange(
+                callback_id2, data))));
+  }
   EXPECT_TRUE(helper2.called() && helper2.same_as_expected());
   EXPECT_FALSE(helper.called());
 
   helper.SetExpectedResult(data);
   helper2.SetExpectedResult(data);
-  // Even if a message with |callback_id| arrives. |helper| shouldn't receive
-  // the result.
-  ASSERT_TRUE(plugin_dispatcher()->OnMessageReceived(
-      PpapiPluginMsg_ResourceReply(
-          reply_params,
-          PpapiPluginMsg_DeviceEnumeration_NotifyDeviceChange(
-              callback_id, data))));
+  {
+    ProxyAutoUnlock unlock;
+    // Even if a message with |callback_id| arrives. |helper| shouldn't receive
+    // the result.
+    ASSERT_TRUE(plugin_dispatcher()->OnMessageReceived(
+        PpapiPluginMsg_ResourceReply(
+            reply_params,
+            PpapiPluginMsg_DeviceEnumeration_NotifyDeviceChange(
+                callback_id, data))));
+  }
   EXPECT_FALSE(helper2.called());
   EXPECT_FALSE(helper.called());
 
@@ -373,12 +393,15 @@ TEST_F(DeviceEnumerationResourceHelperTest, MonitorDeviceChange) {
   sink().ClearMessages();
 
   helper2.SetExpectedResult(data);
-  // |helper2| shouldn't receive any result any more.
-  ASSERT_TRUE(plugin_dispatcher()->OnMessageReceived(
-      PpapiPluginMsg_ResourceReply(
-          reply_params,
-          PpapiPluginMsg_DeviceEnumeration_NotifyDeviceChange(
-              callback_id2, data))));
+  {
+    ProxyAutoUnlock unlock;
+    // |helper2| shouldn't receive any result any more.
+    ASSERT_TRUE(plugin_dispatcher()->OnMessageReceived(
+        PpapiPluginMsg_ResourceReply(
+            reply_params,
+            PpapiPluginMsg_DeviceEnumeration_NotifyDeviceChange(
+                callback_id2, data))));
+  }
   EXPECT_FALSE(helper2.called());
 }
 
diff --git a/ppapi/proxy/file_chooser_resource_unittest.cc b/ppapi/proxy/file_chooser_resource_unittest.cc
index 49c95da..3ff1022 100644
--- a/ppapi/proxy/file_chooser_resource_unittest.cc
+++ b/ppapi/proxy/file_chooser_resource_unittest.cc
@@ -6,10 +6,10 @@
 #include "ppapi/c/dev/ppb_file_chooser_dev.h"
 #include "ppapi/c/pp_errors.h"
 #include "ppapi/proxy/file_chooser_resource.h"
+#include "ppapi/proxy/locking_resource_releaser.h"
 #include "ppapi/proxy/ppapi_messages.h"
 #include "ppapi/proxy/ppapi_proxy_test.h"
 #include "ppapi/shared_impl/proxy_lock.h"
-#include "ppapi/shared_impl/scoped_pp_resource.h"
 #include "ppapi/shared_impl/scoped_pp_var.h"
 #include "ppapi/shared_impl/var.h"
 #include "ppapi/thunk/thunk.h"
@@ -67,7 +67,7 @@ bool CheckParseAcceptType(const std::string& input,
 TEST_F(FileChooserResourceTest, Show) {
   const PPB_FileChooser_Dev_0_6* chooser_iface =
       thunk::GetPPB_FileChooser_Dev_0_6_Thunk();
-  ScopedPPResource res(ScopedPPResource::PassRef(),
+  LockingResourceReleaser res(
       chooser_iface->Create(pp_instance(), PP_FILECHOOSERMODE_OPEN,
                             PP_MakeUndefined()));
 
@@ -77,7 +77,7 @@ TEST_F(FileChooserResourceTest, Show) {
   output.user_data = &dest;
 
   int32_t result = chooser_iface->Show(
-      res, output, PP_MakeCompletionCallback(&DoNothingCallback, NULL));
+      res.get(), output, PP_MakeCompletionCallback(&DoNothingCallback, NULL));
   ASSERT_EQ(PP_OK_COMPLETIONPENDING, result);
 
   // Should have sent a "show" message.
@@ -105,7 +105,7 @@ TEST_F(FileChooserResourceTest, Show) {
 
   // Should have populated our vector.
   ASSERT_EQ(1u, dest.size());
-  ScopedPPResource dest_deletor(dest[0]);  // Ensure it's cleaned up.
+  LockingResourceReleaser dest_deletor(dest[0]);  // Ensure it's cleaned up.
 
   const PPB_FileRef_1_0* file_ref_iface = thunk::GetPPB_FileRef_1_0_Thunk();
   EXPECT_EQ(PP_FILESYSTEMTYPE_EXTERNAL,
diff --git a/ppapi/proxy/flash_resource_unittest.cc b/ppapi/proxy/flash_resource_unittest.cc
index ab22077..1a5e7c6 100644
--- a/ppapi/proxy/flash_resource_unittest.cc
+++ b/ppapi/proxy/flash_resource_unittest.cc
@@ -5,9 +5,9 @@
 #include "ppapi/c/dev/ppb_video_capture_dev.h"
 #include "ppapi/c/pp_errors.h"
 #include "ppapi/c/private/ppb_flash.h"
+#include "ppapi/proxy/locking_resource_releaser.h"
 #include "ppapi/proxy/ppapi_messages.h"
 #include "ppapi/proxy/ppapi_proxy_test.h"
-#include "ppapi/shared_impl/scoped_pp_resource.h"
 #include "ppapi/thunk/thunk.h"
 
 namespace ppapi {
@@ -43,7 +43,7 @@ TEST_F(FlashResourceTest, EnumerateVideoCaptureDevices) {
   sink().AddFilter(&enumerate_video_devices_handler);
 
   // Set up the arguments to the call.
-  ScopedPPResource video_capture(ScopedPPResource::PassRef(),
+  LockingResourceReleaser video_capture(
       ::ppapi::thunk::GetPPB_VideoCapture_Dev_0_3_Thunk()->Create(
           pp_instance()));
   std::vector<PP_Resource> unused;
diff --git a/ppapi/proxy/locking_resource_releaser.h b/ppapi/proxy/locking_resource_releaser.h
new file mode 100644
index 0000000..d390ac4
--- /dev/null
+++ b/ppapi/proxy/locking_resource_releaser.h
@@ -0,0 +1,41 @@
+// Copyright (c) 2013 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef PPAPI_PROXY_LOCKING_RESOURCE_RELEASER_H_
+#define PPAPI_PROXY_LOCKING_RESOURCE_RELEASER_H_
+
+#include "ppapi/shared_impl/ppapi_globals.h"
+#include "ppapi/shared_impl/proxy_lock.h"
+#include "ppapi/shared_impl/resource_tracker.h"
+
+namespace ppapi {
+namespace proxy {
+
+// LockingResourceReleaser is a simple RAII class for releasing a resource at
+// the end of scope. This acquires the ProxyLock before releasing the resource.
+// It is for use in unit tests. Most proxy or implementation code should use
+// ScopedPPResource instead. Unit tests sometimes can't use ScopedPPResource
+// because it asserts that the ProxyLock is already held.
+class LockingResourceReleaser {
+ public:
+  explicit LockingResourceReleaser(PP_Resource resource)
+      : resource_(resource) {
+  }
+  ~LockingResourceReleaser() {
+    ProxyAutoLock lock;
+    PpapiGlobals::Get()->GetResourceTracker()->ReleaseResource(resource_);
+  }
+
+  PP_Resource get() { return resource_; }
+
+ private:
+  PP_Resource resource_;
+
+  DISALLOW_COPY_AND_ASSIGN(LockingResourceReleaser);
+};
+
+}  // namespace proxy
+}  // namespace ppapi
+
+#endif  // PPAPI_PROXY_LOCKING_RESOURCE_RELEASER_H_
diff --git a/ppapi/proxy/plugin_globals.cc b/ppapi/proxy/plugin_globals.cc
index 2e18ac5f..3083832 100644
--- a/ppapi/proxy/plugin_globals.cc
+++ b/ppapi/proxy/plugin_globals.cc
@@ -49,40 +49,36 @@ PluginGlobals* PluginGlobals::plugin_globals_ = NULL;
 PluginGlobals::PluginGlobals()
     : ppapi::PpapiGlobals(),
       plugin_proxy_delegate_(NULL),
-      callback_tracker_(new CallbackTracker),
-      loop_for_main_thread_(
-          new MessageLoopResource(MessageLoopResource::ForMainThread())) {
-#if defined(ENABLE_PEPPER_THREADING)
-  enable_threading_ = true;
-#else
-  enable_threading_ = false;
-#endif
-
+      callback_tracker_(new CallbackTracker) {
   DCHECK(!plugin_globals_);
   plugin_globals_ = this;
+
+  // ResourceTracker asserts that we have the lock when we add new resources,
+  // so we lock when creating the MessageLoopResource even though there is no
+  // chance of race conditions.
+  ProxyAutoLock lock;
+  loop_for_main_thread_ =
+      new MessageLoopResource(MessageLoopResource::ForMainThread());
 }
 
 PluginGlobals::PluginGlobals(PerThreadForTest per_thread_for_test)
     : ppapi::PpapiGlobals(per_thread_for_test),
       plugin_proxy_delegate_(NULL),
       callback_tracker_(new CallbackTracker) {
-#if defined(ENABLE_PEPPER_THREADING)
-  enable_threading_ = true;
-#else
-  enable_threading_ = false;
-#endif
   DCHECK(!plugin_globals_);
 }
 
 PluginGlobals::~PluginGlobals() {
   DCHECK(plugin_globals_ == this || !plugin_globals_);
-  // Release the main-thread message loop. We should have the last reference
-  // count, so this will delete the MessageLoop resource. We do this before
-  // we clear plugin_globals_, because the Resource destructor tries to access
-  // this PluginGlobals.
-  DCHECK(!loop_for_main_thread_ || loop_for_main_thread_->HasOneRef());
-  loop_for_main_thread_ = NULL;
-
+  {
+    ProxyAutoLock lock;
+    // Release the main-thread message loop. We should have the last reference
+    // count, so this will delete the MessageLoop resource. We do this before
+    // we clear plugin_globals_, because the Resource destructor tries to access
+    // this PluginGlobals.
+    DCHECK(!loop_for_main_thread_ || loop_for_main_thread_->HasOneRef());
+    loop_for_main_thread_ = NULL;
+  }
   plugin_globals_ = NULL;
 }
 
@@ -131,9 +127,7 @@ void PluginGlobals::PreCacheFontForFlash(const void* logfontw) {
 }
 
 base::Lock* PluginGlobals::GetProxyLock() {
-  if (enable_threading_)
-    return &proxy_lock_;
-  return NULL;
+  return &proxy_lock_;
 }
 
 void PluginGlobals::LogWithSource(PP_Instance instance,
diff --git a/ppapi/proxy/plugin_globals.h b/ppapi/proxy/plugin_globals.h
index 4da6d5f..37fdc1a 100644
--- a/ppapi/proxy/plugin_globals.h
+++ b/ppapi/proxy/plugin_globals.h
@@ -114,10 +114,6 @@ class PPAPI_PROXY_EXPORT PluginGlobals : public PpapiGlobals {
   // The embedder should call this function when the command line is known.
   void set_command_line(const std::string& c) { command_line_ = c; }
 
-  // Sets whether threadsafety is supported. Defaults to whether the
-  // ENABLE_PEPPER_THREADING build flag is set.
-  void set_enable_threading(bool enable) { enable_threading_ = enable; }
-
  private:
   class BrowserSender;
 
@@ -131,7 +127,6 @@ class PPAPI_PROXY_EXPORT PluginGlobals : public PpapiGlobals {
   PluginVarTracker plugin_var_tracker_;
   scoped_refptr<CallbackTracker> callback_tracker_;
 
-  bool enable_threading_;  // Indicates whether we'll use the lock.
   base::Lock proxy_lock_;
 
   scoped_ptr<base::ThreadLocalStorage::Slot> msg_loop_slot_;
diff --git a/ppapi/proxy/plugin_resource_tracker.cc b/ppapi/proxy/plugin_resource_tracker.cc
index fb81857..12e9d3f 100644
--- a/ppapi/proxy/plugin_resource_tracker.cc
+++ b/ppapi/proxy/plugin_resource_tracker.cc
@@ -17,7 +17,7 @@
 namespace ppapi {
 namespace proxy {
 
-PluginResourceTracker::PluginResourceTracker() {
+PluginResourceTracker::PluginResourceTracker() : ResourceTracker(THREAD_SAFE) {
 }
 
 PluginResourceTracker::~PluginResourceTracker() {
diff --git a/ppapi/proxy/plugin_resource_tracker_unittest.cc b/ppapi/proxy/plugin_resource_tracker_unittest.cc
index 59b64db..9a60864 100644
--- a/ppapi/proxy/plugin_resource_tracker_unittest.cc
+++ b/ppapi/proxy/plugin_resource_tracker_unittest.cc
@@ -9,6 +9,7 @@
 #include "ppapi/proxy/plugin_resource_tracker.h"
 #include "ppapi/proxy/ppapi_messages.h"
 #include "ppapi/proxy/ppapi_proxy_test.h"
+#include "ppapi/shared_impl/proxy_lock.h"
 
 namespace ppapi {
 namespace proxy {
@@ -40,6 +41,8 @@ class PluginResourceTrackerTest : public PluginProxyTest {
 };
 
 TEST_F(PluginResourceTrackerTest, PluginResourceForHostResource) {
+  ProxyAutoLock lock;
+
   PP_Resource host_resource = 0x5678;
 
   HostResource serialized;
diff --git a/ppapi/proxy/plugin_var_tracker.cc b/ppapi/proxy/plugin_var_tracker.cc
index cff8fa1..14a8584 100644
--- a/ppapi/proxy/plugin_var_tracker.cc
+++ b/ppapi/proxy/plugin_var_tracker.cc
@@ -31,7 +31,7 @@ bool PluginVarTracker::HostVar::operator<(const HostVar& other) const {
   return host_object_id < other.host_object_id;
 }
 
-PluginVarTracker::PluginVarTracker() {
+PluginVarTracker::PluginVarTracker() : VarTracker(THREAD_SAFE) {
 }
 
 PluginVarTracker::~PluginVarTracker() {
@@ -39,7 +39,7 @@ PluginVarTracker::~PluginVarTracker() {
 
 PP_Var PluginVarTracker::ReceiveObjectPassRef(const PP_Var& host_var,
                                               PluginDispatcher* dispatcher) {
-  DCHECK(CalledOnValidThread());
+  CheckThreadingPreconditions();
   DCHECK(host_var.type == PP_VARTYPE_OBJECT);
 
   // Get the object.
@@ -65,7 +65,7 @@ PP_Var PluginVarTracker::ReceiveObjectPassRef(const PP_Var& host_var,
 PP_Var PluginVarTracker::TrackObjectWithNoReference(
     const PP_Var& host_var,
     PluginDispatcher* dispatcher) {
-  DCHECK(CalledOnValidThread());
+  CheckThreadingPreconditions();
   DCHECK(host_var.type == PP_VARTYPE_OBJECT);
 
   // Get the object.
@@ -83,7 +83,7 @@ PP_Var PluginVarTracker::TrackObjectWithNoReference(
 
 void PluginVarTracker::StopTrackingObjectWithNoReference(
     const PP_Var& plugin_var) {
-  DCHECK(CalledOnValidThread());
+  CheckThreadingPreconditions();
   DCHECK(plugin_var.type == PP_VARTYPE_OBJECT);
 
   VarMap::iterator found = GetLiveVar(plugin_var);
@@ -98,8 +98,7 @@ void PluginVarTracker::StopTrackingObjectWithNoReference(
 }
 
 PP_Var PluginVarTracker::GetHostObject(const PP_Var& plugin_object) const {
-  DCHECK(CalledOnValidThread());
-
+  CheckThreadingPreconditions();
   if (plugin_object.type != PP_VARTYPE_OBJECT) {
     NOTREACHED();
     return PP_MakeUndefined();
@@ -120,8 +119,7 @@ PP_Var PluginVarTracker::GetHostObject(const PP_Var& plugin_object) const {
 
 PluginDispatcher* PluginVarTracker::DispatcherForPluginObject(
     const PP_Var& plugin_object) const {
-  DCHECK(CalledOnValidThread());
-
+  CheckThreadingPreconditions();
   if (plugin_object.type != PP_VARTYPE_OBJECT)
     return NULL;
 
@@ -137,7 +135,7 @@ PluginDispatcher* PluginVarTracker::DispatcherForPluginObject(
 
 void PluginVarTracker::ReleaseHostObject(PluginDispatcher* dispatcher,
                                          const PP_Var& host_object) {
-  DCHECK(CalledOnValidThread());
+  CheckThreadingPreconditions();
   DCHECK(host_object.type == PP_VARTYPE_OBJECT);
 
   // Convert the host object to a normal var valid in the plugin.
diff --git a/ppapi/proxy/ppapi_proxy_test.cc b/ppapi/proxy/ppapi_proxy_test.cc
index 00947e3..9034d8f 100644
--- a/ppapi/proxy/ppapi_proxy_test.cc
+++ b/ppapi/proxy/ppapi_proxy_test.cc
@@ -171,6 +171,8 @@ Dispatcher* PluginProxyTestHarness::GetDispatcher() {
 void PluginProxyTestHarness::SetUpHarness() {
   // These must be first since the dispatcher set-up uses them.
   CreatePluginGlobals();
+  // Some of the methods called during set-up check that the lock is held.
+  ProxyAutoLock lock;
 
   resource_tracker().DidCreateInstance(pp_instance());
 
@@ -196,6 +198,8 @@ void PluginProxyTestHarness::SetUpHarnessWithChannel(
     bool is_client) {
   // These must be first since the dispatcher set-up uses them.
   CreatePluginGlobals();
+  // Some of the methods called during set-up check that the lock is held.
+  ProxyAutoLock lock;
 
   resource_tracker().DidCreateInstance(pp_instance());
   plugin_delegate_mock_.Init(ipc_message_loop, shutdown_event);
@@ -214,10 +218,15 @@ void PluginProxyTestHarness::SetUpHarnessWithChannel(
 }
 
 void PluginProxyTestHarness::TearDownHarness() {
-  plugin_dispatcher_->DidDestroyInstance(pp_instance());
-  plugin_dispatcher_.reset();
+  {
+    // Some of the methods called during tear-down check that the lock is held.
+    ProxyAutoLock lock;
+
+    plugin_dispatcher_->DidDestroyInstance(pp_instance());
+    plugin_dispatcher_.reset();
 
-  resource_tracker().DidDeleteInstance(pp_instance());
+    resource_tracker().DidDeleteInstance(pp_instance());
+  }
   plugin_globals_.reset();
 }
 
diff --git a/ppapi/proxy/ppb_var_unittest.cc b/ppapi/proxy/ppb_var_unittest.cc
index bf6147d..0ed0f47 100644
--- a/ppapi/proxy/ppb_var_unittest.cc
+++ b/ppapi/proxy/ppb_var_unittest.cc
@@ -164,11 +164,7 @@ class RemoveRefVarThreadDelegate : public base::PlatformThread::Delegate {
 
 }  // namespace
 
-#ifdef ENABLE_PEPPER_THREADING
 TEST_F(PPB_VarTest, Threads) {
-#else
-TEST_F(PPB_VarTest, DISABLED_Threads) {
-#endif
   std::vector<base::PlatformThreadHandle> create_var_threads(kNumThreads);
   std::vector<CreateVarThreadDelegate> create_var_delegates;
   // The strings that the threads will re-extract from Vars (so we can check
diff --git a/ppapi/proxy/ppp_instance_private_proxy_unittest.cc b/ppapi/proxy/ppp_instance_private_proxy_unittest.cc
index df1af0c..317f4d4 100644
--- a/ppapi/proxy/ppp_instance_private_proxy_unittest.cc
+++ b/ppapi/proxy/ppp_instance_private_proxy_unittest.cc
@@ -147,15 +147,7 @@ class PPP_Instance_Private_ProxyTest : public TwoWayTest {
 
 }  // namespace
 
-// TODO(raymes): This #ifdef is only here because we check the state of the
-// plugin globals on the main thread, rather than the plugin thread which causes
-// the thread checker to fail. Once ENABLE_PEPPER_THREADING is the default,
-// this will be safe to do anyway, so we can remove this.
-#ifdef ENABLE_PEPPER_THREADING
 TEST_F(PPP_Instance_Private_ProxyTest, PPPInstancePrivate) {
-#else
-TEST_F(PPP_Instance_Private_ProxyTest, DISABLED_PPPInstancePrivate) {
-#endif
   // This test controls its own instance; we can't use the one that
   // PluginProxyTestHarness provides.
   ASSERT_NE(kInstance, pp_instance());
diff --git a/ppapi/proxy/ppp_instance_proxy_unittest.cc b/ppapi/proxy/ppp_instance_proxy_unittest.cc
index 91775b0..e2df26e 100644
--- a/ppapi/proxy/ppp_instance_proxy_unittest.cc
+++ b/ppapi/proxy/ppp_instance_proxy_unittest.cc
@@ -10,10 +10,10 @@
 #include "ppapi/c/ppb_url_loader.h"
 #include "ppapi/c/ppp_instance.h"
 #include "ppapi/c/private/ppb_flash_fullscreen.h"
+#include "ppapi/proxy/locking_resource_releaser.h"
 #include "ppapi/proxy/ppapi_messages.h"
 #include "ppapi/proxy/ppapi_proxy_test.h"
 #include "ppapi/shared_impl/ppb_view_shared.h"
-#include "ppapi/shared_impl/scoped_pp_resource.h"
 
 namespace ppapi {
 namespace proxy {
@@ -163,11 +163,10 @@ TEST_F(PPP_Instance_ProxyTest, PPPInstance1_0) {
   data.clip_rect = expected_clip;
   data.device_scale = 1.0f;
   ResetReceived();
-  ScopedPPResource view_resource(
-      ScopedPPResource::PassRef(),
+  LockingResourceReleaser view_resource(
       (new PPB_View_Shared(OBJECT_IS_IMPL,
                            expected_instance, data))->GetReference());
-  ppp_instance->DidChangeView(expected_instance, view_resource);
+  ppp_instance->DidChangeView(expected_instance, view_resource.get());
   did_change_view_called.Wait();
   EXPECT_EQ(received_instance, expected_instance);
   EXPECT_EQ(received_position.point.x, expected_position.point.x);
diff --git a/ppapi/proxy/printing_resource_unittest.cc b/ppapi/proxy/printing_resource_unittest.cc
index 980147f..ed96364 100644
--- a/ppapi/proxy/printing_resource_unittest.cc
+++ b/ppapi/proxy/printing_resource_unittest.cc
@@ -7,11 +7,11 @@
 #include "base/message_loop.h"
 #include "ppapi/c/dev/ppb_printing_dev.h"
 #include "ppapi/c/pp_errors.h"
+#include "ppapi/proxy/locking_resource_releaser.h"
 #include "ppapi/proxy/printing_resource.h"
 #include "ppapi/proxy/ppapi_messages.h"
 #include "ppapi/proxy/ppapi_proxy_test.h"
 #include "ppapi/thunk/thunk.h"
-#include "ppapi/shared_impl/scoped_pp_resource.h"
 
 namespace ppapi {
 namespace proxy {
@@ -47,13 +47,12 @@ TEST_F(PrintingResourceTest, GetDefaultPrintSettings) {
 
   const PPB_Printing_Dev_0_7* printing_iface =
       thunk::GetPPB_Printing_Dev_0_7_Thunk();
-  ScopedPPResource res(ScopedPPResource::PassRef(),
-                       printing_iface->Create(pp_instance()));
+  LockingResourceReleaser res(printing_iface->Create(pp_instance()));
 
   PP_PrintSettings_Dev output_settings;
 
   int32_t result = printing_iface->GetDefaultPrintSettings(
-      res, &output_settings, PP_MakeCompletionCallback(&Callback, NULL));
+      res.get(), &output_settings, PP_MakeCompletionCallback(&Callback, NULL));
   ASSERT_EQ(PP_OK_COMPLETIONPENDING, result);
 
   // Should have sent a "GetDefaultPrintSettings" message.
diff --git a/ppapi/proxy/websocket_resource_unittest.cc b/ppapi/proxy/websocket_resource_unittest.cc
index 56012b3..89ea831 100644
--- a/ppapi/proxy/websocket_resource_unittest.cc
+++ b/ppapi/proxy/websocket_resource_unittest.cc
@@ -7,10 +7,14 @@
 #include "ppapi/c/pp_errors.h"
 #include "ppapi/c/ppb_websocket.h"
 #include "ppapi/c/ppb_var.h"
+#include "ppapi/proxy/locking_resource_releaser.h"
 #include "ppapi/proxy/websocket_resource.h"
 #include "ppapi/proxy/ppapi_messages.h"
 #include "ppapi/proxy/ppapi_proxy_test.h"
+#include "ppapi/shared_impl/ppapi_globals.h"
 #include "ppapi/shared_impl/ppb_var_shared.h"
+#include "ppapi/shared_impl/proxy_lock.h"
+#include "ppapi/shared_impl/resource_tracker.h"
 #include "ppapi/shared_impl/scoped_pp_resource.h"
 #include "ppapi/shared_impl/scoped_pp_var.h"
 #include "ppapi/shared_impl/tracked_callback.h"
@@ -59,11 +63,10 @@ TEST_F(WebSocketResourceTest, Connect) {
   PP_Var url_var = MakeStringVar(url);
   PP_Var protocols[] = { MakeStringVar(protocol0), MakeStringVar(protocol1) };
 
-  ScopedPPResource res(ScopedPPResource::PassRef(),
-                       websocket_iface->Create(pp_instance()));
+  LockingResourceReleaser res(websocket_iface->Create(pp_instance()));
 
-  int32_t result =
-      websocket_iface->Connect(res, url_var, protocols, 2, MakeCallback());
+  int32_t result = websocket_iface->Connect(res.get(), url_var, protocols, 2,
+                                            MakeCallback());
   ASSERT_EQ(PP_OK_COMPLETIONPENDING, result);
 
   // Should be sent a "Connect" message.
@@ -94,18 +97,17 @@ TEST_F(WebSocketResourceTest, UnsolicitedReplies) {
   const PPB_WebSocket_1_0* websocket_iface =
       thunk::GetPPB_WebSocket_1_0_Thunk();
 
-  ScopedPPResource res(ScopedPPResource::PassRef(),
-                       websocket_iface->Create(pp_instance()));
+  LockingResourceReleaser res(websocket_iface->Create(pp_instance()));
 
   // Check if BufferedAmountReply is handled.
-  ResourceMessageReplyParams reply_params(res, 0);
+  ResourceMessageReplyParams reply_params(res.get(), 0);
   reply_params.set_result(PP_OK);
   ASSERT_TRUE(plugin_dispatcher()->OnMessageReceived(
       PpapiPluginMsg_ResourceReply(
           reply_params,
           PpapiPluginMsg_WebSocket_BufferedAmountReply(19760227u))));
 
-  uint64_t amount = websocket_iface->GetBufferedAmount(res);
+  uint64_t amount = websocket_iface->GetBufferedAmount(res.get());
   EXPECT_EQ(19760227u, amount);
 
   // Check if StateReply is handled.
@@ -115,7 +117,7 @@ TEST_F(WebSocketResourceTest, UnsolicitedReplies) {
           PpapiPluginMsg_WebSocket_StateReply(
               static_cast<int32_t>(PP_WEBSOCKETREADYSTATE_CLOSING)))));
 
-  PP_WebSocketReadyState state = websocket_iface->GetReadyState(res);
+  PP_WebSocketReadyState state = websocket_iface->GetReadyState(res.get());
   EXPECT_EQ(PP_WEBSOCKETREADYSTATE_CLOSING, state);
 }
 
@@ -126,12 +128,11 @@ TEST_F(WebSocketResourceTest, MessageError) {
   std::string url("ws://ws.google.com");
   PP_Var url_var = MakeStringVar(url);
 
-  ScopedPPResource res(ScopedPPResource::PassRef(),
-                       websocket_iface->Create(pp_instance()));
+  LockingResourceReleaser res(websocket_iface->Create(pp_instance()));
 
   // Establish the connection virtually.
   int32_t result =
-      websocket_iface->Connect(res, url_var, NULL, 0, MakeCallback());
+      websocket_iface->Connect(res.get(), url_var, NULL, 0, MakeCallback());
   ASSERT_EQ(PP_OK_COMPLETIONPENDING, result);
 
   ResourceMessageCallParams params;
@@ -150,11 +151,11 @@ TEST_F(WebSocketResourceTest, MessageError) {
   EXPECT_TRUE(g_callback_called);
 
   PP_Var message;
-  result = websocket_iface->ReceiveMessage(res, &message, MakeCallback());
+  result = websocket_iface->ReceiveMessage(res.get(), &message, MakeCallback());
   EXPECT_FALSE(g_callback_called);
 
   // Synthesize a WebSocket_ErrorReply message.
-  ResourceMessageReplyParams error_reply_params(res, 0);
+  ResourceMessageReplyParams error_reply_params(res.get(), 0);
   error_reply_params.set_result(PP_OK);
   ASSERT_TRUE(plugin_dispatcher()->OnMessageReceived(
       PpapiPluginMsg_ResourceReply(error_reply_params,
diff --git a/ppapi/shared_impl/resource_tracker.cc b/ppapi/shared_impl/resource_tracker.cc
index a448ce4..a46c823 100644
--- a/ppapi/shared_impl/resource_tracker.cc
+++ b/ppapi/shared_impl/resource_tracker.cc
@@ -15,17 +15,23 @@
 
 namespace ppapi {
 
-ResourceTracker::ResourceTracker()
+ResourceTracker::ResourceTracker(ThreadMode thread_mode)
     : last_resource_value_(0),
       ALLOW_THIS_IN_INITIALIZER_LIST(weak_ptr_factory_(this)) {
+  if (thread_mode == SINGLE_THREADED)
+    thread_checker_.reset(new base::ThreadChecker);
 }
 
 ResourceTracker::~ResourceTracker() {
 }
 
-Resource* ResourceTracker::GetResource(PP_Resource res) const {
-  CHECK(thread_checker_.CalledOnValidThread());
+void ResourceTracker::CheckThreadingPreconditions() const {
+  DCHECK(!thread_checker_ || thread_checker_->CalledOnValidThread());
   ProxyLock::AssertAcquired();
+}
+
+Resource* ResourceTracker::GetResource(PP_Resource res) const {
+  CheckThreadingPreconditions();
   ResourceMap::const_iterator i = live_resources_.find(res);
   if (i == live_resources_.end())
     return NULL;
@@ -33,7 +39,7 @@ Resource* ResourceTracker::GetResource(PP_Resource res) const {
 }
 
 void ResourceTracker::AddRefResource(PP_Resource res) {
-  CHECK(thread_checker_.CalledOnValidThread());
+  CheckThreadingPreconditions();
   DLOG_IF(ERROR, !CheckIdType(res, PP_ID_TYPE_RESOURCE))
       << res << " is not a PP_Resource.";
   ResourceMap::iterator i = live_resources_.find(res);
@@ -55,7 +61,7 @@ void ResourceTracker::AddRefResource(PP_Resource res) {
 }
 
 void ResourceTracker::ReleaseResource(PP_Resource res) {
-  CHECK(thread_checker_.CalledOnValidThread());
+  CheckThreadingPreconditions();
   DLOG_IF(ERROR, !CheckIdType(res, PP_ID_TYPE_RESOURCE))
       << res << " is not a PP_Resource.";
   ResourceMap::iterator i = live_resources_.find(res);
@@ -86,7 +92,7 @@ void ResourceTracker::ReleaseResourceSoon(PP_Resource res) {
 }
 
 void ResourceTracker::DidCreateInstance(PP_Instance instance) {
-  CHECK(thread_checker_.CalledOnValidThread());
+  CheckThreadingPreconditions();
   // Due to the infrastructure of some tests, the instance is registered
   // twice in a few cases. It would be nice not to do that and assert here
   // instead.
@@ -96,7 +102,7 @@ void ResourceTracker::DidCreateInstance(PP_Instance instance) {
 }
 
 void ResourceTracker::DidDeleteInstance(PP_Instance instance) {
-  CHECK(thread_checker_.CalledOnValidThread());
+  CheckThreadingPreconditions();
   InstanceMap::iterator found_instance = instance_map_.find(instance);
 
   // Due to the infrastructure of some tests, the instance is unregistered
@@ -151,7 +157,7 @@ void ResourceTracker::DidDeleteInstance(PP_Instance instance) {
 }
 
 int ResourceTracker::GetLiveObjectsForInstance(PP_Instance instance) const {
-  CHECK(thread_checker_.CalledOnValidThread());
+  CheckThreadingPreconditions();
   InstanceMap::const_iterator found = instance_map_.find(instance);
   if (found == instance_map_.end())
     return 0;
@@ -159,7 +165,7 @@ int ResourceTracker::GetLiveObjectsForInstance(PP_Instance instance) const {
 }
 
 PP_Resource ResourceTracker::AddResource(Resource* object) {
-  CHECK(thread_checker_.CalledOnValidThread());
+  CheckThreadingPreconditions();
   // If the plugin manages to create too many resources, don't do crazy stuff.
   if (last_resource_value_ == kMaxPPId)
     return 0;
@@ -191,7 +197,7 @@ PP_Resource ResourceTracker::AddResource(Resource* object) {
 }
 
 void ResourceTracker::RemoveResource(Resource* object) {
-  CHECK(thread_checker_.CalledOnValidThread());
+  CheckThreadingPreconditions();
   PP_Resource pp_resource = object->pp_resource();
   InstanceMap::iterator found = instance_map_.find(object->pp_instance());
   if (found != instance_map_.end())
diff --git a/ppapi/shared_impl/resource_tracker.h b/ppapi/shared_impl/resource_tracker.h
index f5f790c..54e6daf 100644
--- a/ppapi/shared_impl/resource_tracker.h
+++ b/ppapi/shared_impl/resource_tracker.h
@@ -10,6 +10,7 @@
 #include "base/basictypes.h"
 #include "base/hash_tables.h"
 #include "base/memory/linked_ptr.h"
+#include "base/memory/scoped_ptr.h"
 #include "base/memory/weak_ptr.h"
 #include "base/threading/thread_checker.h"
 #include "base/threading/thread_checker_impl.h"
@@ -23,7 +24,12 @@ class Resource;
 
 class PPAPI_SHARED_EXPORT ResourceTracker {
  public:
-  ResourceTracker();
+  // A SINGLE_THREADED ResourceTracker will use a thread-checker to make sure
+  // it's always invoked on the same thread on which it was constructed. A
+  // THREAD_SAFE ResourceTracker will check that the ProxyLock is held. See
+  // CheckThreadingPreconditions() for more details.
+  enum ThreadMode { SINGLE_THREADED, THREAD_SAFE };
+  explicit ResourceTracker(ThreadMode thread_mode);
   virtual ~ResourceTracker();
 
   // The returned pointer will be NULL if there is no resource. The reference
@@ -53,6 +59,10 @@ class PPAPI_SHARED_EXPORT ResourceTracker {
   // This calls AddResource and RemoveResource.
   friend class Resource;
 
+  // On the host-side, make sure we are called on the right thread. On the
+  // plugin side, make sure we have the proxy lock.
+  void CheckThreadingPreconditions() const;
+
   // Adds the given resource to the tracker, associating it with the instance
   // stored in the resource object. The new resource ID is returned, and the
   // resource will have 0 plugin refcount. This is called by the resource
@@ -98,19 +108,11 @@ class PPAPI_SHARED_EXPORT ResourceTracker {
 
   base::WeakPtrFactory<ResourceTracker> weak_ptr_factory_;
 
-  // TODO(raymes): We won't need to do thread checks once pepper calls are
-  // allowed off of the main thread.
-  // See http://code.google.com/p/chromium/issues/detail?id=92909.
-#ifdef ENABLE_PEPPER_THREADING
-  base::ThreadCheckerDoNothing thread_checker_;
-#else
-  // TODO(raymes): We've seen plugins (Flash) creating resources from random
-  // threads. Let's always crash for now in this case. Once we have a handle
-  // over how common this is, we can change ThreadCheckerImpl->ThreadChecker
-  // so that we only crash in debug mode. See
-  // https://code.google.com/p/chromium/issues/detail?id=146415.
-  base::ThreadCheckerImpl thread_checker_;
-#endif
+  // On the host side, we want to check that we are only called on the main
+  // thread. This is to protect us from accidentally using the tracker from
+  // other threads (especially the IO thread). On the plugin side, the tracker
+  // is protected by the proxy lock and is thread-safe, so this will be NULL.
+  scoped_ptr<base::ThreadChecker> thread_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(ResourceTracker);
 };
diff --git a/ppapi/shared_impl/test_globals.cc b/ppapi/shared_impl/test_globals.cc
index 913d53c..6c6af5b 100644
--- a/ppapi/shared_impl/test_globals.cc
+++ b/ppapi/shared_impl/test_globals.cc
@@ -8,11 +8,13 @@ namespace ppapi {
 
 TestGlobals::TestGlobals()
     : ppapi::PpapiGlobals(),
+      resource_tracker_(ResourceTracker::THREAD_SAFE),
       callback_tracker_(new CallbackTracker) {
 }
 
 TestGlobals::TestGlobals(PpapiGlobals::PerThreadForTest per_thread_for_test)
     : ppapi::PpapiGlobals(per_thread_for_test),
+      resource_tracker_(ResourceTracker::THREAD_SAFE),
       callback_tracker_(new CallbackTracker) {
 }
 
diff --git a/ppapi/shared_impl/test_globals.h b/ppapi/shared_impl/test_globals.h
index 5579b09..c4d4aa1 100644
--- a/ppapi/shared_impl/test_globals.h
+++ b/ppapi/shared_impl/test_globals.h
@@ -16,7 +16,7 @@ namespace ppapi {
 
 class TestVarTracker : public VarTracker {
  public:
-  TestVarTracker() {}
+  TestVarTracker() : VarTracker(THREAD_SAFE) {}
   virtual ~TestVarTracker() {}
   virtual ArrayBufferVar* CreateArrayBuffer(uint32 size_in_bytes) OVERRIDE {
     return NULL;
diff --git a/ppapi/shared_impl/var_tracker.cc b/ppapi/shared_impl/var_tracker.cc
index 434121e..26160f4 100644
--- a/ppapi/shared_impl/var_tracker.cc
+++ b/ppapi/shared_impl/var_tracker.cc
@@ -29,22 +29,27 @@ VarTracker::VarInfo::VarInfo(Var* v, int input_ref_count)
       track_with_no_reference_count(0) {
 }
 
-VarTracker::VarTracker() : last_var_id_(0) {
+VarTracker::VarTracker(ThreadMode thread_mode) : last_var_id_(0) {
+  if (thread_mode == SINGLE_THREADED)
+    thread_checker_.reset(new base::ThreadChecker);
 }
 
 VarTracker::~VarTracker() {
 }
 
-int32 VarTracker::AddVar(Var* var) {
-  DCHECK(CalledOnValidThread());
+void VarTracker::CheckThreadingPreconditions() const {
+  DCHECK(!thread_checker_ || thread_checker_->CalledOnValidThread());
   ProxyLock::AssertAcquired();
+}
+
+int32 VarTracker::AddVar(Var* var) {
+  CheckThreadingPreconditions();
 
   return AddVarInternal(var, ADD_VAR_TAKE_ONE_REFERENCE);
 }
 
 Var* VarTracker::GetVar(int32 var_id) const {
-  DCHECK(CalledOnValidThread());
-  ProxyLock::AssertAcquired();
+  CheckThreadingPreconditions();
 
   VarMap::const_iterator result = live_vars_.find(var_id);
   if (result == live_vars_.end())
@@ -53,8 +58,7 @@ Var* VarTracker::GetVar(int32 var_id) const {
 }
 
 Var* VarTracker::GetVar(const PP_Var& var) const {
-  DCHECK(CalledOnValidThread());
-  ProxyLock::AssertAcquired();
+  CheckThreadingPreconditions();
 
   if (!IsVarTypeRefcounted(var.type))
     return NULL;
@@ -62,8 +66,7 @@ Var* VarTracker::GetVar(const PP_Var& var) const {
 }
 
 bool VarTracker::AddRefVar(int32 var_id) {
-  DCHECK(CalledOnValidThread());
-  ProxyLock::AssertAcquired();
+  CheckThreadingPreconditions();
 
   DLOG_IF(ERROR, !CheckIdType(var_id, PP_ID_TYPE_VAR))
       << var_id << " is not a PP_Var ID.";
@@ -88,8 +91,7 @@ bool VarTracker::AddRefVar(int32 var_id) {
 }
 
 bool VarTracker::AddRefVar(const PP_Var& var) {
-  DCHECK(CalledOnValidThread());
-  ProxyLock::AssertAcquired();
+  CheckThreadingPreconditions();
 
   if (!IsVarTypeRefcounted(var.type))
     return true;
@@ -97,8 +99,7 @@ bool VarTracker::AddRefVar(const PP_Var& var) {
 }
 
 bool VarTracker::ReleaseVar(int32 var_id) {
-  DCHECK(CalledOnValidThread());
-  ProxyLock::AssertAcquired();
+  CheckThreadingPreconditions();
 
   DLOG_IF(ERROR, !CheckIdType(var_id, PP_ID_TYPE_VAR))
       << var_id << " is not a PP_Var ID.";
@@ -129,8 +130,7 @@ bool VarTracker::ReleaseVar(int32 var_id) {
 }
 
 bool VarTracker::ReleaseVar(const PP_Var& var) {
-  DCHECK(CalledOnValidThread());
-  ProxyLock::AssertAcquired();
+  CheckThreadingPreconditions();
 
   if (!IsVarTypeRefcounted(var.type))
     return false;
@@ -154,8 +154,7 @@ VarTracker::VarMap::iterator VarTracker::GetLiveVar(int32 id) {
 }
 
 int VarTracker::GetRefCountForObject(const PP_Var& plugin_object) {
-  DCHECK(CalledOnValidThread());
-  ProxyLock::AssertAcquired();
+  CheckThreadingPreconditions();
 
   VarMap::iterator found = GetLiveVar(plugin_object);
   if (found == live_vars_.end())
@@ -165,8 +164,7 @@ int VarTracker::GetRefCountForObject(const PP_Var& plugin_object) {
 
 int VarTracker::GetTrackedWithNoReferenceCountForObject(
     const PP_Var& plugin_object) {
-  DCHECK(CalledOnValidThread());
-  ProxyLock::AssertAcquired();
+  CheckThreadingPreconditions();
 
   VarMap::iterator found = GetLiveVar(plugin_object);
   if (found == live_vars_.end())
@@ -188,8 +186,7 @@ bool VarTracker::IsVarTypeRefcounted(PP_VarType type) const {
 }
 
 PP_Var VarTracker::MakeArrayBufferPPVar(uint32 size_in_bytes) {
-  DCHECK(CalledOnValidThread());
-  ProxyLock::AssertAcquired();
+  CheckThreadingPreconditions();
 
   scoped_refptr<ArrayBufferVar> array_buffer(CreateArrayBuffer(size_in_bytes));
   if (!array_buffer)
@@ -199,8 +196,7 @@ PP_Var VarTracker::MakeArrayBufferPPVar(uint32 size_in_bytes) {
 
 PP_Var VarTracker::MakeArrayBufferPPVar(uint32 size_in_bytes,
                                         const void* data) {
-  DCHECK(CalledOnValidThread());
-  ProxyLock::AssertAcquired();
+  CheckThreadingPreconditions();
 
   ArrayBufferVar* array_buffer = MakeArrayBufferVar(size_in_bytes, data);
   return array_buffer ? array_buffer->GetPPVar() : PP_MakeNull();
@@ -208,8 +204,7 @@ PP_Var VarTracker::MakeArrayBufferPPVar(uint32 size_in_bytes,
 
 ArrayBufferVar* VarTracker::MakeArrayBufferVar(uint32 size_in_bytes,
                                                const void* data) {
-  DCHECK(CalledOnValidThread());
-  ProxyLock::AssertAcquired();
+  CheckThreadingPreconditions();
 
   ArrayBufferVar* array_buffer(CreateArrayBuffer(size_in_bytes));
   if (!array_buffer)
@@ -220,7 +215,7 @@ ArrayBufferVar* VarTracker::MakeArrayBufferVar(uint32 size_in_bytes,
 
 PP_Var VarTracker::MakeArrayBufferPPVar(uint32 size_in_bytes,
                                         base::SharedMemoryHandle handle) {
-  DCHECK(CalledOnValidThread());
+  CheckThreadingPreconditions();
 
   scoped_refptr<ArrayBufferVar> array_buffer(
       CreateShmArrayBuffer(size_in_bytes, handle));
@@ -230,8 +225,7 @@ PP_Var VarTracker::MakeArrayBufferPPVar(uint32 size_in_bytes,
 }
 
 std::vector<PP_Var> VarTracker::GetLiveVars() {
-  DCHECK(CalledOnValidThread());
-  ProxyLock::AssertAcquired();
+  CheckThreadingPreconditions();
 
   std::vector<PP_Var> var_vector;
   var_vector.reserve(live_vars_.size());
diff --git a/ppapi/shared_impl/var_tracker.h b/ppapi/shared_impl/var_tracker.h
index 32acdeb..a9d92ea 100644
--- a/ppapi/shared_impl/var_tracker.h
+++ b/ppapi/shared_impl/var_tracker.h
@@ -10,8 +10,9 @@
 #include "base/basictypes.h"
 #include "base/hash_tables.h"
 #include "base/memory/ref_counted.h"
+#include "base/memory/scoped_ptr.h"
 #include "base/shared_memory.h"
-#include "base/threading/non_thread_safe.h"
+#include "base/threading/thread_checker.h"
 #include "ppapi/c/pp_instance.h"
 #include "ppapi/c/pp_module.h"
 #include "ppapi/c/pp_resource.h"
@@ -36,16 +37,14 @@ class Var;
 // This class maintains the "track_with_no_reference_count" but doesn't do
 // anything with it other than call virtual functions. The interesting parts
 // are added by the PluginObjectVar derived from this class.
-class PPAPI_SHARED_EXPORT VarTracker
-#ifdef ENABLE_PEPPER_THREADING
-    : NON_EXPORTED_BASE(public base::NonThreadSafeDoNothing) {
-#else
-    // TODO(dmichael): Remove the thread checking when calls are allowed off the
-    // main thread (crbug.com/92909).
-    : NON_EXPORTED_BASE(public base::NonThreadSafe) {
-#endif
+class PPAPI_SHARED_EXPORT VarTracker {
  public:
-  VarTracker();
+  // A SINGLE_THREADED VarTracker will use a thread-checker to make sure it's
+  // always invoked on the same thread on which it was constructed. A
+  // THREAD_SAFE VarTracker will check that the ProxyLock is held. See
+  // CheckThreadingPreconditions() for more details.
+  enum ThreadMode { SINGLE_THREADED, THREAD_SAFE };
+  explicit VarTracker(ThreadMode thread_mode);
   virtual ~VarTracker();
 
   // Called by the Var object to add a new var to the tracker.
@@ -148,6 +147,10 @@ class PPAPI_SHARED_EXPORT VarTracker
     ADD_VAR_CREATE_WITH_NO_REFERENCE
   };
 
+  // On the host-side, make sure we are called on the right thread. On the
+  // plugin side, make sure we have the proxy lock.
+  void CheckThreadingPreconditions() const;
+
   // Implementation of AddVar that allows the caller to specify whether the
   // initial refcount of the added object will be 0 or 1.
   //
@@ -198,6 +201,12 @@ class PPAPI_SHARED_EXPORT VarTracker
       uint32 size_in_bytes,
       base::SharedMemoryHandle handle) = 0;
 
+  // On the host side, we want to check that we are only called on the main
+  // thread. This is to protect us from accidentally using the tracker from
+  // other threads (especially the IO thread). On the plugin side, the tracker
+  // is protected by the proxy lock and is thread-safe, so this will be NULL.
+  scoped_ptr<base::ThreadChecker> thread_checker_;
+
   DISALLOW_COPY_AND_ASSIGN(VarTracker);
 };
 
diff --git a/ppapi/tests/test_case.h b/ppapi/tests/test_case.h
index c2379d1..9ffc8f6 100644
--- a/ppapi/tests/test_case.h
+++ b/ppapi/tests/test_case.h
@@ -137,7 +137,6 @@ class TestCase {
   // Run the given test method on a background thread and return the result.
   template <class T>
   std::string RunOnThread(std::string(T::*test_to_run)()) {
-#ifdef ENABLE_PEPPER_THREADING
     if (!testing_interface_) {
       return "Testing blocking callbacks requires the testing interface. In "
              "Chrome, use the --enable-pepper-testing flag.";
@@ -152,10 +151,6 @@ class TestCase {
     RunOnThreadInternal(&ThreadedTestRunner<T>::ThreadFunction, &runner,
                         testing_interface_);
     return runner.result();
-#else
-    // If threading's not enabled, just treat it as success.
-    return std::string();
-#endif
   }
 
   // Pointer to the instance that owns us.
diff --git a/webkit/plugins/plugin_switches.cc b/webkit/plugins/plugin_switches.cc
index dda1acc..bb1b3b4 100644
--- a/webkit/plugins/plugin_switches.cc
+++ b/webkit/plugins/plugin_switches.cc
@@ -15,9 +15,6 @@ const char kDisablePepper3d[] = "disable-pepper-3d";
 // "Command-line" arguments for the PPAPI Flash; used for debugging options.
 const char kPpapiFlashArgs[] = "ppapi-flash-args";
 
-// Set to true to not allow threadsafety support in native Pepper plugins.
-const char kDisablePepperThreading[] = "disable-pepper-threading";
-
 #if defined(OS_WIN)
 // Used by the plugins_test when testing the older WMP plugin to force the new
 // plugin to not get loaded.
diff --git a/webkit/plugins/plugin_switches.h b/webkit/plugins/plugin_switches.h
index a195094..08ed5ae 100644
--- a/webkit/plugins/plugin_switches.h
+++ b/webkit/plugins/plugin_switches.h
@@ -13,7 +13,6 @@ namespace switches {
 WEBKIT_PLUGINS_EXPORT extern const char kDebugPluginLoading[];
 WEBKIT_PLUGINS_EXPORT extern const char kDisablePepper3d[];
 WEBKIT_PLUGINS_EXPORT extern const char kPpapiFlashArgs[];
-WEBKIT_PLUGINS_EXPORT extern const char kDisablePepperThreading[];
 
 #if defined(OS_WIN)
 WEBKIT_PLUGINS_EXPORT extern const char kUseOldWMPPlugin[];
diff --git a/webkit/plugins/ppapi/host_globals.cc b/webkit/plugins/ppapi/host_globals.cc
index 6370269..15dedf2 100644
--- a/webkit/plugins/ppapi/host_globals.cc
+++ b/webkit/plugins/ppapi/host_globals.cc
@@ -25,6 +25,7 @@
 using ppapi::CheckIdType;
 using ppapi::MakeTypedId;
 using ppapi::PPIdType;
+using ppapi::ResourceTracker;
 using WebKit::WebConsoleMessage;
 using WebKit::WebString;
 
@@ -74,14 +75,17 @@ WebConsoleMessage MakeLogMessage(PP_LogLevel level,
 
 HostGlobals* HostGlobals::host_globals_ = NULL;
 
-HostGlobals::HostGlobals() : ::ppapi::PpapiGlobals() {
+HostGlobals::HostGlobals()
+    : ::ppapi::PpapiGlobals(),
+      resource_tracker_(ResourceTracker::SINGLE_THREADED) {
   DCHECK(!host_globals_);
   host_globals_ = this;
 }
 
 HostGlobals::HostGlobals(
     ::ppapi::PpapiGlobals::PerThreadForTest per_thread_for_test)
-    : ::ppapi::PpapiGlobals(per_thread_for_test) {
+    : ::ppapi::PpapiGlobals(per_thread_for_test),
+      resource_tracker_(ResourceTracker::SINGLE_THREADED) {
   DCHECK(!host_globals_);
 }
 
diff --git a/webkit/plugins/ppapi/host_var_tracker.cc b/webkit/plugins/ppapi/host_var_tracker.cc
index 5d53499..99518fd 100644
--- a/webkit/plugins/ppapi/host_var_tracker.cc
+++ b/webkit/plugins/ppapi/host_var_tracker.cc
@@ -16,7 +16,9 @@ using ppapi::NPObjectVar;
 namespace webkit {
 namespace ppapi {
 
-HostVarTracker::HostVarTracker() : last_shared_memory_map_id_(0) {
+HostVarTracker::HostVarTracker()
+  : VarTracker(SINGLE_THREADED),
+    last_shared_memory_map_id_(0) {
 }
 
 HostVarTracker::~HostVarTracker() {
@@ -33,7 +35,7 @@ ArrayBufferVar* HostVarTracker::CreateShmArrayBuffer(
 }
 
 void HostVarTracker::AddNPObjectVar(NPObjectVar* object_var) {
-  DCHECK(CalledOnValidThread());
+  CheckThreadingPreconditions();
 
   InstanceMap::iterator found_instance = instance_map_.find(
       object_var->pp_instance());
@@ -53,7 +55,7 @@ void HostVarTracker::AddNPObjectVar(NPObjectVar* object_var) {
 }
 
 void HostVarTracker::RemoveNPObjectVar(NPObjectVar* object_var) {
-  DCHECK(CalledOnValidThread());
+  CheckThreadingPreconditions();
 
   InstanceMap::iterator found_instance = instance_map_.find(
       object_var->pp_instance());
@@ -78,7 +80,7 @@ void HostVarTracker::RemoveNPObjectVar(NPObjectVar* object_var) {
 
 NPObjectVar* HostVarTracker::NPObjectVarForNPObject(PP_Instance instance,
                                                     NPObject* np_object) {
-  DCHECK(CalledOnValidThread());
+  CheckThreadingPreconditions();
 
   InstanceMap::iterator found_instance = instance_map_.find(instance);
   if (found_instance == instance_map_.end())
@@ -93,7 +95,7 @@ NPObjectVar* HostVarTracker::NPObjectVarForNPObject(PP_Instance instance,
 }
 
 int HostVarTracker::GetLiveNPObjectVarsForInstance(PP_Instance instance) const {
-  DCHECK(CalledOnValidThread());
+  CheckThreadingPreconditions();
 
   InstanceMap::const_iterator found = instance_map_.find(instance);
   if (found == instance_map_.end())
@@ -102,7 +104,7 @@ int HostVarTracker::GetLiveNPObjectVarsForInstance(PP_Instance instance) const {
 }
 
 void HostVarTracker::DidDeleteInstance(PP_Instance instance) {
-  DCHECK(CalledOnValidThread());
+  CheckThreadingPreconditions();
 
   InstanceMap::iterator found_instance = instance_map_.find(instance);
   if (found_instance == instance_map_.end())