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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at */
#ifndef mozilla_dom_workers_jsexecutionmanager_h__
#define mozilla_dom_workers_jsexecutionmanager_h__
#include <stdint.h>
#include <deque>
#include "MainThreadUtils.h"
#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/CondVar.h"
#include "mozilla/Mutex.h"
#include "mozilla/RefPtr.h"
#include "nsISupports.h"
class nsIGlobalObject;
namespace mozilla {
class ErrorResult;
namespace dom {
class WorkerPrivate;
// The code in this file is responsible for throttling JS execution. It does
// this by introducing a JSExecutionManager class. An execution manager may be
// applied to any number of worker threads or a DocGroup on the main thread.
// JS environments associated with a JS execution manager may only execute on a
// certain amount of CPU cores in parallel.
// Whenever the main thread, or a worker thread begins executing JS it should
// make sure AutoRequestJSThreadExecution is present on the stack, in practice
// this is done by it being part of AutoEntryScript.
// Whenever the main thread may end up blocking on the activity of a worker
// thread, it should make sure to have an AutoYieldJSThreadExecution object
// on the stack.
// Whenever a worker thread may end up blocking on the main thread or the
// activity of another worker thread, it should make sure to have an
// AutoYieldJSThreadExecution object on the stack.
// Failure to do this may result in a deadlock. When a deadlock occurs due
// to these circumstances we will crash after 20 seconds.
// For the main thread this class should only be used in the case of an
// emergency surrounding exploitability of SharedArrayBuffers. A single
// execution manager will then be shared between all Workers and the main
// thread doc group containing the SharedArrayBuffer and ensure all this code
// only runs in a serialized manner. On the main thread we therefore may have
// 1 execution manager per DocGroup, as this is the granularity at which
// SharedArrayBuffers may be present.
class AutoRequestJSThreadExecution;
class AutoYieldJSThreadExecution;
// This class is used to regulate JS execution when for whatever reason we wish
// to throttle execution of multiple JS environments to occur with a certain
// maximum of synchronously executing threads. This should be used through
// the stack helper classes.
class JSExecutionManager {
explicit JSExecutionManager(int32_t aMaxRunning = 1)
: mMaxRunning(aMaxRunning) {}
enum class RequestState { Granted, ExecutingAlready };
static void Initialize();
static void Shutdown();
static JSExecutionManager* GetSABSerializationManager();
friend class AutoRequestJSThreadExecution;
friend class AutoYieldJSThreadExecution;
~JSExecutionManager() = default;
// Methods used by Auto*JSThreadExecution
// Request execution permission, returns ExecutingAlready if execution was
// already granted or does not apply to this thread.
RequestState RequestJSThreadExecution();
// Yield JS execution, this asserts that permission is actually granted.
void YieldJSThreadExecution();
// Yield JS execution if permission was granted. This returns false if no
// permission is granted. This method is needed because an execution manager
// may have been set in between the ctor and dtor of
// AutoYieldJSThreadExecution.
bool YieldJSThreadExecutionIfGranted();
RequestState RequestJSThreadExecutionMainThread();
// Execution manager currently managing the main thread.
// MainThread access only.
static JSExecutionManager* mCurrentMTManager;
// Workers waiting to be given permission for execution.
// Guarded by mExecutionQueueMutex.
std::deque<WorkerPrivate*> mExecutionQueue
// Number of threads currently executing concurrently for this manager.
// Guarded by mExecutionQueueMutex.
int32_t mRunning MOZ_GUARDED_BY(mExecutionQueueMutex) = 0;
// Number of threads allowed to run concurrently for environments managed
// by this manager.
// Guarded by mExecutionQueueMutex.
int32_t mMaxRunning MOZ_GUARDED_BY(mExecutionQueueMutex) = 1;
// Mutex that guards the execution queue and associated state.
Mutex mExecutionQueueMutex =
// ConditionVariables that blocked threads wait for.
CondVar mExecutionQueueCondVar =
CondVar{mExecutionQueueMutex, "JSExecutionManager::sExecutionQueueMutex"};
// Whether the main thread is currently executing for this manager.
// MainThread access only.
bool mMainThreadIsExecuting = false;
// Whether the main thread is currently awaiting permission to execute. Main
// thread execution is always prioritized.
// Guarded by mExecutionQueueMutex.
bool mMainThreadAwaitingExecution MOZ_GUARDED_BY(mExecutionQueueMutex) =
// Helper for managing execution requests and allowing re-entrant permission
// requests.
class MOZ_STACK_CLASS AutoRequestJSThreadExecution {
explicit AutoRequestJSThreadExecution(nsIGlobalObject* aGlobalObject,
bool aIsMainThread);
~AutoRequestJSThreadExecution() {
if (mExecutionGrantingManager) {
if (mIsMainThread) {
if (mOldGrantingManager) {
JSExecutionManager::mCurrentMTManager = mOldGrantingManager;
// The manager we obtained permission from. nullptr if permission was already
// granted.
RefPtr<JSExecutionManager> mExecutionGrantingManager;
// The manager we had permission from before, and where permission should be
// re-requested upon destruction.
RefPtr<JSExecutionManager> mOldGrantingManager;
// We store this for performance reasons.
bool mIsMainThread;
// Class used to wrap code which essentially exits JS execution and may block
// on other threads.
class MOZ_STACK_CLASS AutoYieldJSThreadExecution {
~AutoYieldJSThreadExecution() {
if (mExecutionGrantingManager) {
if (NS_IsMainThread()) {
JSExecutionManager::mCurrentMTManager = mExecutionGrantingManager;
// Set to the granting manager if we were granted permission here.
RefPtr<JSExecutionManager> mExecutionGrantingManager;
} // namespace dom
} // namespace mozilla
#endif // mozilla_dom_workers_jsexecutionmanager_h__