Source/JavaScriptCore/runtime/JSLock.h

/*
 * Copyright (C) 2005, 2008, 2009 Apple Inc. All rights reserved.
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Library General Public
 * License as published by the Free Software Foundation; either
 * version 2 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Library General Public License for more details.
 *
 * You should have received a copy of the GNU Library General Public License
 * along with this library; see the file COPYING.LIB.  If not, write to
 * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
 * Boston, MA 02110-1301, USA.
 *
 */

#ifndef JSLock_h
#define JSLock_h

#include <wtf/Assertions.h>
#include <wtf/Noncopyable.h>
#include <wtf/RefPtr.h>
#include <wtf/TCSpinLock.h>
#include <wtf/Threading.h>

namespace JSC {

    // To make it safe to use JavaScript on multiple threads, it is
    // important to lock before doing anything that allocates a
    // JavaScript data structure or that interacts with shared state
    // such as the protect count hash table. The simplest way to lock
    // is to create a local JSLockHolder object in the scope where the lock 
    // must be held and pass it the context that requires protection. 
    // The lock is recursive so nesting is ok. The JSLock 
    // object also acts as a convenience short-hand for running important
    // initialization routines.

    // To avoid deadlock, sometimes it is necessary to temporarily
    // release the lock. Since it is recursive you actually have to
    // release all locks held by your thread. This is safe to do if
    // you are executing code that doesn't require the lock, and you
    // reacquire the right number of locks at the end. You can do this
    // by constructing a locally scoped JSLock::DropAllLocks object. The 
    // DropAllLocks object takes care to release the JSLock only if your
    // thread acquired it to begin with.

    class ExecState;
    class VM;

    // This class is used to protect the initialization of the legacy single 
    // shared VM.
    class GlobalJSLock {
        WTF_MAKE_NONCOPYABLE(GlobalJSLock);
    public:
        JS_EXPORT_PRIVATE GlobalJSLock();
        JS_EXPORT_PRIVATE ~GlobalJSLock();

        static void initialize();
    private:
        static Mutex* s_sharedInstanceLock;
    };

    class JSLockHolder {
    public:
        JS_EXPORT_PRIVATE JSLockHolder(VM*);
        JS_EXPORT_PRIVATE JSLockHolder(VM&);
        JS_EXPORT_PRIVATE JSLockHolder(ExecState*);

        JS_EXPORT_PRIVATE ~JSLockHolder();
    private:
        void init();

        RefPtr<VM> m_vm;
    };

    class JSLock : public ThreadSafeRefCounted<JSLock> {
        WTF_MAKE_NONCOPYABLE(JSLock);
    public:
        JSLock(VM*);
        JS_EXPORT_PRIVATE ~JSLock();

        JS_EXPORT_PRIVATE void lock();
        JS_EXPORT_PRIVATE void unlock();

        static void lock(ExecState*);
        static void unlock(ExecState*);
        static void lock(VM&);
        static void unlock(VM&);

        VM* vm() { return m_vm; }

        JS_EXPORT_PRIVATE bool currentThreadIsHoldingLock();

        unsigned dropAllLocks(SpinLock&);
        unsigned dropAllLocksUnconditionally(SpinLock&);
        void grabAllLocks(unsigned lockCount, SpinLock&);

        void willDestroyVM(VM*);

        class DropAllLocks {
            WTF_MAKE_NONCOPYABLE(DropAllLocks);
        public:
            // By default, we release all locks conditionally. Some clients, such as Mobile Safari,
            // may require that we release all locks unconditionally.
            enum AlwaysDropLocksTag { DontAlwaysDropLocks = 0, AlwaysDropLocks };
            JS_EXPORT_PRIVATE DropAllLocks(ExecState*, AlwaysDropLocksTag = DontAlwaysDropLocks);
            JS_EXPORT_PRIVATE DropAllLocks(VM*, AlwaysDropLocksTag = DontAlwaysDropLocks);
            JS_EXPORT_PRIVATE ~DropAllLocks();
            
        private:
            intptr_t m_lockCount;
            RefPtr<VM> m_vm;
        };

    private:
        SpinLock m_spinLock;
        Mutex m_lock;
        ThreadIdentifier m_ownerThread;
        intptr_t m_lockCount;
        unsigned m_lockDropDepth;
        VM* m_vm;
    };

} // namespace

#endif // JSLock_h