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1 /*
2  * Copyright (C) 2005, 2008, 2009 Apple Inc. All rights reserved.
3  *
4  * This library is free software; you can redistribute it and/or
5  * modify it under the terms of the GNU Library General Public
6  * License as published by the Free Software Foundation; either
7  * version 2 of the License, or (at your option) any later version.
8  *
9  * This library is distributed in the hope that it will be useful,
10  * but WITHOUT ANY WARRANTY; without even the implied warranty of
11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
12  * Library General Public License for more details.
13  *
14  * You should have received a copy of the GNU Library General Public License
15  * along with this library; see the file COPYING.LIB.  If not, write to
16  * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
17  * Boston, MA 02110-1301, USA.
18  *
19  */
20 
21 #ifndef JSLock_h
22 #define JSLock_h
23 
24 #include <wtf/Assertions.h>
25 #include <wtf/Noncopyable.h>
26 
27 namespace JSC {
28 
29     // To make it safe to use JavaScript on multiple threads, it is
30     // important to lock before doing anything that allocates a
31     // JavaScript data structure or that interacts with shared state
32     // such as the protect count hash table. The simplest way to lock
33     // is to create a local JSLock object in the scope where the lock
34     // must be held. The lock is recursive so nesting is ok. The JSLock
35     // object also acts as a convenience short-hand for running important
36     // initialization routines.
37 
38     // To avoid deadlock, sometimes it is necessary to temporarily
39     // release the lock. Since it is recursive you actually have to
40     // release all locks held by your thread. This is safe to do if
41     // you are executing code that doesn't require the lock, and you
42     // reacquire the right number of locks at the end. You can do this
43     // by constructing a locally scoped JSLock::DropAllLocks object. The
44     // DropAllLocks object takes care to release the JSLock only if your
45     // thread acquired it to begin with.
46 
47     // For contexts other than the single shared one, implicit locking is not done,
48     // but we still need to perform all the counting in order to keep debug
49     // assertions working, so that clients that use the shared context don't break.
50 
51     class ExecState;
52     class JSGlobalData;
53 
54     enum JSLockBehavior { SilenceAssertionsOnly, LockForReal };
55 
56     class JSLock {
57         WTF_MAKE_NONCOPYABLE(JSLock);
58     public:
59         JSLock(ExecState*);
60         JSLock(JSGlobalData*);
61 
JSLock(JSLockBehavior lockBehavior)62         JSLock(JSLockBehavior lockBehavior)
63             : m_lockBehavior(lockBehavior)
64         {
65 #ifdef NDEBUG
66             // Locking "not for real" is a debug-only feature.
67             if (lockBehavior == SilenceAssertionsOnly)
68                 return;
69 #endif
70             lock(lockBehavior);
71         }
72 
~JSLock()73         ~JSLock()
74         {
75 #ifdef NDEBUG
76             // Locking "not for real" is a debug-only feature.
77             if (m_lockBehavior == SilenceAssertionsOnly)
78                 return;
79 #endif
80             unlock(m_lockBehavior);
81         }
82 
83         static void lock(JSLockBehavior);
84         static void unlock(JSLockBehavior);
85         static void lock(ExecState*);
86         static void unlock(ExecState*);
87 
88         static intptr_t lockCount();
89         static bool currentThreadIsHoldingLock();
90 
91         JSLockBehavior m_lockBehavior;
92 
93         class DropAllLocks {
94             WTF_MAKE_NONCOPYABLE(DropAllLocks);
95         public:
96             DropAllLocks(ExecState* exec);
97             DropAllLocks(JSLockBehavior);
98             ~DropAllLocks();
99 
100         private:
101             intptr_t m_lockCount;
102             JSLockBehavior m_lockBehavior;
103         };
104     };
105 
106 } // namespace
107 
108 #endif // JSLock_h
109