2 * Copyright (C) 2005, 2008 Apple Inc. All rights reserved.
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.
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 NU
12 * Library General Public License for more details.
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
25 #include "CallFrame.h"
27 #include "ScopeChain.h"
35 // JSLock is only needed to support an obsolete execution model where JavaScriptCore
36 // automatically protected against concurrent access from multiple threads.
37 // So it's safe to disable it on non-mac platforms where we don't have native pthreads.
38 #if (OS(DARWIN) || USE(PTHREADS))
40 // Acquire this mutex before accessing lock-related data.
41 static pthread_mutex_t JSMutex = PTHREAD_MUTEX_INITIALIZER;
43 // Thread-specific key that tells whether a thread holds the JSMutex, and how many times it was taken recursively.
44 pthread_key_t JSLockCount;
46 static void createJSLockCount()
48 pthread_key_create(&JSLockCount, 0);
51 pthread_once_t createJSLockCountOnce = PTHREAD_ONCE_INIT;
53 // Lock nesting count.
54 intptr_t JSLock::lockCount()
56 pthread_once(&createJSLockCountOnce, createJSLockCount);
58 return reinterpret_cast<intptr_t>(pthread_getspecific(JSLockCount));
61 static void setLockCount(intptr_t count)
64 pthread_setspecific(JSLockCount, reinterpret_cast<void*>(count));
67 JSLock::JSLock(ExecState* exec)
68 : m_lockBehavior(exec->globalData().isSharedInstance() ? LockForReal : SilenceAssertionsOnly)
73 JSLock::JSLock(JSGlobalData* globalData)
74 : m_lockBehavior(globalData->isSharedInstance() ? LockForReal : SilenceAssertionsOnly)
79 void JSLock::lock(JSLockBehavior lockBehavior)
82 // Locking "not for real" is a debug-only feature.
83 if (lockBehavior == SilenceAssertionsOnly)
87 pthread_once(&createJSLockCountOnce, createJSLockCount);
89 intptr_t currentLockCount = lockCount();
90 if (!currentLockCount && lockBehavior == LockForReal) {
92 result = pthread_mutex_lock(&JSMutex);
95 setLockCount(currentLockCount + 1);
98 void JSLock::unlock(JSLockBehavior lockBehavior)
103 // Locking "not for real" is a debug-only feature.
104 if (lockBehavior == SilenceAssertionsOnly)
108 intptr_t newLockCount = lockCount() - 1;
109 setLockCount(newLockCount);
110 if (!newLockCount && lockBehavior == LockForReal) {
112 result = pthread_mutex_unlock(&JSMutex);
117 void JSLock::lock(ExecState* exec)
119 lock(exec->globalData().isSharedInstance() ? LockForReal : SilenceAssertionsOnly);
122 void JSLock::unlock(ExecState* exec)
124 unlock(exec->globalData().isSharedInstance() ? LockForReal : SilenceAssertionsOnly);
127 bool JSLock::currentThreadIsHoldingLock()
129 pthread_once(&createJSLockCountOnce, createJSLockCount);
130 return !!pthread_getspecific(JSLockCount);
133 // This is fairly nasty. We allow multiple threads to run on the same
134 // context, and we do not require any locking semantics in doing so -
135 // clients of the API may simply use the context from multiple threads
136 // concurently, and assume this will work. In order to make this work,
137 // We lock the context when a thread enters, and unlock it when it leaves.
138 // However we do not only unlock when the thread returns from its
139 // entry point (evaluate script or call function), we also unlock the
140 // context if the thread leaves JSC by making a call out to an external
141 // function through a callback.
143 // All threads using the context share the same JS stack (the RegisterFile).
144 // Whenever a thread calls into JSC it starts using the RegisterFile from the
145 // previous 'high water mark' - the maximum point the stack has ever grown to
146 // (returned by RegisterFile::end()). So if a first thread calls out to a
147 // callback, and a second thread enters JSC, then also exits by calling out
148 // to a callback, we can be left with stackframes from both threads in the
149 // RegisterFile. As such, a problem may occur should the first thread's
150 // callback complete first, and attempt to return to JSC. Were we to allow
151 // this to happen, and were its stack to grow further, then it may potentially
152 // write over the second thread's call frames.
154 // In avoid JS stack corruption we enforce a policy of only ever allowing two
155 // threads to use a JS context concurrently, and only allowing the second of
156 // these threads to execute until it has completed and fully returned from its
157 // outermost call into JSC. We enforce this policy using 'lockDropDepth'. The
158 // first time a thread exits it will call DropAllLocks - which will do as expected
159 // and drop locks allowing another thread to enter. Should another thread, or the
160 // same thread again, enter JSC (through evaluate script or call function), and exit
161 // again through a callback, then the locks will not be dropped when DropAllLocks
162 // is called (since lockDropDepth is non-zero). Since this thread is still holding
163 // the locks, only it will re able to re-enter JSC (either be returning from the
164 // callback, or by re-entering through another call to evaulate script or call
167 // This policy is slightly more restricive than it needs to be for correctness -
168 // we could validly allow futher entries into JSC from other threads, we only
169 // need ensure that callbacks return in the reverse chronological order of the
170 // order in which they were made - though implementing the less restrictive policy
171 // would likely increase complexity and overhead.
173 static unsigned lockDropDepth = 0;
175 JSLock::DropAllLocks::DropAllLocks(ExecState* exec)
176 : m_lockBehavior(exec->globalData().isSharedInstance() ? LockForReal : SilenceAssertionsOnly)
178 pthread_once(&createJSLockCountOnce, createJSLockCount);
180 if (lockDropDepth++) {
185 m_lockCount = JSLock::lockCount();
186 for (intptr_t i = 0; i < m_lockCount; i++)
187 JSLock::unlock(m_lockBehavior);
190 JSLock::DropAllLocks::DropAllLocks(JSLockBehavior JSLockBehavior)
191 : m_lockBehavior(JSLockBehavior)
193 pthread_once(&createJSLockCountOnce, createJSLockCount);
195 if (lockDropDepth++) {
200 // It is necessary to drop even "unreal" locks, because having a non-zero lock count
201 // will prevent a real lock from being taken.
203 m_lockCount = JSLock::lockCount();
204 for (intptr_t i = 0; i < m_lockCount; i++)
205 JSLock::unlock(m_lockBehavior);
208 JSLock::DropAllLocks::~DropAllLocks()
210 for (intptr_t i = 0; i < m_lockCount; i++)
211 JSLock::lock(m_lockBehavior);
216 #else // (OS(DARWIN) || USE(PTHREADS))
218 JSLock::JSLock(ExecState*)
219 : m_lockBehavior(SilenceAssertionsOnly)
223 // If threading support is off, set the lock count to a constant value of 1 so ssertions
224 // that the lock is held don't fail
225 intptr_t JSLock::lockCount()
230 bool JSLock::currentThreadIsHoldingLock()
235 void JSLock::lock(JSLockBehavior)
239 void JSLock::unlock(JSLockBehavior)
243 void JSLock::lock(ExecState*)
247 void JSLock::unlock(ExecState*)
251 JSLock::DropAllLocks::DropAllLocks(ExecState*)
255 JSLock::DropAllLocks::DropAllLocks(JSLockBehavior)
259 JSLock::DropAllLocks::~DropAllLocks()
263 #endif // (OS(DARWIN) || USE(PTHREADS))