62
62
JSLock::JSLock(ExecState* exec)
63
: m_lockingForReal(exec->globalData().isSharedInstance)
63
: m_lockBehavior(exec->globalData().isSharedInstance ? LockForReal : SilenceAssertionsOnly)
65
lock(m_lockingForReal);
68
void JSLock::lock(bool lockForReal)
68
void JSLock::lock(JSLockBehavior lockBehavior)
71
71
// Locking "not for real" is a debug-only feature.
72
if (lockBehavior == SilenceAssertionsOnly)
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pthread_once(&createJSLockCountOnce, createJSLockCount);
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78
intptr_t currentLockCount = lockCount();
79
if (!currentLockCount && lockForReal) {
79
if (!currentLockCount && lockBehavior == LockForReal) {
81
81
result = pthread_mutex_lock(&JSMutex);
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119
return !!pthread_getspecific(JSLockCount);
122
// This is fairly nasty. We allow multiple threads to run on the same
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// context, and we do not require any locking semantics in doing so -
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// clients of the API may simply use the context from multiple threads
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// concurently, and assume this will work. In order to make this work,
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// We lock the context when a thread enters, and unlock it when it leaves.
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// However we do not only unlock when the thread returns from its
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// entry point (evaluate script or call function), we also unlock the
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// context if the thread leaves JSC by making a call out to an external
130
// function through a callback.
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// All threads using the context share the same JS stack (the RegisterFile).
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// Whenever a thread calls into JSC it starts using the RegisterFile from the
134
// previous 'high water mark' - the maximum point the stack has ever grown to
135
// (returned by RegisterFile::end()). So if a first thread calls out to a
136
// callback, and a second thread enters JSC, then also exits by calling out
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// to a callback, we can be left with stackframes from both threads in the
138
// RegisterFile. As such, a problem may occur should the first thread's
139
// callback complete first, and attempt to return to JSC. Were we to allow
140
// this to happen, and were its stack to grow further, then it may potentially
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// write over the second thread's call frames.
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// In avoid JS stack corruption we enforce a policy of only ever allowing two
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// threads to use a JS context concurrently, and only allowing the second of
145
// these threads to execute until it has completed and fully returned from its
146
// outermost call into JSC. We enforce this policy using 'lockDropDepth'. The
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// first time a thread exits it will call DropAllLocks - which will do as expected
148
// and drop locks allowing another thread to enter. Should another thread, or the
149
// same thread again, enter JSC (through evaluate script or call function), and exit
150
// again through a callback, then the locks will not be dropped when DropAllLocks
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// is called (since lockDropDepth is non-zero). Since this thread is still holding
152
// the locks, only it will re able to re-enter JSC (either be returning from the
153
// callback, or by re-entering through another call to evaulate script or call
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// This policy is slightly more restricive than it needs to be for correctness -
157
// we could validly allow futher entries into JSC from other threads, we only
158
// need ensure that callbacks return in the reverse chronological order of the
159
// order in which they were made - though implementing the less restrictive policy
160
// would likely increase complexity and overhead.
162
static unsigned lockDropDepth = 0;
122
164
JSLock::DropAllLocks::DropAllLocks(ExecState* exec)
123
: m_lockingForReal(exec->globalData().isSharedInstance)
165
: m_lockBehavior(exec->globalData().isSharedInstance ? LockForReal : SilenceAssertionsOnly)
125
167
pthread_once(&createJSLockCountOnce, createJSLockCount);
169
if (lockDropDepth++) {
127
174
m_lockCount = JSLock::lockCount();
128
175
for (intptr_t i = 0; i < m_lockCount; i++)
129
JSLock::unlock(m_lockingForReal);
176
JSLock::unlock(m_lockBehavior);
132
JSLock::DropAllLocks::DropAllLocks(bool lockingForReal)
133
: m_lockingForReal(lockingForReal)
179
JSLock::DropAllLocks::DropAllLocks(JSLockBehavior JSLockBehavior)
180
: m_lockBehavior(JSLockBehavior)
135
182
pthread_once(&createJSLockCountOnce, createJSLockCount);
184
if (lockDropDepth++) {
137
189
// It is necessary to drop even "unreal" locks, because having a non-zero lock count
138
190
// will prevent a real lock from being taken.
140
192
m_lockCount = JSLock::lockCount();
141
193
for (intptr_t i = 0; i < m_lockCount; i++)
142
JSLock::unlock(m_lockingForReal);
194
JSLock::unlock(m_lockBehavior);
145
197
JSLock::DropAllLocks::~DropAllLocks()
147
199
for (intptr_t i = 0; i < m_lockCount; i++)
148
JSLock::lock(m_lockingForReal);
200
JSLock::lock(m_lockBehavior);
153
207
JSLock::JSLock(ExecState*)
154
: m_lockingForReal(false)
208
: m_lockBehavior(SilenceAssertionsOnly)