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<dt>Revision</dt> <dd><a href="http://trac.webkit.org/projects/webkit/changeset/163700">163700</a></dd>
<dt>Author</dt> <dd>mark.lam@apple.com</dd>
<dt>Date</dt> <dd>2014-02-08 00:27:13 -0800 (Sat, 08 Feb 2014)</dd>
</dl>
<h3>Log Message</h3>
<pre>JSLock should not "restore" VM stack values if it did not re-grab locks.
<https://webkit.org/b/128447>
Reviewed by Geoffrey Garen.
In the existing code, if DropAllLocks is instantiate with DontAlwaysDropLocks
in a thread that does not own the JSLock, then a bug will manifest where:
1. The DropAllLocks constructor will save the VM's stackPointerAtEntry,
lastStackTop, and reservedZoneSize even though it will not drop the JSLock.
2. The DropAllLocks destructor will restore those 3 values to the VM even
though the JSLock will not grab its internal lock.
The former only causes busy work but does not impact correctness. The latter
however, will corrupt those 3 VM values which belong to the thread that
actually owns the JSLock.
The fix is to only save the values when the JSLock will actually drop its
internal lock, and only restore the values if it did re-grab the internal lock.
* runtime/JSLock.cpp:
(JSC::JSLock::dropAllLocks):
(JSC::JSLock::dropAllLocksUnconditionally):
(JSC::JSLock::grabAllLocks):
(JSC::JSLock::DropAllLocks::DropAllLocks):
- Moved the saving of VM stack values to dropAllLocks() and
dropAllLocksUnconditionally().
(JSC::JSLock::DropAllLocks::~DropAllLocks):
- Moved the restoring of VM stack values to grabAllLocks().</pre>
<h3>Modified Paths</h3>
<ul>
<li><a href="#trunkSourceJavaScriptCoreChangeLog">trunk/Source/JavaScriptCore/ChangeLog</a></li>
<li><a href="#trunkSourceJavaScriptCoreruntimeJSLockcpp">trunk/Source/JavaScriptCore/runtime/JSLock.cpp</a></li>
</ul>
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<h3>Diff</h3>
<a id="trunkSourceJavaScriptCoreChangeLog"></a>
<div class="modfile"><h4>Modified: trunk/Source/JavaScriptCore/ChangeLog (163699 => 163700)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/JavaScriptCore/ChangeLog 2014-02-08 08:26:57 UTC (rev 163699)
+++ trunk/Source/JavaScriptCore/ChangeLog 2014-02-08 08:27:13 UTC (rev 163700)
</span><span class="lines">@@ -1,3 +1,35 @@
</span><ins>+2014-02-07 Mark Lam <mark.lam@apple.com>
+
+ JSLock should not "restore" VM stack values if it did not re-grab locks.
+ <https://webkit.org/b/128447>
+
+ Reviewed by Geoffrey Garen.
+
+ In the existing code, if DropAllLocks is instantiate with DontAlwaysDropLocks
+ in a thread that does not own the JSLock, then a bug will manifest where:
+
+ 1. The DropAllLocks constructor will save the VM's stackPointerAtEntry,
+ lastStackTop, and reservedZoneSize even though it will not drop the JSLock.
+ 2. The DropAllLocks destructor will restore those 3 values to the VM even
+ though the JSLock will not grab its internal lock.
+
+ The former only causes busy work but does not impact correctness. The latter
+ however, will corrupt those 3 VM values which belong to the thread that
+ actually owns the JSLock.
+
+ The fix is to only save the values when the JSLock will actually drop its
+ internal lock, and only restore the values if it did re-grab the internal lock.
+
+ * runtime/JSLock.cpp:
+ (JSC::JSLock::dropAllLocks):
+ (JSC::JSLock::dropAllLocksUnconditionally):
+ (JSC::JSLock::grabAllLocks):
+ (JSC::JSLock::DropAllLocks::DropAllLocks):
+ - Moved the saving of VM stack values to dropAllLocks() and
+ dropAllLocksUnconditionally().
+ (JSC::JSLock::DropAllLocks::~DropAllLocks):
+ - Moved the restoring of VM stack values to grabAllLocks().
+
</ins><span class="cx"> 2014-02-07 Filip Pizlo <fpizlo@apple.com>
</span><span class="cx">
</span><span class="cx"> Don't throw away code if there is code on the worklists
</span></span></pre></div>
<a id="trunkSourceJavaScriptCoreruntimeJSLockcpp"></a>
<div class="modfile"><h4>Modified: trunk/Source/JavaScriptCore/runtime/JSLock.cpp (163699 => 163700)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/JavaScriptCore/runtime/JSLock.cpp 2014-02-08 08:26:57 UTC (rev 163699)
+++ trunk/Source/JavaScriptCore/runtime/JSLock.cpp 2014-02-08 08:27:13 UTC (rev 163700)
</span><span class="lines">@@ -221,6 +221,11 @@
</span><span class="cx"> if (m_lockDropDepth)
</span><span class="cx"> return 0;
</span><span class="cx">
</span><ins>+ WTFThreadData& threadData = wtfThreadData();
+ threadData.setSavedStackPointerAtVMEntry(m_vm->stackPointerAtVMEntry);
+ threadData.setSavedLastStackTop(m_vm->lastStackTop());
+ threadData.setSavedReservedZoneSize(m_vm->reservedZoneSize());
+
</ins><span class="cx"> // m_lockDropDepth is only incremented if any locks were dropped.
</span><span class="cx"> ++m_lockDropDepth;
</span><span class="cx"> m_lockCount = 0;
</span><span class="lines">@@ -241,6 +246,11 @@
</span><span class="cx"> if (!lockCount || m_ownerThread != WTF::currentThread())
</span><span class="cx"> return 0;
</span><span class="cx">
</span><ins>+ WTFThreadData& threadData = wtfThreadData();
+ threadData.setSavedStackPointerAtVMEntry(m_vm->stackPointerAtVMEntry);
+ threadData.setSavedLastStackTop(m_vm->lastStackTop());
+ threadData.setSavedReservedZoneSize(m_vm->reservedZoneSize());
+
</ins><span class="cx"> // m_lockDropDepth is only incremented if any locks were dropped.
</span><span class="cx"> ++m_lockDropDepth;
</span><span class="cx"> m_lockCount = 0;
</span><span class="lines">@@ -276,6 +286,11 @@
</span><span class="cx"> ASSERT(!m_lockCount);
</span><span class="cx"> m_lockCount = lockCount;
</span><span class="cx"> --m_lockDropDepth;
</span><ins>+
+ WTFThreadData& threadData = wtfThreadData();
+ m_vm->stackPointerAtVMEntry = threadData.savedStackPointerAtVMEntry();
+ m_vm->setLastStackTop(threadData.savedLastStackTop());
+ m_vm->updateStackLimitWithReservedZoneSize(threadData.savedReservedZoneSize());
</ins><span class="cx"> }
</span><span class="cx">
</span><span class="cx"> JSLock::DropAllLocks::DropAllLocks(ExecState* exec, AlwaysDropLocksTag alwaysDropLocks)
</span><span class="lines">@@ -287,12 +302,6 @@
</span><span class="cx"> SpinLock& spinLock = m_vm->apiLock().m_spinLock;
</span><span class="cx"> SpinLockHolder holder(&spinLock);
</span><span class="cx">
</span><del>- WTFThreadData& threadData = wtfThreadData();
-
- threadData.setSavedStackPointerAtVMEntry(m_vm->stackPointerAtVMEntry);
- threadData.setSavedLastStackTop(m_vm->lastStackTop());
- threadData.setSavedReservedZoneSize(m_vm->reservedZoneSize());
-
</del><span class="cx"> if (alwaysDropLocks)
</span><span class="cx"> m_lockCount = m_vm->apiLock().dropAllLocksUnconditionally(spinLock);
</span><span class="cx"> else
</span><span class="lines">@@ -308,12 +317,6 @@
</span><span class="cx"> SpinLock& spinLock = m_vm->apiLock().m_spinLock;
</span><span class="cx"> SpinLockHolder holder(&spinLock);
</span><span class="cx">
</span><del>- WTFThreadData& threadData = wtfThreadData();
-
- threadData.setSavedStackPointerAtVMEntry(m_vm->stackPointerAtVMEntry);
- threadData.setSavedLastStackTop(m_vm->lastStackTop());
- threadData.setSavedReservedZoneSize(m_vm->reservedZoneSize());
-
</del><span class="cx"> if (alwaysDropLocks)
</span><span class="cx"> m_lockCount = m_vm->apiLock().dropAllLocksUnconditionally(spinLock);
</span><span class="cx"> else
</span><span class="lines">@@ -327,12 +330,6 @@
</span><span class="cx"> SpinLock& spinLock = m_vm->apiLock().m_spinLock;
</span><span class="cx"> SpinLockHolder holder(&spinLock);
</span><span class="cx"> m_vm->apiLock().grabAllLocks(m_lockCount, spinLock);
</span><del>-
- WTFThreadData& threadData = wtfThreadData();
-
- m_vm->stackPointerAtVMEntry = threadData.savedStackPointerAtVMEntry();
- m_vm->setLastStackTop(threadData.savedLastStackTop());
- m_vm->updateStackLimitWithReservedZoneSize(threadData.savedReservedZoneSize());
</del><span class="cx"> }
</span><span class="cx">
</span><span class="cx"> } // namespace JSC
</span></span></pre>
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