Changeset 163804 in webkit

Timestamp:

Feb 10, 2014, 11:39:30 AM (11 years ago)

Author:

mark.lam@apple.com

Message:

Remove unnecessary spinLock in JSLock.
<​https://webkit.org/b/128450>

Reviewed by Filip Pizlo.

The JSLock's mutex already provides protection for write access to
JSLock's internal state. The only JSLock state that needs to be read
from any thread including threads that don't own the JSLock is
m_ownerThread, which is used in currentThreadIsHoldingLock() to do an
ownership test on the lock.

It is safe for other threads to read from m_ownerThread because they
only need to know whether its value matches their own thread id
(provided by WTF::currentThread()).

Here are the scenarios for how the ownership test can go:

1. The JSLock has just been initialized and is not owned by any thread.

In this case, m_ownerThread will be 0 and will not match any thread's
thread id. The checking thread will know that it needs to lock the
JSLock before using the VM.

2. The JSLock was previously locked, but now is unlocked.

When we unlock it in JSLock::unlock(), the owner thread clears
m_ownerThread to 0. Hence, this case is the same as (1) above.

3. The JSLock is locked by Thread A. Thread B is checking ownership.

In this case, m_ownerThread will contains the Thread A's thread id.
Thread B will see that the thread id does not match its own and will
proceed to block on the JSLock's mutex to wait for its turn to use
the VM.

With Weak Memory Ordering architectures, Thread A's thread id may
not get written out to memory before Thread B inspects m_ownerThread.
However, though Thread B may not see Thread A's thread id in
m_ownerThread, it will see 0 which is the last value written to it
before the JSLock mutex was unlocked. The mutex unlock would have
executed a memory fence which would have flushed the 0 to
m_ownerThread in memory. Hence, Thread B will know that it does not
own the lock.

Apart from removing the unneeded spin lock code, I also changed the
JSLock code to use currentThreadIsHoldingLock() and setOwnerThread()
instead of accessing m_ownerThread directly.

• runtime/JSLock.cpp:

(JSC::JSLock::JSLock):

(JSC::JSLock::lock):

• Removed spinLock but left the indentation as is to keep the diff to a minimum for better readability. Will unindent in a subsequent patch.

(JSC::JSLock::unlock):

• Before unlocking the mutex, clear m_ownerThread to indicate that the lock is no longer owned.

(JSC::JSLock::currentThreadIsHoldingLock):

• Removed the check of m_lockCount for determining ownership. Checking m_ownerThread is sufficient.

(JSC::JSLock::dropAllLocks):
(JSC::JSLock::dropAllLocksUnconditionally):

• Renamed local locksToDrop to the better name droppedLockCount.
• Clear m_ownerThread since we're unlocking the JSLock.

(JSC::JSLock::grabAllLocks):

• Removed unneeded lock ownership test for lock re-entry case because grabAllLocks() is never used to re-enter a locked JSLock.

(JSC::JSLock::DropAllLocks::DropAllLocks):
(JSC::JSLock::DropAllLocks::~DropAllLocks):

• runtime/JSLock.h:

(JSC::JSLock::setOwnerThread):

Location:

trunk/Source/JavaScriptCore

Files:

• ChangeLog (modified) (1 diff)
• runtime/JSLock.cpp (modified) (13 diffs)
• runtime/JSLock.h (modified) (2 diffs)

trunk/Source/JavaScriptCore/ChangeLog ¶

@@ +1 @@
+2014-02-09  Mark Lam  <mark.lam@apple.com>
+
+        Remove unnecessary spinLock in JSLock.
+        <https://webkit.org/b/128450>
+
+        Reviewed by Filip Pizlo.
+
+        The JSLock's mutex already provides protection for write access to
+        JSLock's internal state. The only JSLock state that needs to be read
+        from any thread including threads that don't own the JSLock is
+        m_ownerThread, which is used in currentThreadIsHoldingLock() to do an
+        ownership test on the lock.
+
+        It is safe for other threads to read from m_ownerThread because they
+        only need to know whether its value matches their own thread id
+        (provided by WTF::currentThread()).
+
+        Here are the scenarios for how the ownership test can go:
+
+        1. The JSLock has just been initialized and is not owned by any thread.
+
+           In this case, m_ownerThread will be 0 and will not match any thread's
+           thread id. The checking thread will know that it needs to lock the
+           JSLock before using the VM.
+
+        2. The JSLock was previously locked, but now is unlocked.
+
+           When we unlock it in JSLock::unlock(), the owner thread clears
+           m_ownerThread to 0. Hence, this case is the same as (1) above.
+
+        3. The JSLock is locked by Thread A. Thread B is checking ownership.
+
+           In this case, m_ownerThread will contains the Thread A's thread id.
+           Thread B will see that the thread id does not match its own and will
+           proceed to block on the JSLock's mutex to wait for its turn to use
+           the VM.
+
+           With Weak Memory Ordering architectures, Thread A's thread id may
+           not get written out to memory before Thread B inspects m_ownerThread.
+           However, though Thread B may not see Thread A's thread id in
+           m_ownerThread, it will see 0 which is the last value written to it
+           before the JSLock mutex was unlocked. The mutex unlock would have
+           executed a memory fence which would have flushed the 0 to
+           m_ownerThread in memory. Hence, Thread B will know that it does not
+           own the lock.
+
+        Apart from removing the unneeded spin lock code, I also changed the
+        JSLock code to use currentThreadIsHoldingLock() and setOwnerThread()
+        instead of accessing m_ownerThread directly.
+
+        * runtime/JSLock.cpp:
+        (JSC::JSLock::JSLock):
+
+        (JSC::JSLock::lock):
+        - Removed spinLock but left the indentation as is to keep the diff to a
+          minimum for better readability. Will unindent in a subsequent patch.
+
+        (JSC::JSLock::unlock):
+        - Before unlocking the mutex, clear m_ownerThread to indicate that the
+          lock is no longer owned.
+
+        (JSC::JSLock::currentThreadIsHoldingLock):
+        - Removed the check of m_lockCount for determining ownership. Checking
+          m_ownerThread is sufficient.
+
+        (JSC::JSLock::dropAllLocks):
+        (JSC::JSLock::dropAllLocksUnconditionally):
+        - Renamed local locksToDrop to the better name droppedLockCount.
+        - Clear m_ownerThread since we're unlocking the JSLock.
+
+        (JSC::JSLock::grabAllLocks):
+        - Removed unneeded lock ownership test for lock re-entry case because
+          grabAllLocks() is never used to re-enter a locked JSLock.
+
+        (JSC::JSLock::DropAllLocks::DropAllLocks):
+        (JSC::JSLock::DropAllLocks::~DropAllLocks):
+
+        * runtime/JSLock.h:
+        (JSC::JSLock::setOwnerThread):
+
 2014-02-10  Filip Pizlo  <fpizlo@apple.com>
 

trunk/Source/JavaScriptCore/runtime/JSLock.cpp ¶

@@ -28 +28 @@
 #include "Operations.h"
 
-#if USE(PTHREADS)
-#include <pthread.h>
-#endif
-
 namespace JSC {
 
@@ -91 +87 @@
     , m_vm(vm)
 {
-    m_spinLock.Init();
 }
 
@@ -107 +102 @@
 {
     ThreadIdentifier currentThread = WTF::currentThread();
-    {
-        SpinLockHolder holder(&m_spinLock);
-        if (m_ownerThread == currentThread && m_lockCount) {
-            m_lockCount++;
-            return;
-        }
+    if (currentThreadIsHoldingLock()) {
+        m_lockCount++;
+        return;
     }
 
     m_lock.lock();
 
-    {
-        SpinLockHolder holder(&m_spinLock);
-        m_ownerThread = currentThread;
-        ASSERT(!m_lockCount);
-        m_lockCount = 1;
-
-        WTFThreadData& threadData = wtfThreadData();
-
-        if (!m_vm->stackPointerAtVMEntry) {
-            m_vm->stackPointerAtVMEntry = &holder; // A proxy for the current stack pointer.
-            threadData.setSavedReservedZoneSize(m_vm->updateStackLimitWithReservedZoneSize(Options::reservedZoneSize()));
-        }
-
-        m_vm->setLastStackTop(threadData.savedLastStackTop());
-    }
+    setOwnerThread(currentThread);
+    ASSERT(!m_lockCount);
+    m_lockCount = 1;
+
+    WTFThreadData& threadData = wtfThreadData();
+
+    if (!m_vm->stackPointerAtVMEntry) {
+        void* p = &p;
+        m_vm->stackPointerAtVMEntry = p; // A proxy for the current stack pointer.
+        threadData.setSavedReservedZoneSize(m_vm->updateStackLimitWithReservedZoneSize(Options::reservedZoneSize()));
+    }
+
+    m_vm->setLastStackTop(threadData.savedLastStackTop());
 }
 
 void JSLock::unlock()
 {
-    SpinLockHolder holder(&m_spinLock);
-    ASSERT(currentThreadIsHoldingLock());
+    RELEASE_ASSERT(currentThreadIsHoldingLock());
 
     m_lockCount--;
@@ -146 +135 @@
             m_vm->updateStackLimitWithReservedZoneSize(wtfThreadData().savedReservedZoneSize());
         }
+        setOwnerThread(0);
         m_lock.unlock();
     }
@@ -162 +152 @@
 bool JSLock::currentThreadIsHoldingLock()
 {
-    return m_lockCount && m_ownerThread == WTF::currentThread();
+    return m_ownerThread == WTF::currentThread();
 }
 
@@ -207 +197 @@
 
 // This function returns the number of locks that were dropped.
-unsigned JSLock::dropAllLocks(SpinLock& spinLock)
-{
-    ASSERT_UNUSED(spinLock, spinLock.IsHeld());
+unsigned JSLock::dropAllLocks()
+{
     // Check if this thread is currently holding the lock.
     // FIXME: Maybe we want to require this, guard with an ASSERT?
-    unsigned locksToDrop = m_lockCount;
-    if (!locksToDrop || m_ownerThread != WTF::currentThread())
+    if (!currentThreadIsHoldingLock())
         return 0;
 
@@ -229 +217 @@
     // m_lockDropDepth is only incremented if any locks were dropped.
     ++m_lockDropDepth;
+
+    unsigned droppedLockCount = m_lockCount;
     m_lockCount = 0;
     if (m_vm) {
@@ -234 +224 @@
         m_vm->updateStackLimitWithReservedZoneSize(wtfThreadData().savedReservedZoneSize());
     }
+    setOwnerThread(0);
     m_lock.unlock();
-    return locksToDrop;
-}
-
-unsigned JSLock::dropAllLocksUnconditionally(SpinLock& spinLock)
-{
-    ASSERT_UNUSED(spinLock, spinLock.IsHeld());
+    return droppedLockCount;
+}
+
+unsigned JSLock::dropAllLocksUnconditionally()
+{
     // Check if this thread is currently holding the lock.
     // FIXME: Maybe we want to require this, guard with an ASSERT?
-    unsigned locksToDrop = m_lockCount;
-    if (!locksToDrop || m_ownerThread != WTF::currentThread())
+    if (!currentThreadIsHoldingLock())
         return 0;
 
@@ -254 +243 @@
     // m_lockDropDepth is only incremented if any locks were dropped.
     ++m_lockDropDepth;
+
+    unsigned droppedLockCount = m_lockCount;
     m_lockCount = 0;
     if (m_vm) {
@@ -259 +250 @@
         m_vm->updateStackLimitWithReservedZoneSize(wtfThreadData().savedReservedZoneSize());
     }
+    setOwnerThread(0);
     m_lock.unlock();
-    return locksToDrop;
-}
-
-void JSLock::grabAllLocks(unsigned droppedLockCount, SpinLock& spinLock)
-{
-    ASSERT(spinLock.IsHeld());
+    return droppedLockCount;
+}
+
+void JSLock::grabAllLocks(unsigned droppedLockCount)
+{
     // If no locks were dropped, nothing to do!
     if (!droppedLockCount)
         return;
 
-    ThreadIdentifier currentThread = WTF::currentThread();
-    // Check if this thread is currently holding the lock.
-    // FIXME: Maybe we want to prohibit this, guard against with an ASSERT?
-    if (m_ownerThread == currentThread && m_lockCount) {
-        m_lockCount += droppedLockCount;
-        --m_lockDropDepth;
-        return;
-    }
-
-    spinLock.Unlock();
+    ASSERT(!currentThreadIsHoldingLock());
+
     m_lock.lock();
-    spinLock.Lock();
-
-    m_ownerThread = currentThread;
+
+    setOwnerThread(WTF::currentThread());
     ASSERT(!m_lockCount);
     m_lockCount = droppedLockCount;
@@ -300 +282 @@
     if (!m_vm)
         return;
-    SpinLock& spinLock = m_vm->apiLock().m_spinLock;
-    SpinLockHolder holder(&spinLock);
 
     if (alwaysDropLocks)
-        m_droppedLockCount = m_vm->apiLock().dropAllLocksUnconditionally(spinLock);
+        m_droppedLockCount = m_vm->apiLock().dropAllLocksUnconditionally();
     else
-        m_droppedLockCount = m_vm->apiLock().dropAllLocks(spinLock);
+        m_droppedLockCount = m_vm->apiLock().dropAllLocks();
 }
 
@@ -315 +295 @@
     if (!m_vm)
         return;
-    SpinLock& spinLock = m_vm->apiLock().m_spinLock;
-    SpinLockHolder holder(&spinLock);
 
     if (alwaysDropLocks)
-        m_droppedLockCount = m_vm->apiLock().dropAllLocksUnconditionally(spinLock);
+        m_droppedLockCount = m_vm->apiLock().dropAllLocksUnconditionally();
     else
-        m_droppedLockCount = m_vm->apiLock().dropAllLocks(spinLock);
+        m_droppedLockCount = m_vm->apiLock().dropAllLocks();
 }
 
@@ -328 +306 @@
     if (!m_vm)
         return;
-    SpinLock& spinLock = m_vm->apiLock().m_spinLock;
-    SpinLockHolder holder(&spinLock);
-    m_vm->apiLock().grabAllLocks(m_droppedLockCount, spinLock);
+    m_vm->apiLock().grabAllLocks(m_droppedLockCount);
 }
 

trunk/Source/JavaScriptCore/runtime/JSLock.h ¶

@@ -25 +25 @@
 #include <wtf/Noncopyable.h>
 #include <wtf/RefPtr.h>
-#include <wtf/TCSpinLock.h>
 #include <wtf/Threading.h>
 
@@ -114 +113 @@
 
     private:
-        unsigned dropAllLocks(SpinLock&);
-        unsigned dropAllLocksUnconditionally(SpinLock&);
-        void grabAllLocks(unsigned lockCount, SpinLock&);
+        void setOwnerThread(ThreadIdentifier owner) { m_ownerThread = owner; }
 
-        SpinLock m_spinLock;
+        unsigned dropAllLocks();
+        unsigned dropAllLocksUnconditionally();
+        void grabAllLocks(unsigned lockCount);
+
         Mutex m_lock;
         ThreadIdentifier m_ownerThread;