diff --git a/xpcom/threads/nsAutoLock.cpp b/xpcom/threads/nsAutoLock.cpp
new file mode 100644
index 000000000000..46dd7798fb39
--- /dev/null
+++ b/xpcom/threads/nsAutoLock.cpp
@@ -0,0 +1,220 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ *
+ * The contents of this file are subject to the Netscape Public License
+ * Version 1.0 (the "NPL"); you may not use this file except in
+ * compliance with the NPL.  You may obtain a copy of the NPL at
+ * http://www.mozilla.org/NPL/
+ *
+ * Software distributed under the NPL is distributed on an "AS IS" basis,
+ * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the NPL
+ * for the specific language governing rights and limitations under the
+ * NPL.
+ *
+ * The Initial Developer of this code under the NPL is Netscape
+ * Communications Corporation.  Portions created by Netscape are
+ * Copyright (C) 1998 Netscape Communications Corporation.  All Rights
+ * Reserved.
+ */
+
+#include "nsAutoLock.h"
+
+#ifdef DEBUG
+
+#include "plhash.h"
+#include "prprf.h"
+#include "prthread.h"
+#include "nsDebug.h"
+#include "nsVector.h"
+
+static PRUintn      LockStackTPI = (PRUintn)-1;
+static PLHashTable* OrderTable = 0;
+
+static const char* LockTypeNames[] = {"Lock", "Monitor", "CMonitor"};
+
+struct nsNamedVector : public nsVector {
+    const char* mName;
+
+    nsNamedVector(const char* name, PRUint32 initialSize = 0)
+        : nsVector(initialSize),
+          mName(name)
+    {
+    }
+};
+
+PR_STATIC_CALLBACK(PLHashNumber)
+_hash_pointer(const void *key)
+{
+    return PLHashNumber(key) >> 2;
+}
+
+static nsNamedVector* GetVector(PLHashTable* table, const void* key)
+{
+    PLHashNumber hash = _hash_pointer(key);
+    PLHashEntry** hep = PL_HashTableRawLookup(table, hash, key);
+    PLHashEntry* he = *hep;
+    if (he)
+        return (nsNamedVector*) he->value;
+    nsNamedVector* vec = new nsNamedVector(0, 1);
+    if (vec)
+        PL_HashTableRawAdd(table, hep, hash, key, vec);
+    return vec;
+}
+
+static PRBool WellOrdered(const void* addr1, const void* addr2,
+                          nsNamedVector** vec1p, nsNamedVector** vec2p)
+{
+    PLHashTable* table = OrderTable;
+    if (!table) return PR_TRUE;
+
+    // Check whether we've already asserted (addr1 < addr2).
+    nsNamedVector* vec1 = GetVector(table, addr1);
+    if (!vec1) return PR_TRUE;
+    for (PRUint32 i = 0, n = vec1->GetSize(); i < n; i++)
+        if (vec1->Get(i) == addr2)
+            return PR_TRUE;
+
+    // Now check for (addr2 < addr1) and return false if so.
+    nsNamedVector* vec2 = GetVector(table, addr2);
+    if (!vec2) return PR_TRUE;
+    for (i = 0, n = vec2->GetSize(); i < n; i++)
+        if (vec2->Get(i) == addr1) {
+            *vec1p = vec1;
+            *vec2p = vec2;
+            return PR_FALSE;
+        }
+
+    // Assert (addr1 < addr2) into the order table.
+    // XXX fix plvector/nsVector to use const void*
+    vec1->Add((void*) addr2);
+    return PR_TRUE;
+}
+
+nsAutoLockBase::nsAutoLockBase(void* addr, nsAutoLockType type)
+{
+    if (LockStackTPI == PRUintn(-1)) {
+        (void) PR_NewThreadPrivateIndex(&LockStackTPI, 0);
+        OrderTable = PL_NewHashTable(64, _hash_pointer,
+                                     PL_CompareValues, PL_CompareValues,
+                                     0, 0);
+    }
+    nsAutoLockBase* stackTop =
+        (nsAutoLockBase*) PR_GetThreadPrivate(LockStackTPI);
+    if (stackTop) {
+        if (stackTop->mAddr == addr) {
+            // Ignore reentry: it's legal for monitors, and NSPR will assert
+            // if you reenter a PRLock.
+        } else {
+            nsNamedVector* vec1;
+            nsNamedVector* vec2;
+            if (!WellOrdered(stackTop->mAddr, addr, &vec1, &vec2)) {
+                char buf[200];
+                PR_snprintf(buf, sizeof buf,
+                            "potential deadlock between %s%s@%p and %s%s@%p",
+                            vec1->mName ? vec1->mName : "",
+                            LockTypeNames[stackTop->mType],
+                            stackTop->mAddr,
+                            vec2->mName ? vec2->mName : "",
+                            LockTypeNames[type],
+                            addr);
+                NS_ERROR(buf);
+            }
+        }
+    }
+    mAddr = addr;
+    mDown = stackTop;
+    mType = type;
+    (void) PR_SetThreadPrivate(LockStackTPI, this);
+}
+
+nsAutoLockBase::~nsAutoLockBase()
+{
+    (void) PR_SetThreadPrivate(LockStackTPI, mDown);
+}
+
+#endif /* DEBUG */
+
+PRMonitor* nsAutoMonitor::NewMonitor(const char* name)
+{
+    PRMonitor* mon = PR_NewMonitor();
+#ifdef DEBUG
+    if (mon && OrderTable) {
+        nsNamedVector* value = new nsNamedVector(name, 0);
+        if (value)
+            PL_HashTableAdd(OrderTable, mon, value);
+    }
+#endif
+    return mon;
+}
+
+#ifdef DEBUG
+PR_STATIC_CALLBACK(PRIntn)
+_purge_mon(PLHashEntry *he, PRIntn cnt, void* arg)
+{
+    nsNamedVector* vec = (nsNamedVector*) he->value;
+
+    if (he->key == arg)
+        return HT_ENUMERATE_REMOVE;
+    for (PRUint32 i = 0, n = vec->GetSize(); i < n; i++) {
+        if (vec->Get(i) == arg) {
+            vec->Remove(i);
+            break;
+        }
+    }
+    return HT_ENUMERATE_NEXT;
+}
+#endif
+
+void nsAutoMonitor::DestroyMonitor(PRMonitor* mon)
+{
+#ifdef DEBUG
+    if (OrderTable)
+        PL_HashTableEnumerateEntries(OrderTable, _purge_mon, mon);
+#endif
+    PR_DestroyMonitor(mon);
+}
+
+void nsAutoMonitor::Enter()
+{
+#ifdef DEBUG
+    nsAutoLockBase* stackTop =
+        (nsAutoLockBase*) PR_GetThreadPrivate(LockStackTPI);
+    NS_ASSERTION(stackTop == mDown, "non-LIFO nsAutoMonitor::Enter");
+    mDown = stackTop;
+    (void) PR_SetThreadPrivate(LockStackTPI, this);
+#endif
+    PR_EnterMonitor(mMonitor);
+}
+
+void nsAutoMonitor::Exit()
+{
+#ifdef DEBUG
+    (void) PR_SetThreadPrivate(LockStackTPI, mDown);
+#endif
+    PRStatus status = PR_ExitMonitor(mMonitor);
+    NS_ASSERTION(status == PR_SUCCESS, "PR_ExitMonitor failed");
+}
+
+// XXX we don't worry about cached monitors being destroyed behind our back.
+// XXX current NSPR (mozilla/nsprpub/pr/src/threads/prcmon.c) never destroys
+// XXX a cached monitor! potential resource pig in conjunction with necko...
+
+void nsAutoCMonitor::Enter()
+{
+#ifdef DEBUG
+    nsAutoLockBase* stackTop =
+        (nsAutoLockBase*) PR_GetThreadPrivate(LockStackTPI);
+    NS_ASSERTION(stackTop == mDown, "non-LIFO nsAutoCMonitor::Enter");
+    mDown = stackTop;
+    (void) PR_SetThreadPrivate(LockStackTPI, this);
+#endif
+    PR_CEnterMonitor(mLockObject);
+}
+
+void nsAutoCMonitor::Exit()
+{
+#ifdef DEBUG
+    (void) PR_SetThreadPrivate(LockStackTPI, mDown);
+#endif
+    PRStatus status = PR_CExitMonitor(mLockObject);
+    NS_ASSERTION(status == PR_SUCCESS, "PR_CExitMonitor failed");
+}