diff --git a/storage/test/gtest/test_deadlock_detector.cpp b/storage/test/gtest/test_deadlock_detector.cpp
index 5fbd3cb1d475..1f09285deaca 100644
--- a/storage/test/gtest/test_deadlock_detector.cpp
+++ b/storage/test/gtest/test_deadlock_detector.cpp
@@ -10,6 +10,7 @@
 // duplication.
 
 #include "mozilla/CondVar.h"
+#include "mozilla/RecursiveMutex.h"
 #include "mozilla/ReentrantMonitor.h"
 #include "SQLiteMutex.h"
 
diff --git a/xpcom/tests/gtest/TestDeadlockDetector.cpp b/xpcom/tests/gtest/TestDeadlockDetector.cpp
index 404a32b18499..f829817dfe03 100644
--- a/xpcom/tests/gtest/TestDeadlockDetector.cpp
+++ b/xpcom/tests/gtest/TestDeadlockDetector.cpp
@@ -12,6 +12,7 @@
 #include "nsMemory.h"
 
 #include "mozilla/CondVar.h"
+#include "mozilla/RecursiveMutex.h"
 #include "mozilla/ReentrantMonitor.h"
 #include "mozilla/Mutex.h"
 
@@ -202,6 +203,31 @@ TEST_F(TESTNAME(DeadlockDetectorTest), TESTNAME(Sanity4DeathTest))
     ASSERT_DEATH_IF_SUPPORTED(Sanity4_Child(), regex);
 }
 
+int
+Sanity5_Child()
+{
+    DisableCrashReporter();
+
+    mozilla::RecursiveMutex m1("dd.sanity4.m1");
+    MUTEX m2("dd.sanity4.m2");
+    m1.Lock();
+    m2.Lock();
+    m1.Lock();
+    return 0;
+}
+
+TEST_F(TESTNAME(DeadlockDetectorTest), TESTNAME(Sanity5DeathTest))
+{
+    const char* const regex =
+        "Re-entering RecursiveMutex after acquiring other resources.*"
+        "###!!! ERROR: Potential deadlock detected.*"
+        "=== Cyclical dependency starts at.*--- RecursiveMutex : dd.sanity4.m1.*"
+        "--- Next dependency:.*--- Mutex : dd.sanity4.m2.*"
+        "=== Cycle completed at.*--- RecursiveMutex : dd.sanity4.m1.*"
+        "###!!! ASSERTION: Potential deadlock detected.*";
+    ASSERT_DEATH_IF_SUPPORTED(Sanity5_Child(), regex);
+}
+
 //-----------------------------------------------------------------------------
 // Multithreaded tests
 
diff --git a/xpcom/tests/gtest/TestRecursiveMutex.cpp b/xpcom/tests/gtest/TestRecursiveMutex.cpp
new file mode 100644
index 000000000000..3f0557aed312
--- /dev/null
+++ b/xpcom/tests/gtest/TestRecursiveMutex.cpp
@@ -0,0 +1,25 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "nsThreadUtils.h"
+#include "mozilla/RecursiveMutex.h"
+#include "gtest/gtest.h"
+
+using mozilla::RecursiveMutex;
+using mozilla::RecursiveMutexAutoLock;
+
+// Basic test to make sure the underlying implementation of RecursiveMutex is,
+// well, actually recursively acquirable.
+
+TEST(RecursiveMutex, SmokeTest)
+{
+  RecursiveMutex mutex("testing mutex");
+
+  RecursiveMutexAutoLock lock1(mutex);
+  RecursiveMutexAutoLock lock2(mutex);
+
+  //...and done.
+}
diff --git a/xpcom/tests/gtest/TestSlicedInputStream.cpp b/xpcom/tests/gtest/TestSlicedInputStream.cpp
index 668e7b933637..3279758e535b 100644
--- a/xpcom/tests/gtest/TestSlicedInputStream.cpp
+++ b/xpcom/tests/gtest/TestSlicedInputStream.cpp
@@ -2,6 +2,7 @@
 
 #include "nsCOMPtr.h"
 #include "nsIInputStream.h"
+#include "nsIPipe.h"
 #include "nsStreamUtils.h"
 #include "nsString.h"
 #include "nsStringStream.h"
diff --git a/xpcom/tests/gtest/moz.build b/xpcom/tests/gtest/moz.build
index db091b5a7a98..6010328a785f 100644
--- a/xpcom/tests/gtest/moz.build
+++ b/xpcom/tests/gtest/moz.build
@@ -32,6 +32,7 @@ UNIFIED_SOURCES += [
     'TestPLDHash.cpp',
     'TestPriorityQueue.cpp',
     'TestRacingServiceManager.cpp',
+    'TestRecursiveMutex.cpp',
     'TestRWLock.cpp',
     'TestSlicedInputStream.cpp',
     'TestSnappyStreams.cpp',
diff --git a/xpcom/threads/BlockingResourceBase.cpp b/xpcom/threads/BlockingResourceBase.cpp
index ff4c83abcb26..88b1ce1b773b 100644
--- a/xpcom/threads/BlockingResourceBase.cpp
+++ b/xpcom/threads/BlockingResourceBase.cpp
@@ -20,6 +20,7 @@
 
 #include "mozilla/CondVar.h"
 #include "mozilla/DeadlockDetector.h"
+#include "mozilla/RecursiveMutex.h"
 #include "mozilla/ReentrantMonitor.h"
 #include "mozilla/Mutex.h"
 #include "mozilla/RWLock.h"
@@ -38,7 +39,7 @@ namespace mozilla {
 // static members
 const char* const BlockingResourceBase::kResourceTypeName[] = {
   // needs to be kept in sync with BlockingResourceType
-  "Mutex", "ReentrantMonitor", "CondVar"
+  "Mutex", "ReentrantMonitor", "CondVar", "RecursiveMutex"
 };
 
 #ifdef DEBUG
@@ -527,6 +528,66 @@ ReentrantMonitor::Wait(PRIntervalTime aInterval)
 }
 
 
+//
+// Debug implementation of RecursiveMutex
+void
+RecursiveMutex::Lock()
+{
+  BlockingResourceBase* chainFront = ResourceChainFront();
+
+  // the code below implements mutex reentrancy semantics
+
+  if (this == chainFront) {
+    // immediately re-entered the mutex: acceptable
+    LockInternal();
+    ++mEntryCount;
+    return;
+  }
+
+  // this is sort of a hack around not recording the thread that
+  // owns this monitor
+  if (chainFront) {
+    for (BlockingResourceBase* br = ResourceChainPrev(chainFront);
+         br;
+         br = ResourceChainPrev(br)) {
+      if (br == this) {
+        NS_WARNING(
+          "Re-entering RecursiveMutex after acquiring other resources.");
+
+        // show the caller why this is potentially bad
+        CheckAcquire();
+
+        LockInternal();
+        ++mEntryCount;
+        return;
+      }
+    }
+  }
+
+  CheckAcquire();
+  LockInternal();
+  NS_ASSERTION(mEntryCount == 0, "RecursiveMutex isn't free!");
+  Acquire();       // protected by us
+  mOwningThread = PR_GetCurrentThread();
+  mEntryCount = 1;
+}
+
+void
+RecursiveMutex::Unlock()
+{
+  if (--mEntryCount == 0) {
+    Release();              // protected by us
+    mOwningThread = nullptr;
+  }
+  UnlockInternal();
+}
+
+void
+RecursiveMutex::AssertCurrentThreadIn()
+{
+  MOZ_ASSERT(IsAcquired() && mOwningThread == PR_GetCurrentThread());
+}
+
 //
 // Debug implementation of CondVar
 nsresult
diff --git a/xpcom/threads/BlockingResourceBase.h b/xpcom/threads/BlockingResourceBase.h
index d70fbcbbf38c..5c8a08524286 100644
--- a/xpcom/threads/BlockingResourceBase.h
+++ b/xpcom/threads/BlockingResourceBase.h
@@ -50,7 +50,7 @@ class BlockingResourceBase
 {
 public:
   // Needs to be kept in sync with kResourceTypeNames.
-  enum BlockingResourceType { eMutex, eReentrantMonitor, eCondVar };
+  enum BlockingResourceType { eMutex, eReentrantMonitor, eCondVar, eRecursiveMutex };
 
   /**
    * kResourceTypeName
diff --git a/xpcom/threads/RecursiveMutex.cpp b/xpcom/threads/RecursiveMutex.cpp
new file mode 100644
index 000000000000..97f5b28d92ee
--- /dev/null
+++ b/xpcom/threads/RecursiveMutex.cpp
@@ -0,0 +1,89 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/RecursiveMutex.h"
+
+#ifdef XP_WIN
+#include <windows.h>
+
+#define NativeHandle(m) (reinterpret_cast<CRITICAL_SECTION*>(&m))
+#endif
+
+namespace mozilla {
+
+RecursiveMutex::RecursiveMutex(const char* aName MOZ_GUARD_OBJECT_NOTIFIER_PARAM_IN_IMPL)
+  : BlockingResourceBase(aName, eRecursiveMutex)
+#ifdef DEBUG
+  , mOwningThread(nullptr)
+  , mEntryCount(0)
+#endif
+{
+  MOZ_GUARD_OBJECT_NOTIFIER_INIT;
+#ifdef XP_WIN
+  // This number was adapted from NSPR.
+  static const DWORD sLockSpinCount = 100;
+
+#if defined(RELEASE_OR_BETA)
+  // Vista and later automatically allocate and subsequently leak a debug info
+  // object for each critical section that we allocate unless we tell the
+  // system not to do that.
+  DWORD flags = CRITICAL_SECTION_NO_DEBUG_INFO;
+#else
+  DWORD flags = 0;
+#endif
+  BOOL r = InitializeCriticalSectionEx(NativeHandle(mMutex),
+                                       sLockSpinCount, flags);
+  MOZ_RELEASE_ASSERT(r);
+#else
+  pthread_mutexattr_t attr;
+
+  MOZ_RELEASE_ASSERT(pthread_mutexattr_init(&attr) == 0,
+                     "pthread_mutexattr_init failed");
+
+  MOZ_RELEASE_ASSERT(pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE) == 0,
+                     "pthread_mutexattr_settype failed");
+
+  MOZ_RELEASE_ASSERT(pthread_mutex_init(&mMutex, &attr) == 0,
+                     "pthread_mutex_init failed");
+
+  MOZ_RELEASE_ASSERT(pthread_mutexattr_destroy(&attr) == 0,
+                     "pthread_mutexattr_destroy failed");
+#endif
+}
+
+RecursiveMutex::~RecursiveMutex()
+{
+#ifdef XP_WIN
+  DeleteCriticalSection(NativeHandle(mMutex));
+#else
+  MOZ_RELEASE_ASSERT(pthread_mutex_destroy(&mMutex) == 0,
+                     "pthread_mutex_destroy failed");
+#endif
+}
+
+void
+RecursiveMutex::LockInternal()
+{
+#ifdef XP_WIN
+  EnterCriticalSection(NativeHandle(mMutex));
+#else
+  MOZ_RELEASE_ASSERT(pthread_mutex_lock(&mMutex) == 0,
+                     "pthread_mutex_lock failed");
+#endif
+}
+
+void
+RecursiveMutex::UnlockInternal()
+{
+#ifdef XP_WIN
+  LeaveCriticalSection(NativeHandle(mMutex));
+#else
+  MOZ_RELEASE_ASSERT(pthread_mutex_unlock(&mMutex) == 0,
+                     "pthread_mutex_unlock failed");
+#endif
+}
+
+} // namespace mozilla
diff --git a/xpcom/threads/RecursiveMutex.h b/xpcom/threads/RecursiveMutex.h
new file mode 100644
index 000000000000..e1b28fafb580
--- /dev/null
+++ b/xpcom/threads/RecursiveMutex.h
@@ -0,0 +1,124 @@
+
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+// A lock that can be acquired multiple times on the same thread.
+
+#ifndef mozilla_RecursiveMutex_h
+#define mozilla_RecursiveMutex_h
+
+#include "mozilla/BlockingResourceBase.h"
+#include "mozilla/GuardObjects.h"
+
+#ifndef XP_WIN
+#include <pthread.h>
+#endif
+
+namespace mozilla {
+
+class RecursiveMutex : public BlockingResourceBase
+{
+public:
+  explicit RecursiveMutex(const char* aName MOZ_GUARD_OBJECT_NOTIFIER_PARAM);
+  ~RecursiveMutex();
+
+#ifdef DEBUG
+  void Lock();
+  void Unlock();
+#else
+  void Lock() { LockInternal(); }
+  void Unlock() { UnlockInternal(); }
+#endif
+
+  void AssertCurrentThreadIn()
+#ifdef DEBUG
+    ;
+#else
+  {
+  }
+#endif
+
+private:
+  RecursiveMutex() = delete;
+  RecursiveMutex(const RecursiveMutex&) = delete;
+  RecursiveMutex& operator=(const RecursiveMutex&) = delete;
+
+  void LockInternal();
+  void UnlockInternal();
+
+#ifdef DEBUG
+  PRThread* mOwningThread;
+  size_t mEntryCount;
+#endif
+
+#if !defined(XP_WIN)
+  pthread_mutex_t mMutex;
+#else
+  // We eschew including windows.h and using CRITICAL_SECTION here so that files
+  // including us don't also pull in windows.h.  Just use a type that's big
+  // enough for CRITICAL_SECTION, and we'll fix it up later.
+  void* mMutex[6];
+#endif
+  MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
+};
+
+class MOZ_RAII RecursiveMutexAutoLock
+{
+public:
+  explicit RecursiveMutexAutoLock(RecursiveMutex& aRecursiveMutex
+                                  MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
+    : mRecursiveMutex(&aRecursiveMutex)
+  {
+    MOZ_GUARD_OBJECT_NOTIFIER_INIT;
+    NS_ASSERTION(mRecursiveMutex, "null mutex");
+    mRecursiveMutex->Lock();
+  }
+
+  ~RecursiveMutexAutoLock(void)
+  {
+    mRecursiveMutex->Unlock();
+  }
+
+private:
+  RecursiveMutexAutoLock() = delete;
+  RecursiveMutexAutoLock(const RecursiveMutexAutoLock&) = delete;
+  RecursiveMutexAutoLock& operator=(const RecursiveMutexAutoLock&) = delete;
+  static void* operator new(size_t) CPP_THROW_NEW;
+
+  mozilla::RecursiveMutex* mRecursiveMutex;
+  MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
+};
+
+class MOZ_RAII RecursiveMutexAutoUnlock
+{
+public:
+  explicit RecursiveMutexAutoUnlock(RecursiveMutex& aRecursiveMutex
+                                    MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
+    : mRecursiveMutex(&aRecursiveMutex)
+  {
+    MOZ_GUARD_OBJECT_NOTIFIER_INIT;
+    NS_ASSERTION(mRecursiveMutex, "null mutex");
+    mRecursiveMutex->Unlock();
+  }
+
+  ~RecursiveMutexAutoUnlock(void)
+  {
+    mRecursiveMutex->Lock();
+  }
+
+private:
+  RecursiveMutexAutoUnlock() = delete;
+  RecursiveMutexAutoUnlock(const RecursiveMutexAutoUnlock&) = delete;
+  RecursiveMutexAutoUnlock& operator=(const RecursiveMutexAutoUnlock&) = delete;
+  static void* operator new(size_t) CPP_THROW_NEW;
+
+  mozilla::RecursiveMutex* mRecursiveMutex;
+  MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
+};
+
+} // namespace mozilla
+
+#endif // mozilla_RecursiveMutex_h
diff --git a/xpcom/threads/ReentrantMonitor.h b/xpcom/threads/ReentrantMonitor.h
index dfc6fe2520a6..75661a1a1f27 100644
--- a/xpcom/threads/ReentrantMonitor.h
+++ b/xpcom/threads/ReentrantMonitor.h
@@ -256,5 +256,4 @@ private:
 
 } // namespace mozilla
 
-
 #endif // ifndef mozilla_ReentrantMonitor_h
diff --git a/xpcom/threads/moz.build b/xpcom/threads/moz.build
index e231da3f52ad..3ded2058980e 100644
--- a/xpcom/threads/moz.build
+++ b/xpcom/threads/moz.build
@@ -49,6 +49,7 @@ EXPORTS.mozilla += [
     'Monitor.h',
     'MozPromise.h',
     'Mutex.h',
+    'RecursiveMutex.h',
     'ReentrantMonitor.h',
     'RWLock.h',
     'SchedulerGroup.h',
@@ -82,6 +83,7 @@ UNIFIED_SOURCES += [
     'nsThreadPool.cpp',
     'nsThreadUtils.cpp',
     'nsTimerImpl.cpp',
+    'RecursiveMutex.cpp',
     'RWLock.cpp',
     'SchedulerGroup.cpp',
     'SharedThreadPool.cpp',