Index: compiler-rt/trunk/lib/sanitizer_common/sanitizer_mac.cc
===================================================================
--- compiler-rt/trunk/lib/sanitizer_common/sanitizer_mac.cc
+++ compiler-rt/trunk/lib/sanitizer_common/sanitizer_mac.cc
@@ -348,20 +348,16 @@
 void BlockingMutex::Lock() {
   CHECK(sizeof(OSSpinLock) <= sizeof(opaque_storage_));
   CHECK_EQ(OS_SPINLOCK_INIT, 0);
-  CHECK_NE(owner_, (uptr)pthread_self());
+  CHECK_EQ(owner_, 0);
   OSSpinLockLock((OSSpinLock*)&opaque_storage_);
-  CHECK(!owner_);
-  owner_ = (uptr)pthread_self();
 }
 
 void BlockingMutex::Unlock() {
-  CHECK(owner_ == (uptr)pthread_self());
-  owner_ = 0;
   OSSpinLockUnlock((OSSpinLock*)&opaque_storage_);
 }
 
 void BlockingMutex::CheckLocked() {
-  CHECK_EQ((uptr)pthread_self(), owner_);
+  CHECK_NE(*(OSSpinLock*)&opaque_storage_, 0);
 }
 
 u64 NanoTime() {
Index: compiler-rt/trunk/lib/sanitizer_common/sanitizer_mutex.h
===================================================================
--- compiler-rt/trunk/lib/sanitizer_common/sanitizer_mutex.h
+++ compiler-rt/trunk/lib/sanitizer_common/sanitizer_mutex.h
@@ -83,6 +83,14 @@
   BlockingMutex();
   void Lock();
   void Unlock();
+
+  // This function does not guarantee an explicit check that the calling thread
+  // is the thread which owns the mutex. This behavior, while more strictly
+  // correct, causes problems in cases like StopTheWorld, where a parent thread
+  // owns the mutex but a child checks that it is locked. Rather than
+  // maintaining complex state to work around those situations, the check only
+  // checks that the mutex is owned, and assumes callers to be generally
+  // well-behaved.
   void CheckLocked();
  private:
   uptr opaque_storage_[10];