Index: libcxx/trunk/utils/libcxx/test/format.py
===================================================================
--- libcxx/trunk/utils/libcxx/test/format.py
+++ libcxx/trunk/utils/libcxx/test/format.py
@@ -203,9 +203,11 @@
             is_flaky = self._get_parser('FLAKY_TEST.', parsers).getValue()
             max_retry = 3 if is_flaky else 1
             for retry_count in range(max_retry):
+                lit_config._retry_rwlock.acquire(retry_count)
                 cmd, out, err, rc = self.executor.run(exec_path, [exec_path],
                                                       local_cwd, data_files,
                                                       env)
+                lit_config._retry_rwlock.release(retry_count)
                 if rc == 0:
                     res = lit.Test.PASS if retry_count == 0 else lit.Test.FLAKYPASS
                     return res, ''
Index: llvm/trunk/utils/lit/lit/TestRunner.py
===================================================================
--- llvm/trunk/utils/lit/lit/TestRunner.py
+++ llvm/trunk/utils/lit/lit/TestRunner.py
@@ -1200,7 +1200,11 @@
     if hasattr(test.config, 'test_retry_attempts'):
         attempts += test.config.test_retry_attempts
     for i in range(attempts):
+        if hasattr(litConfig, '_retry_rwlock'):
+            litConfig._retry_rwlock.acquire(i)
         res = _runShTest(test, litConfig, useExternalSh, script, tmpBase)
+        if hasattr(litConfig, '_retry_rwlock'):
+            litConfig._retry_rwlock.release(i)
         if res.code != Test.FAIL:
             break
     # If we had to run the test more than once, count it as a flaky pass. These
Index: llvm/trunk/utils/lit/lit/run.py
===================================================================
--- llvm/trunk/utils/lit/lit/run.py
+++ llvm/trunk/utils/lit/lit/run.py
@@ -15,6 +15,7 @@
 
 import multiprocessing
 import lit.Test
+import lit.util
 
 def abort_now():
     """Abort the current process without doing any exception teardown"""
@@ -140,6 +141,9 @@
         # our task completion callback.
         self.display = display
 
+        # FIXME: It might be in wrong place.
+        self.lit_config._retry_rwlock = lit.util.RWLock()
+
         self.failure_count = 0
         self.hit_max_failures = False
         if self.lit_config.singleProcess:
Index: llvm/trunk/utils/lit/lit/util.py
===================================================================
--- llvm/trunk/utils/lit/lit/util.py
+++ llvm/trunk/utils/lit/lit/util.py
@@ -1,6 +1,7 @@
 import errno
 import itertools
 import math
+import multiprocessing
 import numbers
 import os
 import platform
@@ -422,3 +423,55 @@
         psutilProc.kill()
     except psutil.NoSuchProcess:
         pass
+
+class RWLock:
+    """
+    Provide RWLock
+    """
+    def __init__(self):
+        lock =  multiprocessing.RLock()
+        self._lock = lock
+        self._rcond = multiprocessing.Condition(lock)
+        self._wcond = multiprocessing.Condition(lock)
+        self._rval  = multiprocessing.Value('i', 0)
+        self._wval  = multiprocessing.Value('i', 0)
+
+    def read_acquire(self):
+        self._lock.acquire()
+        while self._wval.value != 0:
+            self._rcond.wait()
+        self._rval.value += 1
+        self._lock.release()
+
+    def write_acquire(self):
+        self._lock.acquire()
+        while self._rval.value > 0:
+            self._wcond.wait()
+        assert self._wval.value == 0
+        self._wval.value = 1
+
+    def read_release(self):
+        self._lock.acquire()
+        rval = self._rval.value - 1
+        self._rval.value = rval
+        if rval == 0:
+            self._wcond.notify()
+        self._lock.release()
+
+    def write_release(self):
+        self._wval.value = 0
+        self._wcond.notify()
+        self._rcond.notify_all()
+        self._lock.release()
+
+    def acquire(self, count, exclusive_since=1):
+        if count < exclusive_since:
+            self.read_acquire()
+        else:
+            self.write_acquire()
+
+    def release(self, count, exclusive_since=1):
+        if count < exclusive_since:
+            self.read_release()
+        else:
+            self.write_release()