Index: llvm/trunk/utils/lit/lit/TestingConfig.py
===================================================================
--- llvm/trunk/utils/lit/lit/TestingConfig.py
+++ llvm/trunk/utils/lit/lit/TestingConfig.py
@@ -106,7 +106,7 @@
                  environment, substitutions, unsupported,
                  test_exec_root, test_source_root, excludes,
                  available_features, pipefail, limit_to_features = [],
-                 is_early = False, parallelism_group = ""):
+                 is_early = False, parallelism_group = None):
         self.parent = parent
         self.name = str(name)
         self.suffixes = set(suffixes)
Index: llvm/trunk/utils/lit/lit/run.py
===================================================================
--- llvm/trunk/utils/lit/lit/run.py
+++ llvm/trunk/utils/lit/lit/run.py
@@ -17,6 +17,12 @@
         if self.failedCount == self.maxFailures:
             self.provider.cancel()
 
+# No-operation semaphore for supporting `None` for parallelism_groups.
+#   lit_config.parallelism_groups['my_group'] = None
+class NopSemaphore(object):
+    def acquire(self): pass
+    def release(self): pass
+
 class Run(object):
     """
     This class represents a concrete, configured testing run.
@@ -26,11 +32,10 @@
         self.lit_config = lit_config
         self.tests = tests
         # Set up semaphores to limit parallelism of certain classes of tests.
-        # For example, some ASan tests require lots of virtual memory and run
-        # faster with less parallelism on OS X.
-        self.parallelism_semaphores = \
-                {k: multiprocessing.BoundedSemaphore(v) for k, v in
-                 self.lit_config.parallelism_groups.items()}
+        self.parallelism_semaphores = {
+                k : NopSemaphore() if v is None else
+                    multiprocessing.BoundedSemaphore(v)
+                for k, v in lit_config.parallelism_groups.items()}
 
     def execute_tests_in_pool(self, jobs, max_time):
         # We need to issue many wait calls, so compute the final deadline and
Index: llvm/trunk/utils/lit/tests/Inputs/parallelism-groups/lit.cfg
===================================================================
--- llvm/trunk/utils/lit/tests/Inputs/parallelism-groups/lit.cfg
+++ llvm/trunk/utils/lit/tests/Inputs/parallelism-groups/lit.cfg
@@ -0,0 +1,11 @@
+import lit.formats
+config.name = 'parallelism-groups'
+config.suffixes = ['.txt']
+config.test_format = lit.formats.ShTest()
+config.test_source_root = None
+config.test_exec_root = None
+
+# Should not crash
+lit_config.parallelism_groups['my_group'] = None
+
+config.parallelism_group = 'my_group'
Index: llvm/trunk/utils/lit/tests/Inputs/parallelism-groups/test1.txt
===================================================================
--- llvm/trunk/utils/lit/tests/Inputs/parallelism-groups/test1.txt
+++ llvm/trunk/utils/lit/tests/Inputs/parallelism-groups/test1.txt
@@ -0,0 +1 @@
+# RUN: true
Index: llvm/trunk/utils/lit/tests/Inputs/parallelism-groups/test2.txt
===================================================================
--- llvm/trunk/utils/lit/tests/Inputs/parallelism-groups/test2.txt
+++ llvm/trunk/utils/lit/tests/Inputs/parallelism-groups/test2.txt
@@ -0,0 +1 @@
+# RUN: true
Index: llvm/trunk/utils/lit/tests/parallelism-groups.py
===================================================================
--- llvm/trunk/utils/lit/tests/parallelism-groups.py
+++ llvm/trunk/utils/lit/tests/parallelism-groups.py
@@ -0,0 +1,21 @@
+# Check that we do not crash if a parallelism group is set to None. Permits
+# usage of the following pattern.
+#
+# [lit.common.cfg]
+#   lit_config.parallelism_groups['my_group'] = None
+#   if <condition>:
+#     lit_config.parallelism_groups['my_group'] = 3
+#
+# [project/lit.cfg]
+#   config.parallelism_group = 'my_group'
+#
+# Note: We need at least 2 tests to prevent lit from using "single process
+#       mode", which ignores parallelism groups.
+#
+
+# RUN: %{lit} -j2 %{inputs}/parallelism-groups | FileCheck %s
+
+# CHECK:     -- Testing: 2 tests, 2 threads --
+# CHECK-DAG: PASS: parallelism-groups :: test1.txt
+# CHECK-DAG: PASS: parallelism-groups :: test2.txt
+# CHECK:     Expected Passes    : 2