Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -3090,12 +3090,12 @@
 
 // Thread safety warnings on pass by reference
 def warn_guarded_pass_by_reference : Warning<
-  "passing variable %1 by reference requires holding %0 "
-  "%select{'%2'|'%2' exclusively}3">,
+  "passing variable %1 by %select{reference|non-const reference}3 requires "
+  "holding %0 %select{'%2'|'%2' exclusively}3">,
   InGroup<ThreadSafetyReference>, DefaultIgnore;
 def warn_pt_guarded_pass_by_reference : Warning<
-  "passing the value that %1 points to by reference requires holding %0 "
-  "%select{'%2'|'%2' exclusively}3">,
+  "passing the value that %1 points to by %select{reference|non-const "
+  "reference}3 requires holding %0 %select{'%2'|'%2' exclusively}3">,
   InGroup<ThreadSafetyReference>, DefaultIgnore;
 
 // Imprecise thread safety warnings
Index: lib/Analysis/ThreadSafety.cpp
===================================================================
--- lib/Analysis/ThreadSafety.cpp
+++ lib/Analysis/ThreadSafety.cpp
@@ -2020,8 +2020,12 @@
           const ParmVarDecl *Pvd = FD->getParamDecl(i);
           const Expr *Arg = Exp->getArg(i + Skip);
           QualType Qt = Pvd->getType();
-          if (Qt->isReferenceType())
-            checkAccess(Arg, AK_Read, POK_PassByRef);
+          if (const auto* RT = dyn_cast<const ReferenceType>(&*Qt)) {
+            if (RT->getPointeeType().isConstQualified())
+              checkAccess(Arg, AK_Read, POK_PassByRef);
+            else
+              checkAccess(Arg, AK_Written, POK_PassByRef);
+          }
         }
       }
     }
Index: test/SemaCXX/warn-thread-safety-analysis.cpp
===================================================================
--- test/SemaCXX/warn-thread-safety-analysis.cpp
+++ test/SemaCXX/warn-thread-safety-analysis.cpp
@@ -5026,19 +5026,19 @@
 
   void test1() {
     copy(foo);             // expected-warning {{reading variable 'foo' requires holding mutex 'mu'}}
-    write1(foo);           // expected-warning {{passing variable 'foo' by reference requires holding mutex 'mu'}}
-    write2(10, foo);       // expected-warning {{passing variable 'foo' by reference requires holding mutex 'mu'}}
+    write1(foo);           // expected-warning {{passing variable 'foo' by non-const reference requires holding mutex 'mu' exclusively}}
+    write2(10, foo);       // expected-warning {{passing variable 'foo' by non-const reference requires holding mutex 'mu' exclusively}}
     read1(foo);            // expected-warning {{passing variable 'foo' by reference requires holding mutex 'mu'}}
     read2(10, foo);        // expected-warning {{passing variable 'foo' by reference requires holding mutex 'mu'}}
-    destroy(mymove(foo));  // expected-warning {{passing variable 'foo' by reference requires holding mutex 'mu'}}
+    destroy(mymove(foo));  // expected-warning {{passing variable 'foo' by non-const reference requires holding mutex 'mu' exclusively}}
 
-    mwrite1(foo);           // expected-warning {{passing variable 'foo' by reference requires holding mutex 'mu'}}
-    mwrite2(10, foo);       // expected-warning {{passing variable 'foo' by reference requires holding mutex 'mu'}}
+    mwrite1(foo);           // expected-warning {{passing variable 'foo' by non-const reference requires holding mutex 'mu' exclusively}}
+    mwrite2(10, foo);       // expected-warning {{passing variable 'foo' by non-const reference requires holding mutex 'mu' exclusively}}
     mread1(foo);            // expected-warning {{passing variable 'foo' by reference requires holding mutex 'mu'}}
     mread2(10, foo);        // expected-warning {{passing variable 'foo' by reference requires holding mutex 'mu'}}
 
-    smwrite1(foo);           // expected-warning {{passing variable 'foo' by reference requires holding mutex 'mu'}}
-    smwrite2(10, foo);       // expected-warning {{passing variable 'foo' by reference requires holding mutex 'mu'}}
+    smwrite1(foo);           // expected-warning {{passing variable 'foo' by non-const reference requires holding mutex 'mu' exclusively}}
+    smwrite2(10, foo);       // expected-warning {{passing variable 'foo' by non-const reference requires holding mutex 'mu' exclusively}}
     smread1(foo);            // expected-warning {{passing variable 'foo' by reference requires holding mutex 'mu'}}
     smread2(10, foo);        // expected-warning {{passing variable 'foo' by reference requires holding mutex 'mu'}}
 
@@ -5053,12 +5053,13 @@
     (*this) << foo;          // expected-warning {{passing variable 'foo' by reference requires holding mutex 'mu'}}
 
     copy(*foop);             // expected-warning {{reading the value pointed to by 'foop' requires holding mutex 'mu'}}
-    write1(*foop);           // expected-warning {{passing the value that 'foop' points to by reference requires holding mutex 'mu'}}
-    write2(10, *foop);       // expected-warning {{passing the value that 'foop' points to by reference requires holding mutex 'mu'}}
+    write1(*foop);           // expected-warning {{passing the value that 'foop' points to by non-const reference requires holding mutex 'mu' exclusively}}
+    write2(10, *foop);       // expected-warning {{passing the value that 'foop' points to by non-const reference requires holding mutex 'mu' exclusively}}
     read1(*foop);            // expected-warning {{passing the value that 'foop' points to by reference requires holding mutex 'mu'}}
     read2(10, *foop);        // expected-warning {{passing the value that 'foop' points to by reference requires holding mutex 'mu'}}
-    destroy(mymove(*foop));  // expected-warning {{passing the value that 'foop' points to by reference requires holding mutex 'mu'}}
+    destroy(mymove(*foop));  // expected-warning {{passing the value that 'foop' points to by non-const reference requires holding mutex 'mu' exclusively}}
 
+    // TODO -- sometimes this should warn about writing.
     copy(*foosp);             // expected-warning {{reading the value pointed to by 'foosp' requires holding mutex 'mu'}}
     write1(*foosp);           // expected-warning {{reading the value pointed to by 'foosp' requires holding mutex 'mu'}}
     write2(10, *foosp);       // expected-warning {{reading the value pointed to by 'foosp' requires holding mutex 'mu'}}
@@ -5074,6 +5075,57 @@
     read2(10, *foosp.get());
     destroy(mymove(*foosp.get()));
   }
+
+  void test2() {
+    mu.ReaderLock();
+
+    copy(foo);
+    write1(foo);              // expected-warning {{passing variable 'foo' by non-const reference requires holding mutex 'mu' exclusively}}
+    write2(10, foo);          // expected-warning {{passing variable 'foo' by non-const reference requires holding mutex 'mu' exclusively}}
+    read1(foo);
+    read2(10, foo);
+    destroy(mymove(foo));     // expected-warning {{passing variable 'foo' by non-const reference requires holding mutex 'mu' exclusively}}
+
+    mwrite1(foo);             // expected-warning {{passing variable 'foo' by non-const reference requires holding mutex 'mu' exclusively}}
+    mwrite2(10, foo);         // expected-warning {{passing variable 'foo' by non-const reference requires holding mutex 'mu' exclusively}}
+    mread1(foo);
+    mread2(10, foo);
+
+    smwrite1(foo);            // expected-warning {{passing variable 'foo' by non-const reference requires holding mutex 'mu' exclusively}}
+    smwrite2(10, foo);        // expected-warning {{passing variable 'foo' by non-const reference requires holding mutex 'mu' exclusively}}
+    smread1(foo);
+    smread2(10, foo);
+
+    foo + foo2;
+    foo / foo2;
+    foo * foo2;
+    foo[foo2];
+    (*this) << foo;
+
+    copy(*foop);
+    write1(*foop);            // expected-warning {{passing the value that 'foop' points to by non-const reference requires holding mutex 'mu' exclusively}}
+    write2(10, *foop);        // expected-warning {{passing the value that 'foop' points to by non-const reference requires holding mutex 'mu' exclusively}}
+    read1(*foop);
+    read2(10, *foop);
+    destroy(mymove(*foop));   // expected-warning {{passing the value that 'foop' points to by non-const reference requires holding mutex 'mu' exclusively}}
+
+    // TODO -- these require better smart pointer handling.
+    copy(*foosp);
+    write1(*foosp);
+    write2(10, *foosp);
+    read1(*foosp);
+    read2(10, *foosp);
+    destroy(mymove(*foosp));
+
+    copy(*foosp.get());
+    write1(*foosp.get());
+    write2(10, *foosp.get());
+    read1(*foosp.get());
+    read2(10, *foosp.get());
+    destroy(mymove(*foosp.get()));
+
+    mu.ReaderUnlock();
+  }
 };