Index: compiler-rt/lib/tsan/rtl/tsan_interface_atomic.cpp
===================================================================
--- compiler-rt/lib/tsan/rtl/tsan_interface_atomic.cpp
+++ compiler-rt/lib/tsan/rtl/tsan_interface_atomic.cpp
@@ -402,7 +402,6 @@
 template<typename T>
 static bool AtomicCAS(ThreadState *thr, uptr pc,
     volatile T *a, T *c, T v, morder mo, morder fmo) {
-  (void)fmo;  // Unused because llvm does not pass it yet.
   MemoryWriteAtomic(thr, pc, (uptr)a, SizeLog<T>());
   SyncVar *s = 0;
   bool write_lock = mo != mo_acquire && mo != mo_consume;
@@ -429,6 +428,20 @@
   if (pr == cc)
     return true;
   *c = pr;
+
+  // Honor failure memory order.
+  CHECK(IsLoadOrder(fmo));
+  if (fmo != mo_acquire)
+    return false;
+  SyncVar *sf = ctx->metamap.GetIfExistsAndLock((uptr)a, false);
+  if (sf) {
+    AcquireImpl(thr, pc, &sf->clock);
+    // Re-read under sync mutex because we need a consistent snapshot
+    // of the value and the clock we acquire.
+    *c = NoTsanAtomicLoad(a, fmo);
+    sf->mtx.ReadUnlock();
+    MemoryReadAtomic(thr, pc, (uptr)a, SizeLog<T>());
+  }
   return false;
 }
 
Index: compiler-rt/test/tsan/compare_exchange_release_acquire.cpp
===================================================================
--- /dev/null
+++ compiler-rt/test/tsan/compare_exchange_release_acquire.cpp
@@ -0,0 +1,39 @@
+// RUN: %clangxx_tsan -O1 %s %link_libcxx_tsan -o %t && %run %t 2>&1 | FileCheck %s
+#include <atomic>
+#include <cassert>
+#include <thread>
+#include <stdio.h>
+
+struct node { int val; };
+std::atomic<node*> _node{nullptr};
+
+void f1() {
+  auto n = new node();
+  n->val = 42;
+  _node.store(n, std::memory_order_release);
+}
+
+void f2() {
+  node* expected = nullptr;
+  while (expected == nullptr) {
+    _node.compare_exchange_weak(expected, nullptr, std::memory_order_release,
+                                std::memory_order_acquire);
+  };
+
+  ++expected->val;
+  assert(expected->val == 43);
+}
+
+int main() {
+  std::thread t1(f1);
+  std::thread t2(f2);
+
+  t1.join();
+  t2.join();
+
+  fprintf(stderr, "DONE\n");
+  return 0;
+}
+
+// CHECK-NOT: WARNING: ThreadSanitizer: data race
+// CHECK: DONE
Index: compiler-rt/test/tsan/compare_exchange_release_relaxed.cpp
===================================================================
--- /dev/null
+++ compiler-rt/test/tsan/compare_exchange_release_relaxed.cpp
@@ -0,0 +1,36 @@
+// RUN: %clangxx_tsan -O1 %s %link_libcxx_tsan -o %t && %deflake %run %t 2>&1 | FileCheck %s
+#include <atomic>
+#include <cassert>
+#include <thread>
+
+struct node { int val; };
+std::atomic<node*> _node{nullptr};
+
+void f1() {
+  auto n = new node();
+  n->val = 42;
+  _node.store(n, std::memory_order_release);
+}
+
+void f2() {
+  node* expected = nullptr;
+  while (expected == nullptr) {
+    _node.compare_exchange_weak(expected, nullptr, std::memory_order_release,
+                                std::memory_order_relaxed);
+  };
+
+  ++expected->val;
+  assert(expected->val == 43);
+}
+
+int main() {
+  std::thread t1(f1);
+  std::thread t2(f2);
+
+  t1.join();
+  t2.join();
+
+  return 0;
+}
+
+// CHECK: WARNING: ThreadSanitizer: data race
Index: llvm/test/Instrumentation/ThreadSanitizer/atomic.ll
===================================================================
--- llvm/test/Instrumentation/ThreadSanitizer/atomic.ll
+++ llvm/test/Instrumentation/ThreadSanitizer/atomic.ll
@@ -370,6 +370,14 @@
 ; CHECK-LABEL: atomic8_cas_release
 ; CHECK: call i8 @__tsan_atomic8_compare_exchange_val(i8* %a, i8 0, i8 1, i32 3, i32 0), !dbg
 
+define void @atomic8_cas_rel_acq(i8* %a) nounwind uwtable {
+entry:
+  cmpxchg i8* %a, i8 0, i8 1 release acquire, !dbg !7
+  ret void, !dbg !7
+}
+; CHECK-LABEL: atomic8_cas_rel_acq
+; CHECK: call i8 @__tsan_atomic8_compare_exchange_val(i8* %a, i8 0, i8 1, i32 3, i32 2), !dbg
+
 define void @atomic8_cas_acq_rel(i8* %a) nounwind uwtable {
 entry:
   cmpxchg i8* %a, i8 0, i8 1 acq_rel acquire, !dbg !7