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
@@ -402,11 +402,12 @@
 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;
-  if (mo != mo_relaxed) {
+  bool should_lock = mo != mo_relaxed || fmo != mo_relaxed;
+
+  if (should_lock) {
     s = ctx->metamap.GetOrCreateAndLock(thr, pc, (uptr)a, write_lock);
     thr->fast_state.IncrementEpoch();
     // Can't increment epoch w/o writing to the trace as well.
@@ -420,15 +421,35 @@
   }
   T cc = *c;
   T pr = func_cas(a, cc, v);
-  if (s) {
+  if (pr == cc) {
+    if (!s)
+      return true;
     if (write_lock)
       s->mtx.Unlock();
     else
       s->mtx.ReadUnlock();
-  }
-  if (pr == cc)
     return true;
+  }
+
+  // Honor failure memory order.
   *c = pr;
+  CHECK(IsLoadOrder(fmo));
+  if (!s)
+    return false;
+
+  // 31.7.2.18: "The failure argument shall not be memory_order_release
+  // nor memory_order_acq_rel". LLVM (2021-05) fallbacks to Monotonic (relaxed)
+  // when those are used.
+  if (IsAcquireOrder(fmo))
+    AcquireImpl(thr, pc, &s->clock);
+
+  if (s) {
+    if (write_lock)
+      s->mtx.Unlock();
+    else
+      s->mtx.ReadUnlock();
+  }
+
   return false;
 }
 
Index: compiler-rt/test/tsan/compare_exchange.cpp
===================================================================
--- /dev/null
+++ compiler-rt/test/tsan/compare_exchange.cpp
@@ -0,0 +1,99 @@
+// RUN: %clangxx_tsan -O1 %s %link_libcxx_tsan -DRELEASE_ACQUIRE -o %t && %run %t 2>&1 | FileCheck %s
+// RUN: %clangxx_tsan -O1 %s %link_libcxx_tsan -DRELEASE_RELAXED -o %t && %deflake %run %t 2>&1 | FileCheck --check-prefix=CHECK-REPORT %s
+// RUN: %clangxx_tsan -O1 %s %link_libcxx_tsan -DRELACQ_ACQUIRE -o %t && %run %t 2>&1 | FileCheck %s
+// RUN: %clangxx_tsan -O1 %s %link_libcxx_tsan -DRELACQ_RELAXED -o %t && %run %t 2>&1 | FileCheck %s
+// RUN: %clangxx_tsan -O1 %s %link_libcxx_tsan -DRELEASE_CONSUME -o %t && %deflake %run %t 2>&1 | FileCheck --check-prefix=CHECK-REPORT %s
+// RUN: %clangxx_tsan -O1 %s %link_libcxx_tsan -DSEQ_CST_RELAX -o %t && %run %t 2>&1 | FileCheck %s
+// RUN: %clangxx_tsan -O1 %s %link_libcxx_tsan -DSEQ_CST_ACQUIRE -o %t && %run %t 2>&1 | FileCheck %s
+// RUN: %clangxx_tsan -O1 %s %link_libcxx_tsan -DSEQ_CST_CONSUME -o %t && %run %t 2>&1 | FileCheck %s
+// RUN: %clangxx_tsan -O1 %s %link_libcxx_tsan -DRELAXED_ACQUIRE -o %t && %deflake %run %t 2>&1 | FileCheck --check-prefix=CHECK-REPORT %s
+#include <atomic>
+#include <cassert>
+#include <stdio.h>
+#include <thread>
+
+#ifdef RELEASE_ACQUIRE
+static constexpr auto SuccessOrder = std::memory_order_release;
+static constexpr auto FailureOrder = std::memory_order_acquire;
+#endif
+
+#ifdef RELEASE_RELAXED
+static constexpr auto SuccessOrder = std::memory_order_release;
+static constexpr auto FailureOrder = std::memory_order_relaxed;
+#endif
+
+#ifdef RELACQ_ACQUIRE
+static constexpr auto SuccessOrder = std::memory_order_acq_rel;
+static constexpr auto FailureOrder = std::memory_order_acquire;
+#endif
+
+#ifdef RELACQ_RELAXED
+static constexpr auto SuccessOrder = std::memory_order_acq_rel;
+static constexpr auto FailureOrder = std::memory_order_relaxed;
+#endif
+
+#ifdef RELEASE_CONSUME
+static constexpr auto SuccessOrder = std::memory_order_release;
+static constexpr auto FailureOrder = std::memory_order_consume;
+#endif
+
+#ifdef SEQ_CST_RELAX
+static constexpr auto SuccessOrder = std::memory_order_seq_cst;
+static constexpr auto FailureOrder = std::memory_order_relaxed;
+#endif
+
+#ifdef SEQ_CST_ACQUIRE
+static constexpr auto SuccessOrder = std::memory_order_seq_cst;
+static constexpr auto FailureOrder = std::memory_order_acquire;
+#endif
+
+#ifdef SEQ_CST_CONSUME
+static constexpr auto SuccessOrder = std::memory_order_seq_cst;
+static constexpr auto FailureOrder = std::memory_order_consume;
+#endif
+
+#ifdef RELAXED_ACQUIRE
+static constexpr auto SuccessOrder = std::memory_order_relaxed;
+static constexpr auto FailureOrder = std::memory_order_acquire;
+#endif
+
+// FIXME: both RELEASE_CONSUME and RELAXED_ACQUIRE currently gives
+// false positives due to LLVM (2021-05) fallback to other modes in
+// pre C++17 rules. Fix this test once they get fixed.
+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, SuccessOrder,
+                                FailureOrder);
+  };
+
+  ++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
+
+// CHECK-REPORT: WARNING: ThreadSanitizer: data race
\ No newline at end of file
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