Fix race in the implementation of __tsan_acquire()

compiler-rt/lib/tsan/rtl/tsan_rtl_mutex.cpp

@@ -446,9 +446,9 @@ void Acquire(ThreadState *thr, uptr pc, uptr addr) {
   if (!s)
     return;
   SlotLocker locker(thr);
+  ReadLock lock(&s->mtx);
   if (!s->clock)
     return;
-  ReadLock lock(&s->mtx);
   thr->clock.Acquire(s->clock);
 }
 

compiler-rt/test/tsan/compare_exchange_acquire_fence.cpp

@@ -0,0 +1,43 @@
+// RUN: %clangxx_tsan -O1 %s -o %t && %run %t 2>&1
+// This is a correct program and tsan should not report a race.
+//
+// Verify that there is a happens-before relationship between a
+// memory_order_release store that happens as part of a successful
+// compare_exchange_strong(), and an atomic_thread_fence(memory_order_acquire)
+// that happens after a relaxed load.
+
+#include <atomic>
+#include <sanitizer/tsan_interface.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <thread>
+
+std::atomic<bool> a;
+unsigned int b;
+constexpr int loops = 100000;
+
+void Thread1() {
+  for (int i = 0; i < loops; ++i) {
+    while (a.load(std::memory_order_acquire)) {
+    }
+    b = i;
+    bool expected = false;
+    a.compare_exchange_strong(expected, true, std::memory_order_acq_rel);
+  }
+}
+
+int main() {
+  std::thread t(Thread1);
+  unsigned int sum = 0;
+  for (int i = 0; i < loops; ++i) {
+    while (!a.load(std::memory_order_relaxed)) {
+    }
+    std::atomic_thread_fence(std::memory_order_acquire);
+    __tsan_acquire(&a);
+    sum += b;
+    a.store(false, std::memory_order_release);
+  }
+  t.join();
+  fprintf(stderr, "DONE: %u\n", sum);
+  return 0;
+}