tsan: fix a bug memory access handling

We incorrectly replaced shadow slots
when the new value is not stronger than the old one.
The bug can lead to false negatives.
The bug was detected by Go race test suite:
https://github.com/golang/go/issues/10589



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@236008 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/tsan/rtl/tsan_update_shadow_word_inl.h b/lib/tsan/rtl/tsan_update_shadow_word_inl.h
index c80e0a8..6e3ac2f 100644
--- a/lib/tsan/rtl/tsan_update_shadow_word_inl.h
+++ b/lib/tsan/rtl/tsan_update_shadow_word_inl.h
@@ -38,7 +38,8 @@
     }
     StatInc(thr, StatShadowAnotherThread);
     if (HappensBefore(old, thr)) {
-      StoreIfNotYetStored(sp, &store_word);
+      if (old.IsRWWeakerOrEqual(kAccessIsWrite, kIsAtomic))
+        StoreIfNotYetStored(sp, &store_word);
       break;
     }
     if (old.IsBothReadsOrAtomic(kAccessIsWrite, kIsAtomic))
diff --git a/test/tsan/mop1.c b/test/tsan/mop1.c
new file mode 100644
index 0000000..e61c5b8
--- /dev/null
+++ b/test/tsan/mop1.c
@@ -0,0 +1,40 @@
+// RUN: %clang_tsan -O1 %s -o %t && %deflake %run %t | FileCheck %s
+#include "test.h"
+
+// We want to establish the following sequence of accesses to X:
+// - main thread writes X
+// - thread2 reads X, this read happens-before the write in main thread
+// - thread1 reads X, this read is concurrent with the write in main thread
+// Write in main thread and read in thread1 should be detected as a race.
+// Previously tsan replaced write by main thread with read by thread1,
+// as the result the race was not detected.
+
+volatile long X, Y, Z;
+
+void *Thread1(void *x) {
+  barrier_wait(&barrier);
+  barrier_wait(&barrier);
+  Y = X;
+  return NULL;
+}
+
+void *Thread2(void *x) {
+  Z = X;
+  barrier_wait(&barrier);
+  return NULL;
+}
+
+int main() {
+  barrier_init(&barrier, 2);
+  pthread_t t[2];
+  pthread_create(&t[0], 0, Thread1, 0);
+  X = 42;
+  barrier_wait(&barrier);
+  pthread_create(&t[1], 0, Thread2, 0);
+  pthread_join(t[0], 0);
+  pthread_join(t[1], 0);
+  return 0;
+}
+
+// CHECK: WARNING: ThreadSanitizer: data race
+