tsan: fix another false positive related to open/close

[dvyukov]

Summary

The false positive fixed by commit f831d6fc80
("tsan: fix false positive during fd close") still happens episodically
on the added more stressful test which does just open/close.

I don't have a coherent explanation as to what exactly happens
but the fix fixes the false positive on this test as well.
The issue may be related to lost writes during asynchronous MADV_DONTNEED.
I've debugged similar unexplainable false positive related to freed and
reused memory and at the time the only possible explanation I found is that
an asynchronous MADV_DONTNEED may lead to lost writes. That's why commit
302ec7b9bc ("tsan: add memory_limit_mb flag") added StopTheWorld around
the memory flush, but unfortunately the commit does not capture these findings.

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

static void init(ThreadState *thr, uptr pc, int fd, FdSync *s,
   } else if (flags()->io_sync == 1) {
     d->sync = s;
   } else if (flags()->io_sync == 2) {
     unref(thr, pc, s);
     d->sync = &fdctx.globsync;
   }
   d->creation_tid = thr->tid;
   d->creation_stack = CurrentStackId(thr, pc);
+  // This prevents false positives on fd_close_norace3.cpp test.
+  // The mechanics of the false positive are not completely clear,
+  // but it happens only if global reset is enabled (flush_memory_ms=1)
+  // and may be related to lost writes during asynchronous MADV_DONTNEED.
+  SlotLocker locker(thr);
   if (write) {
     // To catch races between fd usage and open.
     MemoryRangeImitateWrite(thr, pc, (uptr)d, 8);
   } else {
     // See the dup-related comment in FdClose.
-    MemoryAccess(thr, pc, (uptr)d, 8, kAccessRead);
+    MemoryAccess(thr, pc, (uptr)d, 8, kAccessRead | kAccessSlotLocked);
   }
 }
 
 void FdInit() {
   atomic_store(&fdctx.globsync.rc, (u64)-1, memory_order_relaxed);
   atomic_store(&fdctx.filesync.rc, (u64)-1, memory_order_relaxed);
   atomic_store(&fdctx.socksync.rc, (u64)-1, memory_order_relaxed);
 }

compiler-rt/test/tsan/fd_close_norace3.cpp

+// RUN: %clangxx_tsan -O1 %s -o %t && %env_tsan_opts=flush_memory_ms=1 %run %t 2>&1 | FileCheck %s
+#include "test.h"
+#include <fcntl.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+void *Thread(void *stop) {
+  while (!__atomic_load_n((int *)stop, __ATOMIC_RELAXED))
+    close(open("/dev/null", O_RDONLY));
+  return 0;
+}
+
+int main() {
+  int stop = 0;
+  const int kThreads = 10;
+  pthread_t th[kThreads];
+  for (int i = 0; i < kThreads; i++)
+    pthread_create(&th[i], 0, Thread, &stop);
+  sleep(5);
+  __atomic_store_n(&stop, 1, __ATOMIC_RELAXED);
+  for (int i = 0; i < kThreads; i++)
+    pthread_join(th[i], 0);
+  fprintf(stderr, "DONE\n");
+}
+
+// CHECK-NOT: WARNING: ThreadSanitizer: data race
+// CHECK: DONE