[tsan] Handle dispatch_once on OS X

Reimplement dispatch_once in an interceptor to solve these issues that may produce false positives with TSan on OS X:

1) there is a racy load inside an inlined part of dispatch_once,
2) the fast path in dispatch_once doesn't perform an acquire load, so we don't properly synchronize the initialization and subsequent uses of whatever is initialized,
3) dispatch_once is already used in a lot of already-compiled code, so TSan doesn't see the inlined fast-path.

This patch uses a trick to avoid ever taking the fast path (by never storing ~0 into the predicate), which means the interceptor will always be called even from already-compiled code. Within the interceptor, our own atomic reads and writes are not written into shadow cells, so the race in the inlined part is not reported (because the accesses are only loads).

Differential Revision: http://reviews.llvm.org/D14811

llvm-svn: 253552

compiler-rt/lib/tsan/CMakeLists.txt

@@ -47,6 +47,7 @@ set(TSAN_CXX_SOURCES
 
 if(APPLE)
   list(APPEND TSAN_SOURCES
+    rtl/tsan_libdispatch_mac.cc
     rtl/tsan_platform_mac.cc
     rtl/tsan_platform_posix.cc)
 elseif(UNIX)

compiler-rt/lib/tsan/rtl/tsan_interceptors.cc

@@ -264,17 +264,6 @@ ScopedInterceptor::~ScopedInterceptor() {
   }
 }
 
-#define SCOPED_TSAN_INTERCEPTOR(func, ...) \
-    SCOPED_INTERCEPTOR_RAW(func, __VA_ARGS__); \
-    if (REAL(func) == 0) { \
-      Report("FATAL: ThreadSanitizer: failed to intercept %s\n", #func); \
-      Die(); \
-    }                                                    \
-    if (thr->ignore_interceptors || thr->in_ignored_lib) \
-      return REAL(func)(__VA_ARGS__); \
-/**/
-
-#define TSAN_INTERCEPTOR(ret, func, ...) INTERCEPTOR(ret, func, __VA_ARGS__)
 #define TSAN_INTERCEPT(func) INTERCEPT_FUNCTION(func)
 #if SANITIZER_FREEBSD
 # define TSAN_INTERCEPT_VER(func, ver) INTERCEPT_FUNCTION(func)

compiler-rt/lib/tsan/rtl/tsan_interceptors.h

@@ -26,6 +26,18 @@ class ScopedInterceptor {
     (void)pc; \
 /**/
 
+#define SCOPED_TSAN_INTERCEPTOR(func, ...) \
+    SCOPED_INTERCEPTOR_RAW(func, __VA_ARGS__); \
+    if (REAL(func) == 0) { \
+      Report("FATAL: ThreadSanitizer: failed to intercept %s\n", #func); \
+      Die(); \
+    }                                                    \
+    if (thr->ignore_interceptors || thr->in_ignored_lib) \
+      return REAL(func)(__VA_ARGS__); \
+/**/
+
+#define TSAN_INTERCEPTOR(ret, func, ...) INTERCEPTOR(ret, func, __VA_ARGS__)
+
 #if SANITIZER_FREEBSD
 #define __libc_free __free
 #define __libc_malloc __malloc

compiler-rt/lib/tsan/rtl/tsan_libdispatch_mac.cc

@@ -0,0 +1,72 @@
+//===-- tsan_libdispatch_mac.cc -------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file is a part of ThreadSanitizer (TSan), a race detector.
+//
+// Mac-specific libdispatch (GCD) support.
+//===----------------------------------------------------------------------===//
+
+#include "sanitizer_common/sanitizer_platform.h"
+#if SANITIZER_MAC
+
+#include "sanitizer_common/sanitizer_common.h"
+#include "interception/interception.h"
+#include "tsan_interceptors.h"
+#include "tsan_platform.h"
+#include "tsan_rtl.h"
+
+#include <dispatch/dispatch.h>
+#include <pthread.h>
+
+namespace __tsan {
+
+// GCD's dispatch_once implementation has a fast path that contains a racy read
+// and it's inlined into user's code. Furthermore, this fast path doesn't
+// establish a proper happens-before relations between the initialization and
+// code following the call to dispatch_once. We could deal with this in
+// instrumented code, but there's not much we can do about it in system
+// libraries. Let's disable the fast path (by never storing the value ~0 to
+// predicate), so the interceptor is always called, and let's add proper release
+// and acquire semantics. Since TSan does not see its own atomic stores, the
+// race on predicate won't be reported - the only accesses to it that TSan sees
+// are the loads on the fast path. Loads don't race. Secondly, dispatch_once is
+// both a macro and a real function, we want to intercept the function, so we
+// need to undefine the macro.
+#undef dispatch_once
+TSAN_INTERCEPTOR(void, dispatch_once, dispatch_once_t *predicate,
+                 dispatch_block_t block) {
+  SCOPED_TSAN_INTERCEPTOR(dispatch_once, predicate, block);
+  atomic_uint32_t *a = reinterpret_cast<atomic_uint32_t *>(predicate);
+  u32 v = atomic_load(a, memory_order_acquire);
+  if (v == 0 &&
+      atomic_compare_exchange_strong(a, &v, 1, memory_order_relaxed)) {
+    block();
+    Release(thr, pc, (uptr)a);
+    atomic_store(a, 2, memory_order_release);
+  } else {
+    while (v != 2) {
+      internal_sched_yield();
+      v = atomic_load(a, memory_order_acquire);
+    }
+    Acquire(thr, pc, (uptr)a);
+  }
+}
+
+#undef dispatch_once_f
+TSAN_INTERCEPTOR(void, dispatch_once_f, dispatch_once_t *predicate,
+                 void *context, dispatch_function_t function) {
+  SCOPED_TSAN_INTERCEPTOR(dispatch_once_f, predicate, context, function);
+  WRAP(dispatch_once)(predicate, ^(void) {
+    function(context);
+  });
+}
+
+}  // namespace __tsan
+
+#endif  // SANITIZER_MAC

compiler-rt/test/tsan/Darwin/gcd-once.mm

@@ -0,0 +1,55 @@
+// RUN: %clang_tsan %s -o %t -framework Foundation
+// RUN: %run %t 2>&1
+
+#import <Foundation/Foundation.h>
+
+#import "../test.h"
+
+static const long kNumThreads = 4;
+
+long global;
+long global2;
+
+static dispatch_once_t once_token;
+static dispatch_once_t once_token2;
+
+void f(void *) {
+  global2 = 42;
+  usleep(100000);
+}
+
+void *Thread(void *a) {
+  barrier_wait(&barrier);
+
+  dispatch_once(&once_token, ^{
+    global = 42;
+    usleep(100000);
+  });
+  long x = global;
+
+  dispatch_once_f(&once_token2, NULL, f);
+  long x2 = global2;
+
+  fprintf(stderr, "global = %ld\n", x);
+  fprintf(stderr, "global2 = %ld\n", x2);
+  return 0;
+}
+
+int main() {
+  fprintf(stderr, "Hello world.\n");
+  barrier_init(&barrier, kNumThreads);
+
+  pthread_t t[kNumThreads];
+  for (int i = 0; i < kNumThreads; i++) {
+    pthread_create(&t[i], 0, Thread, 0);
+  }
+  for (int i = 0; i < kNumThreads; i++) {
+    pthread_join(t[i], 0);
+  }
+
+  fprintf(stderr, "Done.\n");
+}
+
+// CHECK: Hello world.
+// CHECK: Done.
+// CHECK-NOT: WARNING: ThreadSanitizer