[PATCH] D21609: [tsan] Intercept libcxx __release_shared to avoid false positive with weak_ptrs and destructors in C++

[Dmitry Vyukov via llvm-commits]

dvyukov accepted this revision.
dvyukov added a comment.
This revision is now accepted and ready to land.

LGTM if you agree to change acq_rel to release


================
Comment at: lib/tsan/rtl/tsan_interceptors_mac.cc:309
@@ +308,3 @@
+
+  if (__tsan_atomic64_fetch_add(&o->shared_owners, -1, mo_acq_rel) == 0) {
+    o->on_zero_shared();
----------------
Since we are reimplementing the whole function we can actually do better than the original. The problem (for tsan) with mo_acq_rel is that all threads synchronize with each other, while only the last thread needs to synchronize with all other threads. So I would suggest to do:

  if (__tsan_atomic64_fetch_add(&o->shared_owners, -1, mo_release) == 0) {
    Acquire(thr, pc, &o->shared_owners);
    o->on_zero_shared();
    if (__tsan_atomic64_fetch_add(&o->shared_weak_owners, -1, mo_release) == 0)
      Acquire(thr, pc, &o->shared_weak_owners);
      o->on_zero_shared_weak();
    }
  }



http://reviews.llvm.org/D21609