[Bug sanitizer/59215] New: tsan: warning in shared_ptr_base.h

["oleg at smolsky dot net" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59215

            Bug ID: 59215
           Summary: tsan: warning in shared_ptr_base.h
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: sanitizer
          Assignee: unassigned at gcc dot gnu.org
          Reporter: oleg at smolsky dot net
                CC: dodji at gcc dot gnu.org, dvyukov at gcc dot gnu.org,
                    jakub at gcc dot gnu.org, kcc at gcc dot gnu.org

I just got what appears to be a false positive in GCC's own implementation of
shared_ptr ref counts:

Read of size 4 by thread T21:
  #0 _M_add_ref_lock ...gcc/include/c++/4.8.x-google/bits/shared_ptr_base.h:236

Previous atomic write of size 4 by main thread:
  #0 __tsan_atomic32_fetch_add ??:0 (libtsan.so.0+0x00000000d3e5)
  #1 __exchange_and_add_dispatch
...gcc/include/c++/4.8.x-google/ext/atomicity.h:49
  ....

The atomic write is obvious - it's an "up ref". The read, however, should have
just worked as the variable is (well, is supposed to be) atomic.

I don't get it... is the tool missing the atomic manipulation? Or is the
library missing the correct annotation? I'm leaning towards the latter:

in ...gcc/include/c++/4.8.x-google/x86_64-unknown-linux/bits/atomic_word.h

       typedef int _Atomic_word;

Should this be std::atomic<int> ?