Thread-safety of a profiled binary (and GCOV runtime library)

[Martin Liška]

Hello.

I've just analyzed PR68080, which exposes 2 interesting problems we have:

1) Majority of instrumented profiling code is not thread-safe, for instance edge profiler:

  PROF_edge_counter_11 = __gcov0._ZL20__gthread_mutex_lockP15pthread_mutex_t[0];
  PROF_edge_counter_12 = PROF_edge_counter_11 + 1;
  __gcov0._ZL20__gthread_mutex_lockP15pthread_mutex_t[0] = PROF_edge_counter_12;

I'm thinking about introducing a param (maybe option), that would instrument counter manipulation
in a thread-safe way:

  __sync_fetch_and_add_8 (&__gcov0._ZL20__gthread_mutex_lockP15pthread_mutex_t[0], 1);

That would also require very similar support for gcov run-time library.

2) The test-case creates a detached thread which is not properly joined. In such situation, the main
thread reaches gcov_exit, calculates for instance PROFILE_SUMMARY and serializes counters to a file.
However, as the created thread is still running, counters are still updated. The leads to a corrupted
profile file.

I'm wondering whether to prevent that? I can imagine we can lock the streaming code, where any profile
updating thread would not be allowed to do that.

Thanks,
Martin