libunwind GetStackTrace implementation doesn't allow concurrent invocations

[toddlipcon]

The GetStackTrace implementation uses a global atomic to prevent multiple threads from calling into libunwind at the same time:

  if (sync_val_compare_and_swap(&g_now_entering, false, true)) {
    return 0;
  }

The comment, however, says that the issue it's trying to protect from is reentrancy bugs, not concurrency:

// Sometimes, we can try to get a stack trace from within a stack
// trace, because libunwind can call mmap (maybe indirectly via an
// internal mmap based memory allocator), and that mmap gets trapped
// and causes a stack-trace request.  If were to try to honor that
// recursive request, we'd end up with infinite recursion or deadlock.
// Luckily, it's safe to ignore those subsequent traces.  In such
// cases, we return 0 to indicate the situation.

Given this, it makes more sense for the flag to be a threadlocal rather than a global.

[shinh]

Yeah, I think you are right. Maybe we can use GLOG_THREAD_LOCAL_STORAGE, though it's not supported by configure build. PR is welcomed :)

[sergiud]

This should have been fixed by #161. Please reopen if still an issue.