There were multiple steps:
1. Avoid using malloc/free inside libunwind.
2. Avoid using FDECache that required a mutex.
3. Avoid using dl_iterate_phdr (a mutex inside libc).
4. Protection from dereferencing wrong pointers due to incorrect unwind tables.
Most of the changes were integrated to libunwind, but not everything. Example: https://bugs.llvm.org/show_bug.cgi?id=48186