Follow up note:

Note that even with POSIX TLS in use, it isn't entirely safe.  pthread_getspecific() as used by the posix thread code is not required to be async signal safe by the POSIX standard.  :(

The Linux glibc implementation thankfully does not use locks.  But it does suffer from a consistency issue in its current implementation when used asynchronously.  A trivial patch to fix glibc's pthread_getspecific() with a test exists - https://sourceware.org/ml/libc-alpha/2014-12/msg00583.html - but upstream glibc maintainers are obstinate as usual.

Regardless, I believe the worst that happens in that relatively uncommon race condition situation is a faulthandler crash reporting odd information or crashes while crashing.  not a deadlock.

I have not investigated other pthread_getspecific implementations.

The systems I care most about today thankfully run a modified glibc.