Should `atexit_hooks` be guarded during `_atexit`?

[NHDaly]

Two questions, following up from #49774:

1. Is it safe to grab a mutex inside _atexit()? @kpamnany and I were worried about the system being in an undefined state during shutdown, and we aren't sure if we're allowed to grab a mutex. My hope/guess is yes it's fine? But want to confirm.
2. Is it possible for user tasks to still be scheduled during _atexit()?
   • If the user tasks can still be running, they might be adding new atexit_hooks while we are popping them and running them, which would be a data race.
   • If that's possible, we need to @lock atexit_hooks_lock inside _atexit().
   • My preference would be to lock it, either way, just to be safe, assuming 1. is resolved.

Questions best answered by: @vtjnash, @vchuravy, @gbaraldi?

[vtjnash]

Yes, we run these callback hooks quite early, so they can block exit and threads are still chugging away

[NHDaly]

Okay thanks! Then we need to grab a lock. 👍 I'll put up one more PR