malloc->backtrace->dlopen->malloc deadlock

[Roland McGrath]

> Based on the backtrace in the bz, this looks like a different (and
> new) problem from #16159.  The deadlock seems to be happening on the
> mtrace-internal LOCK and not the arena lock.  We never attempted to
> fix that with #16159; our fix was limited to making malloc itself work
> and we never got to any use cases beyond that.

Carlos had previously raised a question about user-supplied malloc
implementations calling dlopen.  That might be the case that seemed most
related to 16573.  But I don't recall if there was a resolution to that.