recursive locks (Duh).
Disable cancellation around the cond_wait() call, since that's also a
cancellation point. Arguably, that would be better handled with
pthread_cleanup_*(), but stubbing those for libc is difficult, and the
current non-exception-based implementation of cleanup handlers is
probably no faster than disabling and reenabling cancellation.
Finally, it only happens in the slow path where the thread is going to
sleep anyway...
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