libc++ implementation of std::condition_variable_any

Condition variables should have have a single order with respect to notify() and unlock_sleep() operations. To achieve this with arbitrary lockables std::condition_variable_any implementations typically use another mutex internally (to ensure atomicity and to sleep on)

If the unlock_sleep() and notify() operations are not atomic with respect to each other you risk a thread unlocking the mutex, another thread signaling and then the original thread going to sleep and never waking up.

I was reading the libstdc++ and libc++ implementations of std::condition_variable_any and noticed this code in the libc++ implementation

{lock_guard<mutex> __lx(*__mut_);}
__cv_.notify_all();

the internal mutex is locked and then immediately unlocked before the signal operation. Doesn't this risk the problem I described above?

libstdc++ seems to have gotten this right