It could be (b) -- it's incorrect because there's a deadlock due to incorrect mutex usage here. It could also be (c) -- it's correct but there's contention or:

It could be (b) -- it's incorrect because there's a deadlock due to incorrect mutex usage here. It could also be (c) -- it's correct but there's contention or: the mutex is too coarse / there's "too much" work being protected by the mutex. But I think your point about matching windows is likely the case (this is how wine implements WaitForMultipleObjects maybe?). The fd exhaustion with FUTEX_FD means they need another way.