- the current concurrency level not to overcommit. On success, the
- number of woken threads is returned, it can be 0.
+ the current concurrency level not to overcommit. On success, a hint of
+ the number of woken threads is returned, it can be 0.
++
+This is only a hint of the number of threads woken up for two reasons. First,
+the kernel could really have woken up a thread, but when it becomes scheduled,
+it could *then* decide that it would overcommit (because some other thread
+unblocked inbetween for example), and block it again.
++
+But it can also lie in the other direction: userland is supposed to account
+for waiting threads. So when we're overcommiting and userland want a waiting
+thread to be unblocked, we actually say we woke none, but still unblock one
+(the famous quarantined threads we talk about above). This allow the userland
+counter of waiting threads to decrease, but we know the thread won't be usable
+so we return 0.