Ah, sorry I didn't work through the math from the commit message. This *appears* very dangerous because on 32-bit it currently implies that there can be no more than 65K contenders. When you do the math (as you pointed out) this is impossible because the 4GB address spaced divided into 2MB chunks yields 2k tasks (much lower than 16k).

I'm not too worried about 64 bit because it's an insane feat if we can spawn 4 billion tasks much less than having them all contend on one mutex.

Regardless, all of this math should be in a large comment explaining why overflow checking isn't necessary.