Summary: Apparently I don't know what I'm doing. But given lock_ is 8bit and the assert, I suppose cast offset_bytes to unsigned is safe.

Reviewed By: dcolascione

Differential Revision: D3010732

fb-gh-sync-id: 1f3440a72d2f0b2fb145ab36966f0382a3009e0c
shipit-source-id: 1f3440a72d2f0b2fb145ab36966f0382a3009e0c