- * Access the page AFTER the futex is queued.
- * Order is important:
- *
- * Userspace waiter: val = var; if (cond(val)) futex_wait(&var, val);
- * Userspace waker: if (cond(var)) { var = new; futex_wake(&var); }
- *
- * The basic logical guarantee of a futex is that it blocks ONLY
- * if cond(var) is known to be true at the time of blocking, for
- * any cond. If we queued after testing *uaddr, that would open
- * a race condition where we could block indefinitely with
- * cond(var) false, which would violate the guarantee.
- *
- * A consequence is that futex_wait() can return zero and absorb
- * a wakeup when *uaddr != val on entry to the syscall. This is
- * rare, but normal.
- *