Implement FUTEX_{WAKE,WAIT}_BITSET by using a `u32` as a data
discriminant value.
Also add a bunch of tests to `usertest` to ensure proper functionality.
This commit refactors the core process representation to decouple
"Identity/Resources" from "Execution/Scheduling". Previously, a
monolithic `Task` struct wrapped in `Arc<SpinLock<>>` caused lock
contention during hot scheduling paths and conflated shared state with
CPU-local state.
The `Task` struct has been split into:
1. `Task` (Shared): Holds process-wide resources (VM, FileTable,
Credentials). Managed via `Arc` and internal fine-grained locking.
2. `OwnedTask` (Private): Holds execution state (Context, v_runtime,
signal mask). Strictly owned by a specific CPU (via the Scheduler) and
accessed lock-free.
Key changes:
* Scheduler:
chedState` now owns tasks via `Box<OwnedTask>`.
- Transitions between `run_queue` and `running_task` involve strictly
moving ownership of the Box, ensuring pointer stability.
- The EEVDF comparison logic now explicitly handles comparisons
between the queued candidates and the currently running task (which is
not in the queue).
* Current Task Access:
- `current()` now returns a `CurrentTaskGuard` which:
1. Disables preemption (preventing context switches while holding
the reference).
2. Performs a runtime borrow check (panic on double-mutable borrow).
3. Dereferences a cached Per-CPU raw pointer for O(1) access.
The `timeout` parameter is only used for `_WAIT` futex ops. For other
ops, the `timeout` parameter is permitted to be an undefied value. The
current implementation would then try to `copy_from_user` using the
garbage pointer and fault, causing a missed wake-up and deadlock the
calling process.
Fix this by only accessing the timeout parmeter for `_WAIT` futex ops
where the parameter's value must be valid.
