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threads(M4): futex_wait/futex_wake, the blocking primitive

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New private syscalls futex_wait(addr, expected, timeout_ns)=40 and
futex_wake(addr, count)=41. A waiter is a .blocked task tagged with
Task.futex_addr (no queue linkage); futex_wait reads the user word under the big
lock and parks only if it still equals expected, so a concurrent wake can't slip
between the check and the block. futex_wake scans the task table and readies up
to count waiters in the same address space. A timed wait also sets wake_at so the
existing wakeExpired times it out; futex_addr staying non-zero (only futex_wake
clears it) distinguishes timeout from a real wake. Waiters park in-kernel, so an
idle core still halts (no busy-wait).

runtime.Thread.Futex mirrors std.Thread.Futex (wait/timedWait/wake). thread-test
gains a futex mode: a waiter parks, the main thread wakes it (serial order
waiting/waking/woke, asserted by the case regex), and timedWait reports a
timeout.

Gate thread-futex PASS (3x); 18 guardrail cases green incl. sleep/event/ipc
blocking paths; build + host tests clean.
This commit is contained in:
daniel
2026-07-20 21:30:57 +01:00
parent 0730e77530
commit b3a8147bd7
8 changed files with 272 additions and 14 deletions
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system/abi.zig
+7
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@@ -66,9 +66,16 @@ pub const SystemCall = enum(u64) {
thread_spawn = 37, // thread_spawn(entry, stack_top, arg, exit_endpoint) -> tid: start a task sharing the caller's address space at `entry` on `stack_top`, `arg` in rdi; exit_endpoint (a handle, or no_cap) is notified when it ends — how join waits (docs/threading.md)
thread_exit = 38, // thread_exit(): end the calling thread, dropping one reference to its address space (destroyed on the last)
current_core = 39, // current_core() -> index: the dense 0-based index of the core the caller is running on (for parallelism/affinity introspection)
futex_wait = 40, // futex_wait(addr, expected, timeout_ns) -> status: if *addr == expected, block until woken or the timeout; returns futex_woken/mismatch/timed_out (docs/threading.md)
futex_wake = 41, // futex_wake(addr, count) -> woken: wake up to `count` tasks blocked in futex_wait on `addr` in this address space
_,
};
/// `futex_wait` return codes (in rax).
pub const futex_woken: u64 = 0; // woken by a futex_wake
pub const futex_mismatch: u64 = 1; // *addr != expected on entry; the caller did not block
pub const futex_timed_out: u64 = 2; // the timeout elapsed before a wake
/// How a process ended — recorded by the kernel at death, queried by the
/// supervisor with `process_exit_reason`, and the input to its restart decision
/// (docs/process-lifecycle.md): a clean exit meant to stop, a fault wants a