add an SMP lock-stress test case

Four pairs push 400k sequenced messages through small channels; checks FIFO order and cross-core execution. Verified to fail with the lock disabled.

src/kernel/tests.zig

@@ -70,6 +70,8 @@ pub fn run(case: []const u8, boot_info: *const BootInfo) void {
         ipcTest();
     } else if (eql(case, "smp")) {
         smpTest();
+    } else if (eql(case, "smp-stress")) {
+        stressTest();
     } else if (eql(case, "fault-ud")) {
         faultInvalidOpcode();
     } else if (eql(case, "fault-pf")) {
@@ -482,6 +484,92 @@ fn smpTest() void {
     result();
 }
 
+// --- SMP stress: hammer the big kernel lock across cores ------------------
+
+const stress_pairs = 4; // producer/consumer pairs (8 tasks; fits the 16-task pool)
+const stress_msgs = 100_000; // messages per pair
+const stress_cap = 4; // small channel -> constant block/wake, more lock churn
+
+var stress_chan = [_]ipc.Channel(u64, stress_cap){.{}} ** stress_pairs;
+var stress_recv = [_]u64{0} ** stress_pairs; // messages received per pair
+var stress_order_ok = [_]bool{true} ** stress_pairs; // FIFO order held per pair
+var stress_cores = [_]bool{false} ** 8; // cores that ran a consumer
+var stress_prod_claim: usize = 0;
+var stress_cons_claim: usize = 0;
+
+fn stressProducer() void {
+    // Claim a unique pair index (atomic: producers start on different cores).
+    const idx = @atomicRmw(usize, &stress_prod_claim, .Add, 1, .monotonic);
+    var v: u64 = 1;
+    while (v <= stress_msgs) : (v += 1) stress_chan[idx].send(v);
+    sched.exit();
+}
+
+fn stressConsumer() void {
+    const idx = @atomicRmw(usize, &stress_cons_claim, .Add, 1, .monotonic);
+    var expected: u64 = 1;
+    while (expected <= stress_msgs) : (expected += 1) {
+        const got = stress_chan[idx].recv();
+        if (got != expected) stress_order_ok[idx] = false; // lost/reordered => lock broke
+        const c = sched.currentCpuIndex();
+        if (c < stress_cores.len) stress_cores[c] = true;
+        stress_recv[idx] = expected;
+    }
+    sched.exit();
+}
+
+/// Stress the big kernel lock under sustained cross-core contention. Each pair drives
+/// `stress_msgs` sequenced messages through a 4-slot channel — every send and recv
+/// takes the lock, and the small buffer forces constant block/wake (so the scheduler
+/// churns too). A single-producer/single-consumer channel must deliver in strict FIFO
+/// order; if the lock let two cores into a critical section at once, the ring buffer
+/// corrupts and the consumer sees a wrong or out-of-order value (or the run hangs /
+/// faults). Passing means ~320k lock acquisitions across the cores stayed consistent.
+fn stressTest() void {
+    log("DANOS-TEST-BEGIN: smp-stress\n", .{});
+    stress_chan = [_]ipc.Channel(u64, stress_cap){.{}} ** stress_pairs;
+    stress_recv = [_]u64{0} ** stress_pairs;
+    stress_order_ok = [_]bool{true} ** stress_pairs;
+    stress_cores = [_]bool{false} ** 8;
+    stress_prod_claim = 0;
+    stress_cons_claim = 0;
+
+    var i: usize = 0;
+    while (i < stress_pairs) : (i += 1) sched.spawn(stressConsumer, 4);
+    i = 0;
+    while (i < stress_pairs) : (i += 1) sched.spawn(stressProducer, 4);
+
+    // Drop below the workers so they get the cores; wake periodically to check for
+    // completion. A broken lock instead hangs here (harness timeout) or faults.
+    sched.setPriority(1);
+    var spins: u64 = 0;
+    while (spins < 40_000_000_000) : (spins += 1) {
+        var done = true;
+        for (stress_recv) |n| {
+            if (n < stress_msgs) done = false;
+        }
+        if (done) break;
+    }
+    sched.setPriority(4);
+
+    var total: u64 = 0;
+    for (stress_recv) |n| total += n;
+    var order_ok = true;
+    for (stress_order_ok) |ok| {
+        if (!ok) order_ok = false;
+    }
+    var cores: u32 = 0;
+    for (stress_cores) |s| {
+        if (s) cores += 1;
+    }
+
+    log("DANOS-STRESS: {d}/{d} pairs complete on {d} cores\n", .{ total, @as(u64, stress_pairs) * stress_msgs, cores });
+    check("every message delivered", total == @as(u64, stress_pairs) * stress_msgs);
+    check("strict FIFO order held (no lock corruption)", order_ok);
+    check("contention was genuinely cross-core", cores >= 2);
+    result();
+}
+
 fn faultInvalidOpcode() void {
     log("DANOS-TEST-BEGIN: fault-ud\n", .{});
     asm volatile ("ud2");