diff --git a/src/kernel/arch/x86_64/cpu.zig b/src/kernel/arch/x86_64/cpu.zig
index 9ef6385..f585391 100644
--- a/src/kernel/arch/x86_64/cpu.zig
+++ b/src/kernel/arch/x86_64/cpu.zig
@@ -282,3 +282,11 @@ pub fn readCr2() u64 {
 pub fn halt() noreturn {
     while (true) asm volatile ("hlt");
 }
+
+/// Spin-wait hint (`pause`). Emitted in the body of a spinlock's busy-wait: it
+/// relaxes the core while it polls a contended lock — yielding pipeline resources
+/// to a hyperthread sibling and easing the cache-coherency traffic on the lock
+/// line. Purely a performance/power hint; correct to omit, but kinder on the bus.
+pub fn cpuRelax() void {
+    asm volatile ("pause");
+}
diff --git a/src/kernel/arch/x86_64/isr.s b/src/kernel/arch/x86_64/isr.s
index 67b149c..0b43191 100644
--- a/src/kernel/arch/x86_64/isr.s
+++ b/src/kernel/arch/x86_64/isr.s
@@ -63,9 +63,14 @@ switch_context:
     ret                     # return into the new task's saved instruction pointer
 
 # task_trampoline: the first thing a freshly-spawned task runs. init_task_stack
-# leaves its entry function in r15. New tasks start with interrupts enabled.
+# leaves its entry function in r15. A fresh task is switched to with the big kernel
+# lock held (the hand-off rule in sync.zig) but has no enter/leave frame of its own,
+# so it releases the lock here before running its body. r15 survives the call (it's
+# callee-saved). New tasks then start with interrupts enabled.
+.extern releaseForFreshTask
 .global task_trampoline
 task_trampoline:
+    call releaseForFreshTask # drop the kernel lock we inherited across the switch
     sti
     call *%r15              # call the task entry (fn() void)
 1:  hlt                     # if the entry returns, idle (still preemptible)
diff --git a/src/kernel/ipc.zig b/src/kernel/ipc.zig
index a7bcd1e..34de07c 100644
--- a/src/kernel/ipc.zig
+++ b/src/kernel/ipc.zig
@@ -11,8 +11,8 @@
 //! When user mode arrives, the same primitive carries messages across the
 //! isolation boundary (with the payload copied between address spaces).
 
-const arch = @import("arch");
 const sched = @import("scheduler.zig");
+const sync = @import("sync.zig");
 
 /// A bounded blocking channel of `capacity` messages of type `T`.
 pub fn Channel(comptime T: type, comptime capacity: usize) type {
@@ -28,7 +28,7 @@ pub fn Channel(comptime T: type, comptime capacity: usize) type {
 
         /// Send a message, blocking while the channel is full.
         pub fn send(self: *Self, msg: T) void {
-            const flags = arch.saveInterrupts();
+            const flags = sync.enter();
             // Recheck the condition in a loop: a wakeup only means "try again"
             // (another waiter may have taken the slot first).
             while (self.count == capacity) sched.waitLocked(&self.not_full);
@@ -36,18 +36,18 @@ pub fn Channel(comptime T: type, comptime capacity: usize) type {
             self.tail = (self.tail + 1) % capacity;
             self.count += 1;
             sched.wakeLocked(&self.not_empty); // a receiver can now proceed
-            arch.restoreInterrupts(flags);
+            sync.leave(flags);
         }
 
         /// Receive a message, blocking while the channel is empty.
         pub fn recv(self: *Self) T {
-            const flags = arch.saveInterrupts();
+            const flags = sync.enter();
             while (self.count == 0) sched.waitLocked(&self.not_empty);
             const msg = self.buffer[self.head];
             self.head = (self.head + 1) % capacity;
             self.count -= 1;
             sched.wakeLocked(&self.not_full); // a sender can now proceed
-            arch.restoreInterrupts(flags);
+            sync.leave(flags);
             return msg;
         }
     };
diff --git a/src/kernel/scheduler.zig b/src/kernel/scheduler.zig
index 2ea3858..c1f4d91 100644
--- a/src/kernel/scheduler.zig
+++ b/src/kernel/scheduler.zig
@@ -9,10 +9,18 @@
 //! Switching happens both cooperatively (`yield`) and preemptively (the timer
 //! calls `tick`). See docs/scheduling.md for the interrupt-flag discipline that
 //! makes those two paths coexist.
+//!
+//! Cross-core safety is the **big kernel lock** (`sync.zig`): every critical
+//! section here runs under it, and it is held across a context switch and released
+//! by the task that resumes (see sync.zig's hand-off rule). On a single core the
+//! lock is never contended, so the behaviour is exactly the old interrupt-flag
+//! model; it's what lets a second core enter `schedule()` without corrupting the
+//! shared queues.
 
 const std = @import("std");
 const arch = @import("arch");
 const heap = @import("heap.zig");
+const sync = @import("sync.zig");
 
 /// Priority level: 0 (lowest) .. 7 (highest). 8 levels total.
 pub const Priority = u3;
@@ -121,19 +129,19 @@ fn schedule() void {
 
 /// Voluntarily give up the CPU to the next ready task.
 pub fn yield() void {
-    const flags = arch.saveInterrupts();
+    const flags = sync.enter();
     schedule();
-    arch.restoreInterrupts(flags);
+    sync.leave(flags);
 }
 
 /// Block the current task for `ms` milliseconds, then let it become runnable
 /// again. The idle task (or other work) runs in the meantime.
 pub fn sleep(ms: u64) void {
-    const flags = arch.saveInterrupts();
+    const flags = sync.enter();
     current.wake_at = arch.millis() + ms;
     current.state = .blocked;
     schedule(); // current is blocked, so schedule() won't re-enqueue it
-    arch.restoreInterrupts(flags);
+    sync.leave(flags);
 }
 
 // --- event-based blocking -------------------------------------------------
@@ -147,9 +155,10 @@ pub const WaitQueue = struct {
     head: ?*Task = null,
 };
 
-/// Block the current task on `wq` and switch away. Precondition: interrupts are
-/// disabled (the caller holds them, so a condition can be checked and the block
-/// committed atomically). On return — when woken — interrupts are still disabled.
+/// Block the current task on `wq` and switch away. Precondition: the big kernel
+/// lock is held (so a condition can be checked and the block committed atomically;
+/// it also keeps local interrupts disabled). On return — when woken — the lock is
+/// still held.
 pub fn waitLocked(wq: *WaitQueue) void {
     current.state = .blocked;
     current.next = wq.head;
@@ -158,7 +167,7 @@ pub fn waitLocked(wq: *WaitQueue) void {
 }
 
 /// Move the highest-priority waiter on `wq` (if any) to the ready queue.
-/// Precondition: interrupts disabled. Does not preempt — the caller decides.
+/// Precondition: the big kernel lock is held. Does not preempt — the caller decides.
 pub fn wakeLocked(wq: *WaitQueue) void {
     // Find the highest-priority waiter (bounded scan) and unlink it.
     var best_prev: ?*Task = null;
@@ -182,20 +191,20 @@ pub fn wakeLocked(wq: *WaitQueue) void {
 
 /// Block on `wq` (a self-contained critical section).
 pub fn wait(wq: *WaitQueue) void {
-    const flags = arch.saveInterrupts();
+    const flags = sync.enter();
     waitLocked(wq);
-    arch.restoreInterrupts(flags);
+    sync.leave(flags);
 }
 
 /// Wake the highest-priority waiter on `wq`, preempting if it outranks us.
 pub fn wake(wq: *WaitQueue) void {
-    const flags = arch.saveInterrupts();
+    const flags = sync.enter();
     wakeLocked(wq);
     // If a higher-priority task is now ready, run it immediately.
     if (highestReadyPriority()) |p| {
         if (p > current.priority) schedule();
     }
-    arch.restoreInterrupts(flags);
+    sync.leave(flags);
 }
 
 fn highestReadyPriority() ?Priority {
@@ -217,10 +226,15 @@ fn wakeExpired() void {
 }
 
 /// Called from the timer interrupt (interrupts already disabled): wake due
-/// sleepers, then preempt.
+/// sleepers, then preempt. Takes the kernel lock like any other critical section,
+/// but releases it *without* touching the interrupt flag — the handler's `iretq`
+/// restores the interrupted context's flags, so re-enabling here would open a
+/// nested-interrupt window before the return.
 pub fn tick() void {
+    _ = sync.enter();
     wakeExpired();
     if (preemption_enabled) schedule();
+    sync.leaveIsr();
 }
 
 /// Enable or disable timer-driven preemption (cooperative-only when off).
@@ -229,9 +243,10 @@ pub fn setPreemption(enabled: bool) void {
 }
 
 /// End the current task and switch away for good; never returns. The task's stack
-/// is leaked for now (no reaper yet).
+/// is leaked for now (no reaper yet). Acquires the kernel lock and hands it off to
+/// the task we switch into (which releases it) — this frame never returns to leave.
 pub fn exit() noreturn {
-    arch.disableInterrupts();
+    _ = sync.enter();
     current.state = .free;
     const next = dequeueHighest() orelse @panic("sched: no task left to run");
     next.state = .running;
diff --git a/src/kernel/sync.zig b/src/kernel/sync.zig
new file mode 100644
index 0000000..ffa03f8
--- /dev/null
+++ b/src/kernel/sync.zig
@@ -0,0 +1,80 @@
+//! The big kernel lock (BKL) — the coarse mutual exclusion that lets more than one
+//! CPU run kernel code safely.
+//!
+//! Until SMP, the kernel's mutual exclusion *was* the interrupt flag: a critical
+//! section did `cli`, and since only one core existed, nothing else could touch
+//! kernel state (the discipline in docs/scheduling.md). That invariant dies the
+//! instant a second core runs kernel code — `cli` on one core does nothing to
+//! another. So the kernel's shared state (the scheduler queues, IPC channels) is
+//! guarded by a spinlock, and the lock is **always held with local interrupts
+//! disabled**, so a core's own timer interrupt can't re-enter the kernel and
+//! deadlock against the lock it already holds.
+//!
+//! This is deliberately *one coarse lock*, not many fine ones: it's philosophically
+//! aligned with a tiny kernel and it keeps the single-core correctness model
+//! (docs/scheduling.md) largely intact — one lock around kernel entry instead of
+//! rethinking every critical section. It's the first-design choice seL4 makes and
+//! docs/smp.md endorses; per-core run queues + fine-grained locking come later, if
+//! contention ever bites. Because the kernel does little, the lock is held briefly.
+//!
+//! **The hand-off rule.** The lock is held *across* a context switch and released
+//! by whichever task resumes, not by the one that switched away. A task that blocks
+//! or yields calls `enter`, mutates the queues, `schedule()`s — switching to another
+//! task *with the lock still held* — and only calls `leave` once it is eventually
+//! resumed and its critical section runs to the end. So every call into `schedule()`
+//! (and thus `switch_context`) happens with the lock held, and every task resumes
+//! from a switch holding it. A freshly-spawned task has no `enter`/`leave` frame to
+//! resume into, so `task_trampoline` releases the lock explicitly on its behalf via
+//! `releaseForFreshTask` before running the task body.
+
+const std = @import("std");
+const arch = @import("arch");
+
+/// 0 = free, 1 = held. A single global lock for the whole kernel.
+var held = std.atomic.Value(u32).init(0);
+
+/// Enter the kernel: disable interrupts on this core, then spin until we own the
+/// lock. Returns the caller's prior interrupt flags for `leave` to restore.
+/// Interrupts stay off for the whole critical section so this core's timer tick
+/// can't try to re-acquire the lock we're holding.
+pub fn enter() u64 {
+    const flags = arch.saveInterrupts();
+    acquire();
+    return flags;
+}
+
+/// Release the lock and restore the interrupt flags `enter` returned (re-enabling
+/// interrupts only if they were on beforehand). The normal exit for a critical
+/// section reached from task context (`yield`, `sleep`, `wait`, `wake`, IPC).
+pub fn leave(flags: u64) void {
+    release();
+    arch.restoreInterrupts(flags);
+}
+
+/// Release the lock but leave interrupts as they are. The exit for a critical
+/// section running inside an interrupt handler (the timer `tick`): the handler's
+/// `iretq` is what restores the interrupted context's flags, so restoring them
+/// here too would open a nested-interrupt window before the return. Release only.
+pub fn leaveIsr() void {
+    release();
+}
+
+/// Release the lock on behalf of a freshly-spawned task. Such a task is switched to
+/// (with the lock held) but has no `enter`/`leave` frame of its own to release
+/// through — `task_trampoline` calls this before running the task body. Interrupts
+/// are enabled separately by the trampoline. Exported for the assembly trampoline.
+export fn releaseForFreshTask() callconv(.c) void {
+    release();
+}
+
+fn acquire() void {
+    // Test-and-test-and-set: try once, then spin read-only until the lock looks
+    // free before retrying the (bus-locked) swap — cheaper on the coherency fabric.
+    while (held.swap(1, .acquire) != 0) {
+        while (held.load(.monotonic) != 0) arch.cpuRelax();
+    }
+}
+
+fn release() void {
+    held.store(0, .release);
+}