sync: add SpinLock (#24788)

Author: kbkpbot

thirdparty/stdatomic/nix/atomic_cpp.h

@@ -120,6 +120,14 @@ using std::memory_order_consume;
 using std::memory_order_relaxed;
 using std::memory_order_release;
 using std::memory_order_seq_cst;
+
+#define memory_order_relaxed std::memory_order_relaxed
+#define memory_order_consume std::memory_order_consume
+#define memory_order_acquire std::memory_order_acquire
+#define memory_order_release std::memory_order_release
+#define memory_order_acq_rel std::memory_order_acq_rel
+#define memory_order_seq_cst std::memory_order_seq_cst
+
 #else /* <atomic> unavailable, possibly because this is C, not C++ */
 #include <sys/types.h>
 #include <stdbool.h>
@@ -266,6 +274,7 @@ typedef enum
     memory_order_acq_rel = __ATOMIC_ACQ_REL,
     memory_order_seq_cst = __ATOMIC_SEQ_CST
 } memory_order;
+
 /*
  * 7.17.4 Fences.
  */

vlib/sync/spinlock_test.v

@@ -0,0 +1,22 @@
+import sync
+
+fn test_spinlock() {
+	mut counter := 0
+	mut s := sync.new_spin_lock()
+	num_threads := 10
+	mut wg := sync.new_waitgroup()
+	wg.add(num_threads)
+
+	for _ in 0 .. num_threads {
+		spawn fn (mut wg sync.WaitGroup, s &sync.SpinLock, counter_ref &int) {
+			defer {
+				s.unlock()
+				wg.done()
+			}
+			s.lock()
+			(*counter_ref)++
+		}(mut wg, s, &counter)
+	}
+	wg.wait()
+	assert counter == num_threads
+}

vlib/sync/sync.c.v

@@ -1,5 +1,7 @@
 module sync
 
+import time
+
 @[noreturn]
 fn cpanic(res int) {
 	panic(unsafe { tos_clone(&u8(C.strerror(res))) })
@@ -15,3 +17,72 @@ fn should_be_zero(res int) {
 		cpanic(res)
 	}
 }
+
+// SpinLock is a mutual exclusion lock that busy-waits (spins) when locked.
+// When one thread holds the lock, any other thread attempting to acquire it
+// will loop repeatedly until the lock becomes available.
+pub struct SpinLock {
+mut:
+	locked  u8     // Lock state: 0 = unlocked, 1 = locked
+	padding [63]u8 // Cache line padding (fills to 64 bytes total)
+}
+
+// new_spin_lock creates and returns a new SpinLock instance initialized to unlocked state
+pub fn new_spin_lock() &SpinLock {
+	mut the_lock := &SpinLock{
+		locked: 0
+	}
+	// Ensure initialization visibility across threads
+	C.atomic_thread_fence(C.memory_order_release)
+	return the_lock
+}
+
+// lock acquires the spin lock. If the lock is currently held by another thread,
+// this function will spin (busy-wait) until the lock becomes available.
+@[inline]
+pub fn (s &SpinLock) lock() {
+	// Expected value starts as unlocked (0)
+	mut expected := u8(0)
+	mut spin_count := 0
+	max_spins := 100
+	base_delay := 100 // nanosecond
+	max_delay := 10000 // nanoseconds (10μs)
+
+	// Busy-wait until lock is acquired
+	for {
+		// Attempt atomic compare-and-swap:
+		// Succeeds if current value matches expected (0),
+		// then swaps to locked (1)
+		if C.atomic_compare_exchange_weak_byte(&s.locked, &expected, 1) {
+			// Prevent critical section reordering
+			C.atomic_thread_fence(C.memory_order_acquire)
+			return
+		}
+
+		spin_count++
+		// Exponential backoff after max_spins
+		if spin_count > max_spins {
+			// Calculate delay with cap: 100ns to 10μs
+			exponent := int_min(spin_count / max_spins, 10)
+			delay := int_min(base_delay * (1 << exponent), max_delay)
+			time.sleep(delay * time.nanosecond)
+		} else {
+			// Reduce power/bus contention during spinning
+			C.cpu_relax()
+		}
+
+		// Refresh lock state before next attempt
+		expected = C.atomic_load_byte(&s.locked)
+	}
+}
+
+// unlock releases the spin lock, making it available to other threads.
+// IMPORTANT: Must only be called by the thread that currently holds the lock.
+@[inline]
+pub fn (s &SpinLock) unlock() {
+	// Ensure critical section completes before release
+	C.atomic_thread_fence(C.memory_order_release)
+
+	// Atomically reset to unlocked state
+	C.atomic_store_byte(&s.locked, 0)
+}