Merge tag 'locking-urgent-2025-03-28' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull misc locking fixes and updates from Ingo Molnar:

 - Fix a locking self-test FAIL on PREEMPT_RT kernels

 - Fix nr_unused_locks accounting bug

 - Simplify the split-lock debugging feature's fast-path

* tag 'locking-urgent-2025-03-28' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  locking/lockdep: Decrease nr_unused_locks if lock unused in zap_class()
  lockdep: Fix wait context check on softirq for PREEMPT_RT
  x86/split_lock: Simplify reenabling

Author: torvalds

arch/x86/kernel/cpu/bus_lock.c

@@ -200,6 +200,26 @@ static void __split_lock_reenable(struct work_struct *work)
  */
 static DEFINE_PER_CPU(struct delayed_work, sl_reenable);
 
+/*
+ * Per-CPU delayed_work can't be statically initialized properly because
+ * the struct address is unknown. Thus per-CPU delayed_work structures
+ * have to be initialized during kernel initialization and after calling
+ * setup_per_cpu_areas().
+ */
+static int __init setup_split_lock_delayed_work(void)
+{
+	unsigned int cpu;
+
+	for_each_possible_cpu(cpu) {
+		struct delayed_work *work = per_cpu_ptr(&sl_reenable, cpu);
+
+		INIT_DELAYED_WORK(work, __split_lock_reenable);
+	}
+
+	return 0;
+}
+pure_initcall(setup_split_lock_delayed_work);
+
 /*
  * If a CPU goes offline with pending delayed work to re-enable split lock
  * detection then the delayed work will be executed on some other CPU. That
@@ -219,15 +239,16 @@ static int splitlock_cpu_offline(unsigned int cpu)
 
 static void split_lock_warn(unsigned long ip)
 {
-	struct delayed_work *work = NULL;
+	struct delayed_work *work;
 	int cpu;
+	unsigned int saved_sld_mitigate = READ_ONCE(sysctl_sld_mitigate);
 
 	if (!current->reported_split_lock)
 		pr_warn_ratelimited("#AC: %s/%d took a split_lock trap at address: 0x%lx\n",
 				    current->comm, current->pid, ip);
 	current->reported_split_lock = 1;
 
-	if (sysctl_sld_mitigate) {
+	if (saved_sld_mitigate) {
 		/*
 		 * misery factor #1:
 		 * sleep 10ms before trying to execute split lock.
@@ -240,18 +261,10 @@ static void split_lock_warn(unsigned long ip)
 		 */
 		if (down_interruptible(&buslock_sem) == -EINTR)
 			return;
-		work = &sl_reenable_unlock;
 	}
 
 	cpu = get_cpu();
-
-	if (!work) {
-		work = this_cpu_ptr(&sl_reenable);
-		/* Deferred initialization of per-CPU struct */
-		if (!work->work.func)
-			INIT_DELAYED_WORK(work, __split_lock_reenable);
-	}
-
+	work = saved_sld_mitigate ? &sl_reenable_unlock : per_cpu_ptr(&sl_reenable, cpu);
 	schedule_delayed_work_on(cpu, work, 2);
 
 	/* Disable split lock detection on this CPU to make progress */

kernel/locking/lockdep.c

@@ -6264,6 +6264,9 @@ static void zap_class(struct pending_free *pf, struct lock_class *class)
 		hlist_del_rcu(&class->hash_entry);
 		WRITE_ONCE(class->key, NULL);
 		WRITE_ONCE(class->name, NULL);
+		/* Class allocated but not used, -1 in nr_unused_locks */
+		if (class->usage_mask == 0)
+			debug_atomic_dec(nr_unused_locks);
 		nr_lock_classes--;
 		__clear_bit(class - lock_classes, lock_classes_in_use);
 		if (class - lock_classes == max_lock_class_idx)

kernel/softirq.c

@@ -126,6 +126,18 @@ static DEFINE_PER_CPU(struct softirq_ctrl, softirq_ctrl) = {
 	.lock	= INIT_LOCAL_LOCK(softirq_ctrl.lock),
 };
 
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+static struct lock_class_key bh_lock_key;
+struct lockdep_map bh_lock_map = {
+	.name			= "local_bh",
+	.key			= &bh_lock_key,
+	.wait_type_outer	= LD_WAIT_FREE,
+	.wait_type_inner	= LD_WAIT_CONFIG, /* PREEMPT_RT makes BH preemptible. */
+	.lock_type		= LD_LOCK_PERCPU,
+};
+EXPORT_SYMBOL_GPL(bh_lock_map);
+#endif
+
 /**
  * local_bh_blocked() - Check for idle whether BH processing is blocked
  *
@@ -148,6 +160,8 @@ void __local_bh_disable_ip(unsigned long ip, unsigned int cnt)
 
 	WARN_ON_ONCE(in_hardirq());
 
+	lock_map_acquire_read(&bh_lock_map);
+
 	/* First entry of a task into a BH disabled section? */
 	if (!current->softirq_disable_cnt) {
 		if (preemptible()) {
@@ -211,6 +225,8 @@ void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
 	WARN_ON_ONCE(in_hardirq());
 	lockdep_assert_irqs_enabled();
 
+	lock_map_release(&bh_lock_map);
+
 	local_irq_save(flags);
 	curcnt = __this_cpu_read(softirq_ctrl.cnt);
 
@@ -261,6 +277,8 @@ static inline void ksoftirqd_run_begin(void)
 /* Counterpart to ksoftirqd_run_begin() */
 static inline void ksoftirqd_run_end(void)
 {
+	/* pairs with the lock_map_acquire_read() in ksoftirqd_run_begin() */
+	lock_map_release(&bh_lock_map);
 	__local_bh_enable(SOFTIRQ_OFFSET, true);
 	WARN_ON_ONCE(in_interrupt());
 	local_irq_enable();