x86/split_lock: Simplify reenabling

When split_lock_mitigate is disabled, each CPU needs its own delayed_work
structure. They are used to reenable split lock detection after its
disabling. But delayed_work structure must be correctly initialized after
its allocation.

Current implementation uses deferred initialization that makes the
split lock handler code unclear. The code can be simplified a bit
if the initialization is moved to the appropriate initcall.

sld_setup() is called before setup_per_cpu_areas(), thus it can't be used
for this purpose, so introduce an independent initcall for
the initialization.

[ mingo: Simplified the 'work' assignment line a bit more. ]

Signed-off-by: Maksim Davydov <davydov-max@yandex-team.ru>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250325085807.171885-1-davydov-max@yandex-team.ru

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 */