Register CPU online/offline callbacks via cpuhp_setup_state_nocalls()
so stack watches are installed/removed dynamically as CPUs come online
or go offline.

When a new CPU comes online, register a hardware breakpoint for the holder,
avoiding races with watch_on()/watch_off() that may run on another CPU. The
watch address will be updated the next time watch_on() is called.

Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>
---
 mm/kstackwatch/watch.c | 52 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 52 insertions(+)

diff --git a/mm/kstackwatch/watch.c b/mm/kstackwatch/watch.c
index 722ffd9fda7c..f32b1e46168c 100644
--- a/mm/kstackwatch/watch.c
+++ b/mm/kstackwatch/watch.c
@@ -89,6 +89,48 @@ static void ksw_watch_on_local_cpu(void *info)
 	WARN(ret, "fail to reinstall HWBP on CPU%d ret %d", cpu, ret);
 }
 
+static int ksw_watch_cpu_online(unsigned int cpu)
+{
+	struct perf_event_attr attr;
+	struct ksw_watchpoint *wp;
+	call_single_data_t *csd;
+	struct perf_event *bp;
+
+	mutex_lock(&all_wp_mutex);
+	list_for_each_entry(wp, &all_wp_list, list) {
+		attr = wp->attr;
+		attr.bp_addr = (u64)&holder;
+		bp = perf_event_create_kernel_counter(&attr, cpu, NULL,
+						      ksw_watch_handler, wp);
+		if (IS_ERR(bp)) {
+			pr_warn("%s failed to create watch on CPU %d: %ld\n",
+				__func__, cpu, PTR_ERR(bp));
+			continue;
+		}
+
+		per_cpu(*wp->event, cpu) = bp;
+		csd = per_cpu_ptr(wp->csd, cpu);
+		INIT_CSD(csd, ksw_watch_on_local_cpu, wp);
+	}
+	mutex_unlock(&all_wp_mutex);
+	return 0;
+}
+
+static int ksw_watch_cpu_offline(unsigned int cpu)
+{
+	struct ksw_watchpoint *wp;
+	struct perf_event *bp;
+
+	mutex_lock(&all_wp_mutex);
+	list_for_each_entry(wp, &all_wp_list, list) {
+		bp = per_cpu(*wp->event, cpu);
+		if (bp)
+			unregister_hw_breakpoint(bp);
+	}
+	mutex_unlock(&all_wp_mutex);
+	return 0;
+}
+
 static void ksw_watch_update(struct ksw_watchpoint *wp, ulong addr, u16 len)
 {
 	call_single_data_t *csd;
@@ -210,6 +252,16 @@ int ksw_watch_init(void)
 	if (ret <= 0)
 		return -EBUSY;
 
+	ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
+					"kstackwatch:online",
+					ksw_watch_cpu_online,
+					ksw_watch_cpu_offline);
+	if (ret < 0) {
+		ksw_watch_free();
+		pr_err("Failed to register CPU hotplug notifier\n");
+		return ret;
+	}
+
 	return 0;
 }
 
-- 
2.43.0