[PATCH v2 3/3] mm/kmemleak: Prevent soft lockup in first object iteration loop of kmemleak_scan()

[Waiman Long <longman@redhat.com>]

The first RCU-based object iteration loop has to modify the object
count. So we cannot skip taking the object lock.

One way to avoid soft lockup is to insert occasional cond_resched()
call into the loop. This cannot be done while holding the RCU read
lock which is to protect objects from being freed. However, taking a
reference to the object will prevent it from being freed. We can then
do a cond_resched() call after every 64k objects safely.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 mm/kmemleak.c | 28 +++++++++++++++++++++++++++-
 1 file changed, 27 insertions(+), 1 deletion(-)

diff --git a/mm/kmemleak.c b/mm/kmemleak.c
index 7dd64139a7c7..abba063ae5ee 100644
--- a/mm/kmemleak.c
+++ b/mm/kmemleak.c
@@ -1417,12 +1417,16 @@ static void kmemleak_scan(void)
 	struct zone *zone;
 	int __maybe_unused i;
 	int new_leaks = 0;
+	int loop1_cnt = 0;
 
 	jiffies_last_scan = jiffies;
 
 	/* prepare the kmemleak_object's */
 	rcu_read_lock();
 	list_for_each_entry_rcu(object, &object_list, object_list) {
+		bool obj_pinned = false;
+
+		loop1_cnt++;
 		raw_spin_lock_irq(&object->lock);
 #ifdef DEBUG
 		/*
@@ -1437,10 +1441,32 @@ static void kmemleak_scan(void)
 #endif
 		/* reset the reference count (whiten the object) */
 		object->count = 0;
-		if (color_gray(object) && get_object(object))
+		if (color_gray(object) && get_object(object)) {
 			list_add_tail(&object->gray_list, &gray_list);
+			obj_pinned = true;
+		}
 
 		raw_spin_unlock_irq(&object->lock);
+
+		/*
+		 * Do a cond_resched() to avoid soft lockup every 64k objects.
+		 * Make sure a reference has been taken so that the object
+		 * won't go away without RCU read lock.
+		 */
+		if (!(loop1_cnt & 0xffff)) {
+			if (!obj_pinned && !get_object(object)) {
+				/* Try the next object instead */
+				loop1_cnt--;
+				continue;
+			}
+
+			rcu_read_unlock();
+			cond_resched();
+			rcu_read_lock();
+
+			if (!obj_pinned)
+				put_object(object);
+		}
 	}
 	rcu_read_unlock();
 
-- 
2.31.1