[PATCH v6 09/16] locking/lockdep: Reuse list entries that are no longer in use

[Bart Van Assche]

Instead of abandoning elements of list_entries[] that are no longer in
use, make alloc_list_entry() reuse array elements that have been freed.

Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Waiman Long <longman@xxxxxxxxxx>
Cc: Johannes Berg <johannes@xxxxxxxxxxxxxxxx>
Signed-off-by: Bart Van Assche <bvanassche@xxxxxxx>
---
 kernel/locking/lockdep.c | 48 +++++++++++++++++++++++++++++++---------
 1 file changed, 38 insertions(+), 10 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 5b142f699503..5e8a3a17bb94 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -45,6 +45,7 @@
 #include <linux/hash.h>
 #include <linux/ftrace.h>
 #include <linux/stringify.h>
+#include <linux/bitmap.h>
 #include <linux/bitops.h>
 #include <linux/gfp.h>
 #include <linux/random.h>
@@ -132,6 +133,7 @@ static inline int debug_locks_off_graph_unlock(void)
 
 unsigned long nr_list_entries;
 static struct lock_list list_entries[MAX_LOCKDEP_ENTRIES];
+static DECLARE_BITMAP(list_entries_in_use, MAX_LOCKDEP_ENTRIES);
 
 /*
  * All data structures here are protected by the global debug_lock.
@@ -296,6 +298,7 @@ static struct pending_free {
 	struct list_head zapped_classes;
 	struct rcu_head	 rcu_head;
 	bool		 scheduled;
+	DECLARE_BITMAP(list_entries_being_freed, MAX_LOCKDEP_ENTRIES);
 } pending_free[2];
 static DECLARE_WAIT_QUEUE_HEAD(rcu_cb);
 
@@ -756,6 +759,19 @@ static bool assign_lock_key(struct lockdep_map *lock)
 	return true;
 }
 
+static bool list_entry_being_freed(int list_entry_idx)
+{
+	struct pending_free *pf;
+	int i;
+
+	for (i = 0, pf = pending_free; i < ARRAY_SIZE(pending_free);
+	     i++, pf++)
+		if (test_bit(list_entry_idx, pf->list_entries_being_freed))
+			return true;
+
+	return false;
+}
+
 /*
  * Initialize the lock_classes[] array elements, the free_lock_classes list
  * and also the pending_free[] array.
@@ -896,7 +912,10 @@ register_lock_class(struct lockdep_map *lock, unsigned int subclass, int force)
  */
 static struct lock_list *alloc_list_entry(void)
 {
-	if (nr_list_entries >= MAX_LOCKDEP_ENTRIES) {
+	int idx = find_first_zero_bit(list_entries_in_use,
+				      ARRAY_SIZE(list_entries));
+
+	if (idx >= ARRAY_SIZE(list_entries)) {
 		if (!debug_locks_off_graph_unlock())
 			return NULL;
 
@@ -904,7 +923,9 @@ static struct lock_list *alloc_list_entry(void)
 		dump_stack();
 		return NULL;
 	}
-	return list_entries + nr_list_entries++;
+	nr_list_entries++;
+	__set_bit(idx, list_entries_in_use);
+	return list_entries + idx;
 }
 
 /*
@@ -1008,7 +1029,7 @@ static inline void mark_lock_accessed(struct lock_list *lock,
 	unsigned long nr;
 
 	nr = lock - list_entries;
-	WARN_ON(nr >= nr_list_entries); /* Out-of-bounds, input fail */
+	WARN_ON(nr >= ARRAY_SIZE(list_entries)); /* Out-of-bounds, input fail */
 	lock->parent = parent;
 	lock->class->dep_gen_id = lockdep_dependency_gen_id;
 }
@@ -1018,7 +1039,7 @@ static inline unsigned long lock_accessed(struct lock_list *lock)
 	unsigned long nr;
 
 	nr = lock - list_entries;
-	WARN_ON(nr >= nr_list_entries); /* Out-of-bounds, input fail */
+	WARN_ON(nr >= ARRAY_SIZE(list_entries)); /* Out-of-bounds, input fail */
 	return lock->class->dep_gen_id == lockdep_dependency_gen_id;
 }
 
@@ -4277,13 +4298,15 @@ static void zap_class(struct pending_free *pf, struct lock_class *class)
 	 * Remove all dependencies this lock is
 	 * involved in:
 	 */
-	for (i = 0, entry = list_entries; i < nr_list_entries; i++, entry++) {
+	for_each_set_bit(i, list_entries_in_use, ARRAY_SIZE(list_entries)) {
+		entry = list_entries + i;
 		if (entry->class != class && entry->links_to != class)
 			continue;
+		if (list_entry_being_freed(i))
+			continue;
+		set_bit(i, pf->list_entries_being_freed);
+		nr_list_entries--;
 		list_del_rcu(&entry->entry);
-		/* Clear .class and .links_to to avoid double removal. */
-		WRITE_ONCE(entry->class, NULL);
-		WRITE_ONCE(entry->links_to, NULL);
 	}
 	if (list_empty(&class->locks_after) &&
 	    list_empty(&class->locks_before)) {
@@ -4325,8 +4348,9 @@ static bool inside_selftest(void)
 }
 
 /*
- * Free all lock classes that are on the pf->zapped_classes list. May be called
- * from RCU callback context.
+ * Free all lock classes that are on the pf->zapped_classes list and also all
+ * list entries that have been marked as being freed. May be called from RCU
+ * callback context.
  */
 static void free_zapped_classes(struct rcu_head *ch)
 {
@@ -4342,6 +4366,9 @@ static void free_zapped_classes(struct rcu_head *ch)
 		reinit_class(class);
 	}
 	list_splice_init(&pf->zapped_classes, &free_lock_classes);
+	bitmap_andnot(list_entries_in_use, list_entries_in_use,
+		      pf->list_entries_being_freed, ARRAY_SIZE(list_entries));
+	bitmap_clear(pf->list_entries_being_freed, 0, ARRAY_SIZE(list_entries));
 	graph_unlock();
 restore_irqs:
 	raw_local_irq_restore(flags);
@@ -4626,6 +4653,7 @@ void __init lockdep_init(void)
 
 	printk(" memory used by lock dependency info: %zu kB\n",
 	       (sizeof(list_entries) +
+		sizeof(list_entries_in_use) +
 		sizeof(lock_classes) +
 		sizeof(classhash_table) +
 		sizeof(chainhash_table) +
-- 
2.20.1.97.g81188d93c3-goog