[PATCH 23/29] mm: shrinker: add refcount and completion_wait fields

[Qi Zheng <zhengqi.arch@bytedance.com>]

This commit introduces refcount and completion_wait
fields to struct shrinker to manage the life cycle
of shrinker instance.

Just a preparation work for implementing the lockless
slab shrink, no functional changes.

Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
---
 include/linux/shrinker.h | 11 +++++++++++
 mm/vmscan.c              |  5 +++++
 2 files changed, 16 insertions(+)

diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
index 4094e4c44e80..7bfeb2f25246 100644
--- a/include/linux/shrinker.h
+++ b/include/linux/shrinker.h
@@ -4,6 +4,8 @@
 
 #include <linux/atomic.h>
 #include <linux/types.h>
+#include <linux/refcount.h>
+#include <linux/completion.h>
 
 /*
  * This struct is used to pass information from page reclaim to the shrinkers.
@@ -70,6 +72,9 @@ struct shrinker {
 	int seeks;	/* seeks to recreate an obj */
 	unsigned flags;
 
+	refcount_t refcount;
+	struct completion completion_wait;
+
 	void *private_data;
 
 	/* These are for internal use */
@@ -118,6 +123,12 @@ struct shrinker *shrinker_alloc_and_init(count_objects_cb count,
 void shrinker_free(struct shrinker *shrinker);
 void unregister_and_free_shrinker(struct shrinker *shrinker);
 
+static inline void shrinker_put(struct shrinker *shrinker)
+{
+	if (refcount_dec_and_test(&shrinker->refcount))
+		complete(&shrinker->completion_wait);
+}
+
 #ifdef CONFIG_SHRINKER_DEBUG
 extern int shrinker_debugfs_add(struct shrinker *shrinker);
 extern struct dentry *shrinker_debugfs_detach(struct shrinker *shrinker,
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 3a8d50ad6ff6..6f9c4750effa 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -740,6 +740,8 @@ void free_prealloced_shrinker(struct shrinker *shrinker)
 void register_shrinker_prepared(struct shrinker *shrinker)
 {
 	down_write(&shrinker_rwsem);
+	refcount_set(&shrinker->refcount, 1);
+	init_completion(&shrinker->completion_wait);
 	list_add_tail(&shrinker->list, &shrinker_list);
 	shrinker->flags |= SHRINKER_REGISTERED;
 	shrinker_debugfs_add(shrinker);
@@ -794,6 +796,9 @@ void unregister_shrinker(struct shrinker *shrinker)
 	if (!(shrinker->flags & SHRINKER_REGISTERED))
 		return;
 
+	shrinker_put(shrinker);
+	wait_for_completion(&shrinker->completion_wait);
+
 	down_write(&shrinker_rwsem);
 	list_del(&shrinker->list);
 	shrinker->flags &= ~SHRINKER_REGISTERED;
-- 
2.30.2