[PATCH 2/8] mm: vmscan: split unregister_shrinker()

[Qi Zheng <qi.zheng@xxxxxxxxx>]

From: Kirill Tkhai <tkhai@xxxxx>

This and the next patches in this series aim to make
time effect of synchronize_srcu() invisible for user.
The patch splits unregister_shrinker() in two functions:

	unregister_shrinker_delayed_initiate()
	unregister_shrinker_delayed_finalize()

and shrinker users may make the second of them to be called
asynchronous (e.g., from workqueue). Next patches make
superblock shrinker to follow this way, so user-visible
umount() time won't contain delays from synchronize_srcu().

Signed-off-by: Kirill Tkhai <tkhai@xxxxx>
Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
---
 include/linux/shrinker.h |  2 ++
 mm/vmscan.c              | 22 ++++++++++++++++++----
 2 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
index 224293b2dd06..e9d5a19d83fe 100644
--- a/include/linux/shrinker.h
+++ b/include/linux/shrinker.h
@@ -102,6 +102,8 @@ extern void register_shrinker_prepared(struct shrinker *shrinker);
 extern int __printf(2, 3) register_shrinker(struct shrinker *shrinker,
 					    const char *fmt, ...);
 extern void unregister_shrinker(struct shrinker *shrinker);
+extern void unregister_shrinker_delayed_initiate(struct shrinker *shrinker);
+extern void unregister_shrinker_delayed_finalize(struct shrinker *shrinker);
 extern void free_prealloced_shrinker(struct shrinker *shrinker);
 extern void synchronize_shrinkers(void);
 
diff --git a/mm/vmscan.c b/mm/vmscan.c
index a773e97e152e..baf8d2327d70 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -799,10 +799,7 @@ int register_shrinker(struct shrinker *shrinker, const char *fmt, ...)
 #endif
 EXPORT_SYMBOL(register_shrinker);
 
-/*
- * Remove one
- */
-void unregister_shrinker(struct shrinker *shrinker)
+void unregister_shrinker_delayed_initiate(struct shrinker *shrinker)
 {
 	struct dentry *debugfs_entry;
 	int debugfs_id;
@@ -819,6 +816,13 @@ void unregister_shrinker(struct shrinker *shrinker)
 	mutex_unlock(&shrinker_mutex);
 
 	shrinker_debugfs_remove(debugfs_entry, debugfs_id);
+}
+EXPORT_SYMBOL(unregister_shrinker_delayed_initiate);
+
+void unregister_shrinker_delayed_finalize(struct shrinker *shrinker)
+{
+	if (!shrinker->nr_deferred)
+		return;
 
 	atomic_inc(&shrinker_srcu_generation);
 	synchronize_srcu(&shrinker_srcu);
@@ -826,6 +830,16 @@ void unregister_shrinker(struct shrinker *shrinker)
 	kfree(shrinker->nr_deferred);
 	shrinker->nr_deferred = NULL;
 }
+EXPORT_SYMBOL(unregister_shrinker_delayed_finalize);
+
+/*
+ * Remove one
+ */
+void unregister_shrinker(struct shrinker *shrinker)
+{
+	unregister_shrinker_delayed_initiate(shrinker);
+	unregister_shrinker_delayed_finalize(shrinker);
+}
 EXPORT_SYMBOL(unregister_shrinker);
 
 /**
-- 
2.30.2