[PATCH RFC 04/10] mm: Split unregister_shrinker()

[Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>]

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 <ktkhai@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 9443cafd1969..92062d1239c2 100644
--- a/include/linux/shrinker.h
+++ b/include/linux/shrinker.h
@@ -86,5 +86,7 @@ extern int prealloc_shrinker(struct shrinker *shrinker);
 extern void register_shrinker_prepared(struct shrinker *shrinker);
 extern int register_shrinker(struct shrinker *shrinker);
 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);
 #endif
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 2dc274a385b9..fba4996dfe25 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -422,10 +422,7 @@ int register_shrinker(struct shrinker *shrinker)
 }
 EXPORT_SYMBOL(register_shrinker);
 
-/*
- * Remove one
- */
-void unregister_shrinker(struct shrinker *shrinker)
+void unregister_shrinker_delayed_initiate(struct shrinker *shrinker)
 {
 	if (!shrinker->nr_deferred)
 		return;
@@ -434,12 +431,29 @@ void unregister_shrinker(struct shrinker *shrinker)
 	mutex_lock(&shrinker_mutex);
 	list_del_rcu(&shrinker->list);
 	mutex_unlock(&shrinker_mutex);
+}
+EXPORT_SYMBOL(unregister_shrinker_delayed_initiate);
+
+void unregister_shrinker_delayed_finalize(struct shrinker *shrinker)
+{
+	if (!shrinker->nr_deferred)
+		return;
 
 	synchronize_srcu(&srcu);
 
 	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);
 
 #define SHRINK_BATCH 128