[PATCH RFC 10/10] fs: Use unregister_shrinker_delayed_{initiate, finalize} for super_block shrinker

[Kirill Tkhai]

Previous patches made all the data, which is touched from
super_cache_count(), destroyed from destroy_super_work():
s_dentry_lru, s_inode_lru and super_block::s_fs_info.

super_cache_scan() can't be called after SB_ACTIVE is cleared
in generic_shutdown_super().

So, it safe to move heavy unregister_shrinker_delayed_finalize()
part to delayed work, i.e. it's safe for parallel do_shrink_slab()
to be executed between unregister_shrinker_delayed_initiate() and
destroy_super_work()->unregister_shrinker_delayed_finalize().

This makes the heavy synchronize_srcu() to do not affect on user-visible
unregistration speed (since now it's executed from workqueue).

All further time-critical for unregistration places may be written
in the same conception.

Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
---
 fs/super.c         |    4 +++-
 include/linux/fs.h |    5 +++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/super.c b/fs/super.c
index c60f092538c7..33e829741ec0 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -165,6 +165,8 @@ static void destroy_super_work(struct work_struct *work)
 							destroy_work);
 	int i;
 
+	unregister_shrinker_delayed_finalize(&s->s_shrink);
+
 	WARN_ON(list_lru_count(&s->s_dentry_lru));
 	WARN_ON(list_lru_count(&s->s_inode_lru));
 	list_lru_destroy(&s->s_dentry_lru);
@@ -334,7 +336,7 @@ void deactivate_locked_super(struct super_block *s)
 	struct file_system_type *fs = s->s_type;
 	if (atomic_dec_and_test(&s->s_active)) {
 		cleancache_invalidate_fs(s);
-		unregister_shrinker(&s->s_shrink);
+		unregister_shrinker_delayed_initiate(&s->s_shrink);
 		fs->kill_sb(s);
 
 		put_filesystem(fs);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 33dfaed0a01a..8a1cd3097eef 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1902,6 +1902,11 @@ struct super_operations {
 	struct dquot **(*get_dquots)(struct inode *);
 #endif
 	int (*bdev_try_to_free_page)(struct super_block*, struct page*, gfp_t);
+	/*
+	 * Shrinker may call these two function on destructing super_block
+	 * till unregister_shrinker_delayed_finalize() has completed
+	 * in destroy_super_work(), and they must care about that.
+	 */
 	long (*nr_cached_objects)(struct super_block *,
 				  struct shrink_control *);
 	long (*free_cached_objects)(struct super_block *,