[PATCH v6 5/7] kernfs: Use a per-fs rwsem to protect per-fs list of kernfs_super_info.

[Imran Khan <imran.f.khan@oracle.com>]

Right now per-fs kernfs_rwsem protects list of kernfs_super_info instances
for a kernfs_root. Since kernfs_rwsem is used to synchronize several other
operations across kernfs and since most of these operations don't impact
kernfs_super_info, we can use a separate per-fs rwsem to synchronize access
to list of kernfs_super_info.
This helps in reducing contention around kernfs_rwsem and also allows
operations that change/access list of kernfs_super_info to proceed without
contending for kernfs_rwsem.

Signed-off-by: Imran Khan <imran.f.khan@oracle.com>
---
 fs/kernfs/dir.c        | 1 +
 fs/kernfs/file.c       | 2 ++
 fs/kernfs/mount.c      | 8 ++++----
 include/linux/kernfs.h | 1 +
 4 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index e6d9772ddb4ca..dc769301ac96b 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -917,6 +917,7 @@ struct kernfs_root *kernfs_create_root(struct kernfs_syscall_ops *scops,
 	idr_init(&root->ino_idr);
 	init_rwsem(&root->kernfs_rwsem);
 	INIT_LIST_HEAD(&root->supers);
+	init_rwsem(&root->supers_rwsem);
 
 	/*
 	 * On 64bit ino setups, id is ino.  On 32bit, low 32bits are ino.
diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index f3ecc6fe8aedc..af2046bc63aa1 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -859,6 +859,7 @@ static void kernfs_notify_workfn(struct work_struct *work)
 	/* kick fsnotify */
 	down_write(&root->kernfs_rwsem);
 
+	down_write(&root->supers_rwsem);
 	list_for_each_entry(info, &kernfs_root(kn)->supers, node) {
 		struct kernfs_node *parent;
 		struct inode *p_inode = NULL;
@@ -894,6 +895,7 @@ static void kernfs_notify_workfn(struct work_struct *work)
 
 		iput(inode);
 	}
+	up_write(&root->supers_rwsem);
 
 	up_write(&root->kernfs_rwsem);
 	kernfs_put(kn);
diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
index 809b738739b18..d35142226c340 100644
--- a/fs/kernfs/mount.c
+++ b/fs/kernfs/mount.c
@@ -347,9 +347,9 @@ int kernfs_get_tree(struct fs_context *fc)
 		}
 		sb->s_flags |= SB_ACTIVE;
 
-		down_write(&root->kernfs_rwsem);
+		down_write(&root->supers_rwsem);
 		list_add(&info->node, &info->root->supers);
-		up_write(&root->kernfs_rwsem);
+		up_write(&root->supers_rwsem);
 	}
 
 	fc->root = dget(sb->s_root);
@@ -376,9 +376,9 @@ void kernfs_kill_sb(struct super_block *sb)
 	struct kernfs_super_info *info = kernfs_info(sb);
 	struct kernfs_root *root = info->root;
 
-	down_write(&root->kernfs_rwsem);
+	down_write(&root->supers_rwsem);
 	list_del(&info->node);
-	up_write(&root->kernfs_rwsem);
+	up_write(&root->supers_rwsem);
 
 	/*
 	 * Remove the superblock from fs_supers/s_instances
diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index 7ee0595b315a2..84653c609a5c0 100644
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -265,6 +265,7 @@ struct kernfs_root {
 
 	wait_queue_head_t	deactivate_waitq;
 	struct rw_semaphore	kernfs_rwsem;
+	struct rw_semaphore	supers_rwsem;
 };
 
 struct kernfs_open_file {
-- 
2.30.2