[SRU][J][PATCH v3 5/6] kernfs: Use a per-fs rwsem to protect per-fs list of kernfs_super_info.

[ghadi.rahme at canonical.com]

From: Imran Khan <imran.f.khan at oracle.com>

BugLink: https://bugs.launchpad.net/bugs/2125142

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 at oracle.com>
Link: https://lore.kernel.org/r/20230309110932.2889010-3-imran.f.khan@oracle.com
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
(cherry picked from commit c9f2dfb7b59e5a6db054f821a6e1a6db8fa57d64)
Signed-off-by: Ghadi Elie Rahme <ghadi.rahme at canonical.com>
---
 fs/kernfs/dir.c             | 1 +
 fs/kernfs/file.c            | 4 ++--
 fs/kernfs/kernfs-internal.h | 1 +
 fs/kernfs/mount.c           | 8 ++++----
 4 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index bcc08c38d38f..636522aa8692 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -941,6 +941,7 @@ struct kernfs_root *kernfs_create_root(struct kernfs_syscall_ops *scops,
 	idr_init(&root->ino_idr);
 	init_rwsem(&root->kernfs_rwsem);
 	init_rwsem(&root->kernfs_iattr_rwsem);
+	init_rwsem(&root->kernfs_supers_rwsem);
 	INIT_LIST_HEAD(&root->supers);
 
 	/*
diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index 9414a7a60a9f..b526b29e5260 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -862,8 +862,8 @@ static void kernfs_notify_workfn(struct work_struct *work)
 
 	root = kernfs_root(kn);
 	/* kick fsnotify */
-	down_write(&root->kernfs_rwsem);
 
+	down_read(&root->kernfs_supers_rwsem);
 	list_for_each_entry(info, &kernfs_root(kn)->supers, node) {
 		struct kernfs_node *parent;
 		struct inode *p_inode = NULL;
@@ -900,7 +900,7 @@ static void kernfs_notify_workfn(struct work_struct *work)
 		iput(inode);
 	}
 
-	up_write(&root->kernfs_rwsem);
+	up_read(&root->kernfs_supers_rwsem);
 	kernfs_put(kn);
 	goto repeat;
 }
diff --git a/fs/kernfs/kernfs-internal.h b/fs/kernfs/kernfs-internal.h
index da34fb2d8ab8..1b9f2b21d21f 100644
--- a/fs/kernfs/kernfs-internal.h
+++ b/fs/kernfs/kernfs-internal.h
@@ -48,6 +48,7 @@ struct kernfs_root {
 	wait_queue_head_t	deactivate_waitq;
 	struct rw_semaphore	kernfs_rwsem;
 	struct rw_semaphore	kernfs_iattr_rwsem;
+	struct rw_semaphore	kernfs_supers_rwsem;
 };
 
 /* +1 to avoid triggering overflow warning when negating it */
diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
index cfa79715fc1a..a54e5ad432dd 100644
--- a/fs/kernfs/mount.c
+++ b/fs/kernfs/mount.c
@@ -346,9 +346,9 @@ int kernfs_get_tree(struct fs_context *fc)
 		}
 		sb->s_flags |= SB_ACTIVE;
 
-		down_write(&root->kernfs_rwsem);
+		down_write(&root->kernfs_supers_rwsem);
 		list_add(&info->node, &info->root->supers);
-		up_write(&root->kernfs_rwsem);
+		up_write(&root->kernfs_supers_rwsem);
 	}
 
 	fc->root = dget(sb->s_root);
@@ -375,9 +375,9 @@ 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->kernfs_supers_rwsem);
 	list_del(&info->node);
-	up_write(&root->kernfs_rwsem);
+	up_write(&root->kernfs_supers_rwsem);
 
 	/*
 	 * Remove the superblock from fs_supers/s_instances
-- 
2.34.1