kernfs: dont take i_lock on inode attr read

The kernfs write lock is held when the kernfs node inode attributes
are updated. Therefore, when either kernfs_iop_getattr() or
kernfs_iop_permission() are called the kernfs node inode attributes
won't change.

Consequently concurrent kernfs_refresh_inode() calls always copy the
same values from the kernfs node.

So there's no need to take the inode i_lock to get consistent values
for generic_fillattr() and generic_permission(), the kernfs read lock
is sufficient.

Cc: Tejun Heo <[email protected]>
Signed-off-by: Ian Kent <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

fs/kernfs/inode.c

@@ -190,10 +190,8 @@ int kernfs_iop_getattr(struct user_namespace *mnt_userns,
 	struct kernfs_root *root = kernfs_root(kn);
 
 	down_read(&root->kernfs_rwsem);
-	spin_lock(&inode->i_lock);
 	kernfs_refresh_inode(kn, inode);
 	generic_fillattr(&init_user_ns, inode, stat);
-	spin_unlock(&inode->i_lock);
 	up_read(&root->kernfs_rwsem);
 
 	return 0;
@@ -288,10 +286,8 @@ int kernfs_iop_permission(struct user_namespace *mnt_userns,
 	root = kernfs_root(kn);
 
 	down_read(&root->kernfs_rwsem);
-	spin_lock(&inode->i_lock);
 	kernfs_refresh_inode(kn, inode);
 	ret = generic_permission(&init_user_ns, inode, mask);
-	spin_unlock(&inode->i_lock);
 	up_read(&root->kernfs_rwsem);
 
 	return ret;