On Wed, Jun 09, 2021 at 04:51:22PM +0800, Ian Kent wrote:
> The inode operations .permission() and .getattr() use the kernfs node
> write lock but all that's needed is to keep the rb tree stable while
> updating the inode attributes as well as protecting the update itself
> against concurrent changes.
Huh? Where does it access the rbtree at all? Confused...
> diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c
> index 3b01e9e61f14e..6728ecd81eb37 100644
> --- a/fs/kernfs/inode.c
> +++ b/fs/kernfs/inode.c
> @@ -172,6 +172,7 @@ static void kernfs_refresh_inode(struct kernfs_node *kn, struct inode *inode)
> {
> struct kernfs_iattrs *attrs = kn->iattr;
>
> + spin_lock(&inode->i_lock);
> inode->i_mode = kn->mode;
> if (attrs)
> /*
> @@ -182,6 +183,7 @@ static void kernfs_refresh_inode(struct kernfs_node *kn, struct inode *inode)
>
> if (kernfs_type(kn) == KERNFS_DIR)
> set_nlink(inode, kn->dir.subdirs + 2);
> + spin_unlock(&inode->i_lock);
> }
Even more so - just what are you serializing here? That code synchronizes inode
metadata with those in kernfs_node. Suppose you've got two threads doing
->permission(); the first one gets through kernfs_refresh_inode() and goes into
generic_permission(). No locks are held, so kernfs_refresh_inode() from another
thread can run in parallel with generic_permission().
If that's not a problem, why two kernfs_refresh_inode() done in parallel would
be a problem?
Thread 1:
permission
done refresh, all locks released now
Thread 2:
change metadata in kernfs_node
Thread 2:
permission
goes into refresh, copying metadata into inode
Thread 1:
generic_permission()
No locks in common between the last two operations, so
we generic_permission() might see partially updated metadata.
Either we don't give a fuck (in which case I don't understand
what purpose does that ->i_lock serve) *or* we need the exclusion
to cover a wider area.
