On Sat, 2021-06-12 at 01:45 +0000, Al Viro wrote:
> 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...
That description's wrong, I'll fix that.
>
> > 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.
