Re: [PATCH v6 5/7] kernfs: use i_lock to protect concurrent inode updates

[Ian Kent <raven@themaw.net>]

On Mon, 2021-06-14 at 09:32 +0800, Ian Kent wrote:
> 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...
> > 
> > > 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.
> 
> This didn't occur to me, obviously.
> 
> It seems to me this can happen with the original code too although
> using a mutex might reduce the likelihood of it happening.
> 
> Still ->permission() is meant to be a read-only function so the VFS
> shouldn't need to care about it.
> 
> Do you have any suggestions on how to handle this.
> Perhaps the only way is to ensure the inode is updated only in
> functions that are expected to do this.

IIRC Greg and Tejun weren't averse to adding a field to the 
struct kernfs_iattrs, but there were concerns about increasing
memory usage.

Because of this I think the best way to handle this would be to
broaden the scope of the i_lock to cover the generic calls in
kernfs_iop_getattr() and kernfs_iop_permission(). The only other
call to kernfs_refresh_inode() is at inode initialization and
then only for I_NEW inodes so that should be ok. Also both
generic_permission() and generic_fillattr() are reading from the
inode so not likely to need to take the i_lock any time soon (is
this a reasonable assumption Al?).

Do you think this is a sensible way to go Al?

Ian