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

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Date:   Sun, 13 Jun 2021 09:31:26 +0800
From:   Ian Kent <raven@...maw.net>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>, Eric Sandeen <sandeen@...deen.net>,
        Fox Chen <foxhlchen@...il.com>,
        Brice Goglin <brice.goglin@...il.com>,
        Rick Lindsley <ricklind@...ux.vnet.ibm.com>,
        David Howells <dhowells@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Carlos Maiolino <cmaiolino@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 5/7] kernfs: use i_lock to protect concurrent inode
 updates

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.