linux-kernel - Re: [RFC/PATCH 2/8] revoke: inode revoke lock V7

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Date:	Wed, 19 Dec 2007 09:30:56 -0600
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	Pekka J Enberg <penberg@...helsinki.fi>
Cc:	serge@...lyn.com, alan@...hat.com, viro@...iv.linux.org.uk,
	hch@...radead.org, peterz@...radead.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC/PATCH 2/8] revoke: inode revoke lock V7

Quoting Pekka J Enberg (penberg@...helsinki.fi):
> Hi Serge,
> 
> (Thanks for looking at this. I appreciate the review!)
> 
> On Mon, 17 Dec 2007, serge@...lyn.com wrote:
> > >  	struct vfsmount *mnt = nd->mnt;
> > > -	struct dentry *dentry = __d_lookup(nd->dentry, name);
> > > +	struct dentry *dentry;
> > >  
> > > +again:
> > > +	dentry  = __d_lookup(nd->dentry, name);
> > >  	if (!dentry)
> > >  		goto need_lookup;
> > > +
> > > +	if (dentry->d_inode && IS_REVOKE_LOCKED(dentry->d_inode)) {
> > 
> > not sure whether this is a problem or not, but dentry->d_inode isn't
> > locked here, right?  So nothing is keeping do_lookup() returning
> > with an inode which gets revoked between here and the return 0
> > a few lines down?
> 
> I assume you mean S_REVOKE_LOCK and not ->i_mutex, right?

No I did mean the i_mutex since you take the i_mutex when you set
S_REVOKE_LOCK.  So between that and the comment above do_lookup(),
I assumed you were trying to lock out concurrent do_lookups() returning
an inode whose revoke is starting at the same time.

But based on your next paragraph it sounds like I misunderstand your
locking.

> The caller is supposed to block open(2) with chmod(2)/chattr(2) so while 
> revoke is in progress, you can get references to the _revoked inode_, 
> which is fine (operations on it will fail with EBADFS). The 
> ->i_revoke_wait bits are there to make sure that while we revoke, you 
> can't get a _new reference_ to the inode until we're done.

And a new reference means through iget(), so if revoke starts
between the IS_REVOKE_LOCKED() check in do_lookup and its return,
it's ok bc we'll get a reference later on?

I'm a little confused but i'll keep looking.

thanks,
-serge
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