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

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Date:	Wed, 19 Dec 2007 14:30:57 -0600
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	Pekka J Enberg <penberg@...helsinki.fi>
Cc:	"Serge E. Hallyn" <serue@...ibm.com>, 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,
> 
> On Wed, 19 Dec 2007, Serge E. Hallyn wrote:
> > > 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.
> 
> No, I only use ->i_mutex for synchronizing the write to ->i_flags.

duh.

> On Wed, 19 Dec 2007, Serge E. Hallyn wrote:
> > > 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?
> 
> Yes, as soon as we unhash the dentries and the inode, do_lookup() will try 
> to find a new inode with iget() but we need to wait before writeback on 
> the revoked inode is finished.

Ok, that makes sense.  I'll let that sit for a short while and look
again :)

thanks,
-serge

> On Wed, 19 Dec 2007, Serge E. Hallyn wrote:
> > I'm a little confused but i'll keep looking.
> 
> I don't blame you. The patch is missing the following "minor detail" which 
> is needed to avoid fs corruption...
> 
> 			Pekka
> 
> Index: 2.6/fs/revoke.c
> ===================================================================
> --- 2.6.orig/fs/revoke.c	2007-12-16 19:57:40.000000000 +0200
> +++ 2.6/fs/revoke.c	2007-12-19 18:03:13.000000000 +0200
> @@ -426,6 +426,8 @@ 	int err = 0;
>  	make_revoked_inode(inode);
>  	remove_inode_hash(inode);
>  	revoke_aliases(inode);
> +
> +	err = write_inode_now(inode, 1);
>  failed:
>  	revoke_unlock(inode);
>  	wake_up(&inode->i_revoke_wait);
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