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Date:   Mon, 23 May 2022 20:07:54 -0400
From:   "J. Bruce Fields" <bfields@fieldses.org>
To:     Jeff Layton <jlayton@kernel.org>
Cc:     Chuck Lever III <chuck.lever@oracle.com>,
        Trond Myklebust <trondmy@hammerspace.com>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH RFC] NFSD: Fix possible sleep during
 nfsd4_release_lockowner()
Message-ID: <20220524000754.GB31155@fieldses.org>
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On Mon, May 23, 2022 at 05:28:16PM -0400, J. Bruce Fields wrote:
> On Mon, May 23, 2022 at 05:15:55PM -0400, Jeff Layton wrote:
> > On Mon, 2022-05-23 at 16:29 -0400, J. Bruce Fields wrote:
> > > On Mon, May 23, 2022 at 03:36:27PM -0400, Jeff Layton wrote:
> > > > The other lockowner _is_ involved. It's the one holding the conflicting
> > > > lock. nfs4_set_lock_denied copies info from the conflicting lockowner
> > > > into the LOCK/LOCKT response. That's safe now because it holds a
> > > > reference to the owner. At one point it wasn't (see commit aef9583b234a4
> > > > "NFSD: Get reference of lockowner when coping file_lock", which fixed
> > > > that).
> > > 
> > > I doubt that commit fixed the whole problem, for what it's worth.  What
> > > if the client holding the conflicting lock expires before we get to
> > > nfs4_set_lock_denied?
> > > 
> > 
> > Good point -- stateowners can't hold a client reference.
> > 
> > clientid_t is 64 bits, so one thing we could do is just keep a copy of
> > that in the lockowner. That way we wouldn't need to dereference
> > so_client in nfs4_set_lock_denied.
> > 
> > Maybe there are better ways to fix it though.
> 
> I kinda feel like lock and testlock should both take a 
> 
> 	struct conflock {
> 		void *data
> 		void (*copier)(struct file_lock, struct conflock)
> 	}
> 
> and the caller should provide a "copier" that they can use to extract
> the data they want while the flc_lock is held.
> 
> I'm probably overthinking it.

But in any case note the problem isn't just the copy of the clientid_t;
nfs4_put_stateowner also dereferences the client.

--b.