linux-kernel - Re: [patch] nfs: fix locking in nfs/inode.c in nfs_free_open

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Message-Id: <1185453451.6585.174.camel@localhost>
Date:	Thu, 26 Jul 2007 08:37:31 -0400
From:	Trond Myklebust <trond.myklebust@....uio.no>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Christian Krafft <krafft@...ibm.com>, linux-kernel@...r.kernel.org
Subject: Re: [patch] nfs: fix locking in nfs/inode.c in
	nfs_free_open_context

On Thu, 2007-07-26 at 13:23 +0200, Arnd Bergmann wrote:
> On Wednesday 25 July 2007, Trond Myklebust wrote:
> > 
> > On Wed, 2007-07-25 at 17:08 +0200, Christian Krafft wrote:
> > 
> > > Obviously the locking code in nfs_free_open_context is wrong.
> > > Checking the list for entries and removing the entry should be an atomic operation.
> > 
> > Wrong. It is quite safe to test the structure member ctx->list for
> > emptiness outside the spinlock because we have an explicit guarantee
> > that nobody else has a reference to this structure, plus the
> > atomic_dec_and_test() in kref_put() has acted as a memory barrier for
> > us.
> 
> Well, the real question then is how the ctx can still be present in the
> nfsi->open_files list. Since we are in nfs_free_open_context(), there
> must not be any pointer to the ctx anywhere, but still we have this other
> thread calling get_nfs_open_context() on it.

Yup. That is definitely a bug. I wish we had a 'kref_put_and_lock' to
deal with these situations where you want to grab a lock atomically with
the last put. It would make krefs a lot more useful...

Trond

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