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

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Message-Id: <200707261323.38310.arnd@arndb.de>
Date:	Thu, 26 Jul 2007 13:23:37 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Trond Myklebust <trond.myklebust@....uio.no>
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 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.

	Arnd <><
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