Subject: Re: [PATCH] nfs: fix bdi_unregister() before sb kill
From: Trond Myklebust <trond.myklebust@fys.uio.no>
To: Jens Axboe <jens.axboe@oracle.com>
Cc: Linux Kernel <linux-kernel@vger.kernel.org>, a.p.zijlstra@chello.nl
In-Reply-To: <20090918150248.GB23126@kernel.dk>
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Date: Fri, 18 Sep 2009 12:19:36 -0400
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On Fri, 2009-09-18 at 17:02 +0200, Jens Axboe wrote:
> On Fri, Sep 18 2009, Jens Axboe wrote:
> > On Thu, Sep 17 2009, Trond Myklebust wrote:
> > > On Thu, 2009-09-17 at 21:40 +0200, Jens Axboe wrote:
> > > > On Thu, Sep 17 2009, Jens Axboe wrote:
> > > > > Hi,
> > > > > 
> > > > > This can cause a hang on NFS umount, since the bdi gets pruned before we
> > > > > flush any pending dirty IO. Peter, can you check whether it fixes your
> > > > > issue?
> > > > 
> > > > There's another problem with NFS && backing devices. NFS may call
> > > > bdi_destroy() on a bdi without ever called bdi_init(). This was always a
> > > > bad idea, now it's an issue.
> > > > 
> > > > So, Trond, can I safely add a server->flags private flag to indicate
> > > > that we have called bdi_init()? Then nfs_free_server() knows when to
> > > > call bdi_destroy(). Seems like the safest fix, since error handling is
> > > > currently 'just call nfs_free_server()'.
> > > 
> > > Urgh... Is there any reason why we can't just move the call to
> > > bdi_init() into nfs_bdi_register()? It seems bizarre to have to
> > > initialise the backing_dev_info twice like this...
> > 
> > No reason at all, I don't know why it was implemented that way
> > originally.
> > 
> > > If we do that, then we can just look at the BDI_registered state flag in
> > > order to figure out if we need to call bdi_unregister()
> > 
> > That's not exactly pretty either, diving into internal bdi details to
> > find out if we did an init/register of the device. BDI_registered is
> > just a debug flag, it may even go away again shortly.
> 
> Trond, we need to make some sort of decision on this very shortly. It
> definitely needs to be fixed for -rc1, it's causing NFS oopses. So lets
> make some sort of call on this and get it added, then you/we/I can
> always pretty it up later.
> 

OK... I think the solution is to move the call to bdi_init() into
nfs_bdi_register(), then move the calls to nfs_bdi_register() into
nfs_set_super() (with an appropriate call to bdi_destroy() if
set_anon_super() fails).

Then we can put bdi_destroy() in place of the call to bdi_unregister()
in nfs_kill_super().

I'm not going to attempt a patch, since I don't have a copy of your
current tree to base it on, but does the above make sense to you?

Cheers
  Trond

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