linux-kernel - Re: [PATCH] nfs: fix bdi_unregister() before sb kill

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Message-ID: <20090918150248.GB23126@kernel.dk>
Date:	Fri, 18 Sep 2009 17:02:48 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Trond Myklebust <trond.myklebust@....uio.no>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>, a.p.zijlstra@...llo.nl
Subject: Re: [PATCH] nfs: fix bdi_unregister() before sb kill

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.

-- 
Jens Axboe

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