linux-kernel - Re: [PATCH] lightnvm: prevent bd removal if busy

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Date:   Sun, 10 Sep 2017 22:14:30 +0300
From:   Rakesh Pandit <rakesh@...era.com>
To:     Matias Bjørling <mb@...htnvm.io>
CC:     <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Javier González <jg@...htnvm.io>
Subject: Re: [PATCH] lightnvm: prevent bd removal if busy

On Fri, Sep 08, 2017 at 12:42:47PM +0200, Matias Bjørling wrote:
> On 09/07/2017 03:58 PM, Rakesh Pandit wrote:
> > Removal of virtual block device by "nvm lnvm remove..." undergoing IO
> > and created by "nvme lnvm create... -t pblk" results in following and
> > is annoying.
> > 
> > 446416.309757] bdi-block not registered
> > [446416.309773] ------------[ cut here ]------------
> > [446416.309780] WARNING: CPU: 3 PID: 4319 at fs/fs-writeback.c:2159 __mark_inode_dirty+0x268/0x340
> > .....
> > 
> > This patch solves this by checking bd_openers for each partition
> > before removal can continue.  Note that this isn't full proof as
> > device can become busy as soon as it's bd_mutex is unlocked but it
> > needn't be full proof either.  It does work for general case where
> > device is mounted and removal can be prevented.
> > 
> > Signed-off-by: Rakesh Pandit <rakesh@...era.com>
[..]
> > +	while ((part = disk_part_iter_next(&piter))) {
> 
> A race condition can occur where disk_part_next tries to pblk (in
> block/genhd.c), and it in the meantime has been set to NULL. Leading to a
> kernel crash. Is there a better way to do it?
> 
> [root@...alhost ~]# nvme lnvm remove -n pblk0
> [ 5262.338647] BUG: unable to handle kernel NULL pointer dereference at
> 0000000000000010
> [ 5262.340769] IP: disk_part_iter_next+0xd3/0xf0

Thanks, indeed partition can go away from our feet if we don't lock
the whole thing from changing and not just individual partition locks.

I have given it another go which should avoid taking mutex locks on
bdev.  Posted V2.