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From: Dan Williams <dan.j.williams@intel.com>
Date: Thu, 26 Jan 2017 08:39:54 -0800
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Subject: Re: [RFC PATCH v2 0/2] block: fix backing_dev_info lifetime
To: Christoph Hellwig <hch@lst.de>
Cc: Jan Kara <jack@suse.cz>,
        Thiago Jung Bauermann <bauerman@linux.vnet.ibm.com>,
        Jens Axboe <axboe@kernel.dk>, Rabin Vincent <rabinv@axis.com>,
        "linux-nvdimm@lists.01.org" <linux-nvdimm@ml01.01.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-block@vger.kernel.org, Andi Kleen <andi@firstfloor.org>,
        Wei Fang <fangwei1@huawei.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Jej B <James.Bottomley@hansenpartnership.com>
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On Thu, Jan 26, 2017 at 5:17 AM, Christoph Hellwig <hch@lst.de> wrote:
> On Thu, Jan 26, 2017 at 11:06:53AM +0100, Jan Kara wrote:
>> Yeah, so my patches (and I suspect your as well), have a problem when the
>> backing_device_info stays around because blkdev inode still exists, device
>> gets removed (e.g. USB disk gets unplugged) but blkdev inode still stays
>> around (there doesn't appear to be anything that would be forcing blkdev
>> inode out of cache on device removal and there cannot be because different
>> processes may hold inode reference) and then some other device gets plugged
>> in and reuses the same MAJOR:MINOR combination. Things get awkward there, I
>> think we need to unhash blkdev inode on device removal but so far I didn't
>> make this work...
>
> The other option is to simply not release the dev_t until the backing_dev
> is gone.

I came to a similar conclusion here:

   https://marc.info/?l=linux-scsi&m=147103737421897&w=4

James had some concerns, but I think its now clear this problem is
bigger than something we can fix locally in scsi.