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Re: [Qemu-devel] [PATCH 3/3] block: remove legacy_dinfo at blk_detach_de

From: Markus Armbruster
Subject: Re: [Qemu-devel] [PATCH 3/3] block: remove legacy_dinfo at blk_detach_dev time
Date: Mon, 21 Mar 2016 18:30:41 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)

Paolo Bonzini <address@hidden> writes:

> On 21/03/2016 17:15, Markus Armbruster wrote:
>> * Explicitly with x-blockdev-del
>>   Fails unless no other reference exists.  Where is the legacy_dinfo
>>   released?
> Can a -drive block device be deleted with x-blockdev-del even?!?

When I wrote my review, I forgot that I expect x-blockdev-del to accept
only backends created with blockdev-add.  With that, my question is
indeed moot.

However, I've now tested my expectation, and it turned out to be wrong.
I'm inclined to call that a bug.

> In other words, you said "This looks like DriveInfo now owns a reference
> to BlockBackend, even though the pointer still goes in the other
> direction".  I say, "I thought this was the idea all along"...

For me, the DriveInfo doesn't own anything, but a BlockBackend may have
a DriveInfo.  Evidence:

* The pointer goes from the BlockBackend to the DriveInfo

* To go back, you search the blk_backends for the one that has the
  DriveInfo.  See blk_by_legacy_dinfo().

* There is no list of DriveInfo.  If you want to find one, you search
  blk_backends.  See drive_get() & friends.

> Shall I add a check to x-blockdev-del that gives an error if the
> BlockBackend has a DriveInfo attached?

Yes, please.  But do double-check with Kevin & Max, who might have
different ideas on blockdev-add/del than I do.

> Paolo
>> * Implicitly via warty automatic deletion
>>   Your PATCH 01 has the necessary replacement of blk_unref(blk) by
>>   blkdev_del_drive(blk) for some devices (virtio-blk.c, scsi-bus.c,
>>   xen_disk,c, piix.c), butas far as I can see not for others such as
>>   nvme.c.

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