[Xen-devel] Re: [PATCH -v2 0/3] xen-blkback: refactor vbd remove/disconnect.

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Aug 03, 2011 at 02:03:14PM +0800, Joe Jin wrote:
> This patchset is a backport and original patch author is Daniel Stodden:
> http://xenbits.xen.org/hg/XCP/linux-2.6.32.pq.hg/file/tip/CA-7672-blkback-shutdown.patch
> 
> Initial issue:
>   When we do block device attach/detach test with below steps, umount hang
>   in guest and the guest unable to shutdown:

So the patchset looks good and it fixes the guest hanging.. but
>   
>   1. start guest with the latest kernel.
>   2. attach new block device by xm block-attach in Dom0

So I think your patch while it fixes this problem it introduces a bug:

I did this in Dom0:

18:10:23 # 5 :~/
> xm block-attach 1 phy:/dev/sda xvda w

and did _not_ attach the disk in the guest. Then I did


18:10:35 # 6 :~/
> xm block-list 1
Vdev  BE handle state evt-ch ring-ref BE-path
51712  0    0     4      18     770   /local/domain/0/backend/vbd/1/51712

18:10:39 # 7 :~/
> xm block-detach 1 51712

18:10:46 # 8 :~/
> xm block-list 1



If I try the same sequence of events with your patch, I get this:

1:28:06 # 1 :~/ 
> xm list
Name                                        ID   Mem VCPUs      State   Time(s)
Domain-0                                     0  1500     4     r-----   1246.6
sda                                          2  2048     2     -b----   1034.7
sdb                                          6  2048     2     -b----      3.4
21:28:09 # 2 :~/ 
> xm block-list 6

21:28:22 # 4 :~/ 
> xm block-attach 6 phy:/dev/sdb xvda w

[did not do anything in the guest]
21:28:33 # 5 :~/ 
> xm block-list 6
Vdev  BE handle state evt-ch ring-ref BE-path
51712  0    0     4      18     770   /local/domain/0/backend/vbd/6/51712  

21:28:37 # 6 :~/ 
> xm block-detach 6 51712
Error: Device 51712 (vbd) could not be disconnected.
Usage: xm block-detach <Domain> <DevId> [-f|--force]

Destroy a domain's virtual block device.

21:30:30 # 7 :~/

Any ideas?
>   3. mount new disk in guest
>   4. execute xm block-detach to detach the block device in dom0 until timeout
>   5. try to unmount the disk in guest, umount hung. at here, any IOs to the 
>      device will hang.
>   
> Root cause:
>   This caused by 'xm block-detach' in Dom0 set backend device's state to
>   'XenbusStateClosing', frontend received the notification and 
>   blkfront_closing() be called, at the moment, the disk still using by guest,
>   so frontend refused to close. In the blkfront_closing(), frontend send a
>   notification to backend said that the its state switched to 'Closing', when
>   backend got the event, it will disconnect from real device, at here any IO
>   request will be stuck, even tried to release the disk by umount.
> 
>   So this may fix either frontend or backend, I have send a fix for frontend:
>   https://lkml.org/lkml/2011/7/8/159
>   Ian think we should fix it from backend and he pointed out Daniel Stodden 
> have
>   submitted a patch(see above link) for xen-blkback, I tried it and it works 
>   well.
> 
> Changes:
>   v2:
>     - Reformat code style.
>     - Per Knoard suggestions, change some int defines to bool.
> 
>  drivers/block/xen-blkback/blkback.c |   10 +--
>  drivers/block/xen-blkback/common.h  |    5 +
>  drivers/block/xen-blkback/xenbus.c  |  203 
> +++++++++++++++++++++++++++++++++++++++++++++++++++++++-------
>  3 files changed, 192 insertions(+), 26 deletions(-)

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