linux-kernel - Re: xen-blkfront: Don't send closing notification to backend in blkfront

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Date:	Sat, 09 Jul 2011 08:26:07 +0800
From:	Joe Jin <joe.jin@...cle.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	Daniel Stodden <daniel.stodden@...rix.com>,
	Jens Axboe <jaxboe@...ionio.com>, annie.li@...cle.com,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Ian Campbell <ian.campbell@...rix.com>,
	Kurt C Hackel <KURT.HACKEL@...cle.com>,
	Greg Marsden <greg.marsden@...cle.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: xen-blkfront: Don't send closing  notification to backend in
 blkfront_closing()

Konrad,

Thanks for the reply, see comments in lines.

On 07/09/11 00:04, Konrad Rzeszutek Wilk wrote:
> On Fri, Jul 08, 2011 at 03:14:29PM +0800, Joe Jin wrote:
>> When we do block attach detach test with below steps, umount hang and the
>> guest unable to shutdown:
>>
>> 1. start guest with the latest kernel.
>> 2. attach new disk by xm-attach in Dom0
>> 3. mount new disk in guest
>> 4. detach the disk by xm-detach in dom0
> 
> I think you mean xm block-detach and xm-attach?

You are right and sorry for confusing.

> 
> I tried with and without your patch and in both cases I get
> this in my guest:
> 
> sh-4.1# mount /dev/xvda /test
> [  385.949749] EXT3-fs: barriers not enabled
> [  385.960173] kjournald starting.  Commit interval 5 seconds
> [  385.960418] EXT3-fs (xvda): using internal journal
> [  385.960427] EXT3-fs (xvda): mounted filesystem with writeback data mode
> sh-4.1# [  411.176887] vbd vbd-51712: 16 Device in use; refusing to close
> 
> The commands on the other side (Dom0) were:
> 
> [root@...009 ~]# xm block-list 6
> Vdev  BE handle state evt-ch ring-ref BE-path
> 51712  0    0     4      12     770   /local/domain/0/backend/vbd/6/51712
> [root@...009 ~]# xm block-detach 6 51712
> Error: Device 51712 (vbd) could not be disconnected. 
> Usage: xm block-detach <Domain> <DevId> [-f|--force]
> 

The error caused by xm block-detach timeout  to waiting the dev's state switch
to Closed.

> Destroy a domain's virtual block device.
> [root@...009 ~]# xm block-detach 6 51712 -f
> 

With "--force", it always success but frontend did not disconnected if device 
opened by someone.

> 
>> 5. umount the partition/disk in guest, command hung. exactly at here, any
>>    IO request to the partition/disk will hang.
> 
> I get that with the patch and without it:
> 
> sh-4.1#
> sh-4.1# [  519.814048] block xvda: device/vbd/51712 was hot-unplugged, 1 stale handles
> 
> sh-4.1# df -h
> Filesystem            Size  Used Avail Use% Mounted on
> none                  490M  120K  490M   1% /dev
> none                  490M  131M  359M  27% /lib/modules/3.0.0-rc6-00052-g3edce4b-dirty
> shm                    10M     0   10M   0% /dev/shm
> var_tmp                10M     0   10M   0% /var/tmp
> /dev/xvda              20G  173M   19G   1% /test
> sh-4.1# umount /test
> 
> Any ideas?

This caused by backend kthread stopped, any IO request to the real device will hang, that
is the patch intend to resolving.

Thanks,
Joe

>>
>> Checking the code we found when xm-detach command set backend state to 
>> Closing, will trigger blkback_changed() -> blkfront_closing() call.
>> At the moment, the disk still opened by guest, so frontend will refuse the 
>> request, but in the blkfront_closing(), it send a notification to backend 
>> said that the frontend 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.
>>
>> Per our test, below patch fix this issue.
>>
>> Signed-off-by: Joe Jin <joe.jin@...cle.com>
>> Signed-off-by: Annie Li <annie.li@...cle.com>
>> ---
>>  xen-blkfront.c |    2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
>> index b536a9c..f6d8ac2 100644
>> --- a/drivers/block/xen-blkfront.c
>> +++ b/drivers/block/xen-blkfront.c
>> @@ -1088,7 +1088,7 @@ blkfront_closing(struct blkfront_info *info)
>>  	if (bdev->bd_openers) {
>>  		xenbus_dev_error(xbdev, -EBUSY,
>>  				 "Device in use; refusing to close");
>> -		xenbus_switch_state(xbdev, XenbusStateClosing);
>> +		xbdev->state = XenbusStateClosing;
>>  	} else {
>>  		xlvbd_release_gendisk(info);
>>  		xenbus_frontend_closed(xbdev);

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