linux-kernel - Re: [Xen-devel] [PATCH 1/1] xen-blkback: stop blkback thread of every queue in xen_blkif

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Date:   Fri, 18 Aug 2017 10:29:15 -0400
From:   annie li <annie.li@...cle.com>
To:     Roger Pau Monné <roger.pau@...rix.com>
Cc:     xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [Xen-devel] [PATCH 1/1] xen-blkback: stop blkback thread of every
 queue in xen_blkif_disconnect


On 8/18/2017 5:14 AM, Roger Pau Monné wrote:
> On Thu, Aug 17, 2017 at 06:43:46PM -0400, Annie Li wrote:
>> If there is inflight I/O in any non-last queue, blkback returns -EBUSY
>> directly, and never stops thread of remaining queue and processs them. When
>> removing vbd device with lots of disk I/O load, some queues with inflight
>> I/O still have blkback thread running even though the corresponding vbd
>> device or guest is gone.
>> And this could cause some problems, for example, if the backend device type
>> is file, some loop devices and blkback thread always lingers there forever
>> after guest is destroyed, and this causes failure of umounting repositories
>> unless rebooting the dom0. So stop all threads properly and return -EBUSY
>> if any queue has inflight I/O.
>>
>> Signed-off-by: Annie Li <annie.li@...cle.com>
>> Reviewed-by: Herbert van den Bergh <herbert.van.den.bergh@...cle.com>
>> Reviewed-by: Bhavesh Davda <bhavesh.davda@...cle.com>
>> Reviewed-by: Adnan Misherfi <adnan.misherfi@...cle.com>
>> ---
>>   drivers/block/xen-blkback/xenbus.c | 10 ++++++++--
>>   1 file changed, 8 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
>> index 792da68..2adb859 100644
>> --- a/drivers/block/xen-blkback/xenbus.c
>> +++ b/drivers/block/xen-blkback/xenbus.c
>> @@ -244,6 +244,7 @@ static int xen_blkif_disconnect(struct xen_blkif *blkif)
>>   {
>>   	struct pending_req *req, *n;
>>   	unsigned int j, r;
>> +	bool busy = false;
>>   
>>   	for (r = 0; r < blkif->nr_rings; r++) {
>>   		struct xen_blkif_ring *ring = &blkif->rings[r];
>> @@ -261,8 +262,10 @@ static int xen_blkif_disconnect(struct xen_blkif *blkif)
>>   		 * don't have any discard_io or other_io requests. So, checking
>>   		 * for inflight IO is enough.
>>   		 */
>> -		if (atomic_read(&ring->inflight) > 0)
>> -			return -EBUSY;
>> +		if (atomic_read(&ring->inflight) > 0) {
>> +			busy = true;
>> +			continue;
>> +		}
> I guess I'm missing something, but I don't see how this is solving the
> problem described in the description.
>
> If the problem is that xen_blkif_disconnect returns without cleaning
> all the queues, this patch keeps the current behavior, just that it
> will try to remove more queues before returning, as opposed to
> returning when finding the first busy queue.
Before checking inflight, following code stops the blkback thread,
                 if (ring->xenblkd) {
                         kthread_stop(ring->xenblkd);
                         wake_up(&ring->shutdown_wq);
                 }
This patch allows thread of every queue has the chance to get stopped. 
Otherwise, only thread of queue before(including) first busy one get 
stopped, threads of remaining queue will still run, and these blkthread 
and corresponding loop device will linger forever even after guest is 
destroyed.

Thanks
Annie