Re: [PATCH] xen/blkfront: restart request queue when there is enough persistent_gnts_c

[David Vrabel <david.vrabel@citrix.com>]

On 12/01/15 11:36, Roger Pau Monné wrote:
> El 12/01/15 a les 8.09, Bob Liu ha escrit:
>>
>> On 01/09/2015 11:51 PM, Roger Pau Monné wrote:
>>> El 06/01/15 a les 14.19, Bob Liu ha escrit:
>>>> When there is no enough free grants, gnttab_alloc_grant_references()
>>>> will fail and block request queue will stop.
>>>> If the system is always lack of grants, blkif_restart_queue_callback() can't be
>>>> scheduled and block request queue can't be restart(block I/O hang).
>>>>
>>>> But when there are former requests complete, some grants may free to
>>>> persistent_gnts_c, we can give the request queue another chance to restart and
>>>> avoid block hang.
>>>>
>>>> Reported-by: Junxiao Bi <junxiao.bi@oracle.com>
>>>> Signed-off-by: Bob Liu <bob.liu@oracle.com>
>>>> ---
>>>>  drivers/block/xen-blkfront.c |   11 +++++++++++
>>>>  1 file changed, 11 insertions(+)
>>>>
>>>> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
>>>> index 2236c6f..dd30f99 100644
>>>> --- a/drivers/block/xen-blkfront.c
>>>> +++ b/drivers/block/xen-blkfront.c
>>>> @@ -1125,6 +1125,17 @@ static void blkif_completion(struct blk_shadow *s, struct blkfront_info *info,
>>>>  			}
>>>>  		}
>>>>  	}
>>>> +
>>>> +	/*
>>>> +	 * Request queue would be stopped if failed to alloc enough grants and
>>>> +	 * won't be restarted until gnttab_free_count >= info->callback->count.
>>>> +	 *
>>>> +	 * But there is another case, once we have enough persistent grants we
>>>> +	 * can try to restart the request queue instead of continue to wait for
>>>> +	 * 'gnttab_free_count'.
>>>> +	 */
>>>> +	if (info->persistent_gnts_c >= info->callback.count)
>>>> +		schedule_work(&info->work);
>>>
>>> I guess I'm missing something here, but blkif_completion is called by
>>> blkif_interrupt, which in turn calls kick_pending_request_queues when
>>> finished, which IMHO should be enough to restart the processing of requests.
>>>
>>
>> You are right, sorry for the mistake.
>>
>> The problem we met was a xenblock I/O hang.
>> Dumped data showed at that time info->persistent_gnt_c = 8, max_gref = 8
>> but block request queue was still stopped.
>> It's very hard to reproduce this issue, we only see it once.
>>
>> I think there might be a race condition:
>>
>> request A                          request B:
>>
>>                                    info->persistent_gnts_c < max_grefs
>>                                    and fail to alloc enough grants
>>
>>
>> ^^^^
>> interrupt happen, blkif_complte():
>> info->persistent_gnts_c++
>> kick_pending_request_queues()
>>
>>                                     stop block request queue
>>                                     added to callback list
>>
>> If the system don't have enough grants(but have enough persistent_gnts),
>> request queue would still hang.
> 
> Not sure how can this happen, blkif_queue_request explicitly checks that
> persistent_gnts_c < max_grefs before adding the callback and stopping
> the request queue, so in your case the queue should not be blocked. Can
> you dump the state of info->connected?

I think Bob has correctly identified a race.

After calling blk_stop_queue(), check info->persistent_gnts again and
restart the queue if there free grefs.

David