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

["Roger Pau Monné" <roger.pau@citrix.com>]

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?

Roger.