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

[Bob Liu <bob.liu@oracle.com>]

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()

If the system don't have enough grants(but have enough persistent_gnts),
request queue would still hang.

-- 
Regards,
-Bob