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Date: Mon, 12 Jan 2015 13:04:51 +0000 From: David Vrabel <david.vrabel@...rix.com> To: Roger Pau Monné <roger.pau@...rix.com>, Bob Liu <bob.liu@...cle.com> CC: <xen-devel@...ts.xen.org>, <konrad.wilk@...cle.com>, <linux-kernel@...r.kernel.org>, <junxiao.bi@...cle.com> Subject: Re: [PATCH] xen/blkfront: restart request queue when there is enough persistent_gnts_c 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@...cle.com> >>>> Signed-off-by: Bob Liu <bob.liu@...cle.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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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